(p. 597 ) Glossary

This is a list of definitions of the more unusual terms that are used and as they apply specifically to the locust. Words or phrases in bold in the definitions are defined elsewhere in their own right.

• 5-HT.

  5-hydroxytryptamine, often called serotonin. Contained in only a few central neurons that have extensive arborisations, and in neurons of the satellite nervous system that form release sites along certain nerves. Acts as a neuromodulator by blocking particular K⁺ channels.

• A4I1.

  An identified interneuron (I) with its cell body in abdominal ganglion 4 (A4), and arbitrarily given the number 1. It receives direct synaptic inputs from hairs on the ventral region of the neck that are sensitive to low velocities of air movements and is probably involved in controlling steering during flight by virtue of its output connections with motor neurons innervating the pleuroaxillary muscle of a wing.

• ACh.

  Acetylcholine. Probably the major transmitter of mechanosensory neurons. Probably also used by many interneurons but this use is not established.

• Acheta domesticus.

  Domestic cricket.

• Adipokinetic hormones.

  A large family of peptides the most well known of which are AKH-I and AKH-II. Others are known by names such as red pigment concentrating hormone and neurohormone D (myotropin I). About 20 members of this family have been sequenced so far. In the locust, AKH-I and -II and at least three other peptides (adipokinetic precursor-related peptides, APRPs) are synthesised in the glandular cells of the corpora cardiaca.

• Air sac.

  A thin walled, expandable extension of the tracheae that inflates and deflates with each cycle of the ventilatory rhythm so that it can smooth the changes in pressure.

• AKH.

  See adipokinetic hormones.

• Alternating tripod.

  The normal gait used by a locust and many other insects during walking, in which three legs are always supporting the weight of the body by their contact with the ground. The sequence of movements involves the middle leg on one side moving in time with the front and hind legs of the opposite side.

• Amacrine.

  A neuron that lacks an axon and which has branches restricted to one region of the central nervous system. The name could describe either a nonspiking or a spiking local interneuron, but is not commonly used in these contexts.

• (p. 598 ) Anastomosis.

  The fusion of two nerves from the same or adjacent ganglia, found most commonly in the abdomen and forming the ladder-like arrangement of the paired paramedian and lateral heart nerves, and in the thorax between N1 of one ganglion and N6 of the next posterior ganglion.

• Antagonist.

  A description of a muscle that works in opposition to another muscle in moving a particular joint. Thus, a muscle that extends a joint will often work as an antagonist to a muscle that flexes the same joint. Sometimes, however, these muscles need to work as synergists. The term is thus not an immutable description of the actions of muscles, but refers to their actions in a particular behavioural context.

• Antennal heart.

  An accessory muscular pump at the base of an antenna which aids in the flow of haemolymph into that antenna.

• Antidromic.

  A spike (action potential) which is conducted in the opposite direction to the normal (orthodromic) direction. This definition means that such spikes are normally only evoked experimentally, but in neurons with distinct spike-initiating zones it is used to describe the two way traffic of the spikes along an axon.

• Apodeme.

  Cuticular structure onto which muscle fibres insert, equivalent therefore to a tendon of a vertebrate. It is also the structure, sometimes called a ligament, to which the dendrites of sensory neurons in some chordotonal organs insert. The apodeme is an extension of cuticle, often from the adjacent section of an appendage, so that the movements of the joint are transmitted to sensory neurons.

• Assistance reflex.

  An adjustment of a movement to a stimulus that results in an enhancement of that movement. It is thus the reverse of a resistance reflex. The same stimuli can elicit either response depending on the behavioural context.

• Azadirachtin.

  A natural substance extracted from the Indian neem tree (Azadirachta indica) that prevents feeding and moulting. Fifth instar larvae treated with azadirachtin remain as larvae and fail to moult into adults that can fly. It may disrupt the synthesis or release of ecdysone. (See precocene.)

• Band.

  A large accumulation of larval locusts moving as a group, progressing by marching and hopping, and feeding prodigiously as they go.

• Basiconic sensilla.

  Sensory receptors with shafts protruding from the surface of the cuticle that are generally shorter than those of trichoid sensilla, set in a membranous socket, and which are innervated by several sensory neurons. They can serve a purely olfactory function, as on the antennae where the different sensory neurons respond to different odours. On the body, limbs and mouthparts, they generally function as contact chemoreceptors, although they may still respond to certain odours, but have one neuron that is a mechanoreceptor.

• Bistable membrane properties.

  See plateau potential.

• Brunner’s organ.

  A small, soft tubercle on the ventral surface of the femur of a hind leg; it is not present on the other legs. It is associated with a group of tactile hairs and campaniform sensilla. When the tibia is fully flexed the tubercle is distorted and may deflect the hairs. No function has been ascribed to this structure.

• C neuron.

  An intersegmental interneuron with its cell body in the mesothoracic ganglion and an axon projecting to the metathoracic ganglion, given the functional name cocking interneuron because of its previously supposed action during kicking and jumping.

• Calliphora erythrocephala.

  Blowfly.

• Campaniform sensillum.

  Sensory receptor set in the cuticle, in which the dendrite of a single sensory neuron is attached to a dome of flexible cuticle in an oval depression surrounded by the normal hard cuticle. It responds to strains and can thus act either as a proprioceptor, monitoring the forces generated by the muscles, or as an exteroceptor (p. 599 ) monitoring externally applied forces. May occur individually on many parts of the body and limbs, and as groups in specific places.

• Catch property.

  Strictly, this should be called ‘catch-like property’ to distinguish it clearly from the catch state of certain molluscan muscles. It refers to the hysteresis in the force generated by a muscle that depends on the past history of its motor impulses. The force that is generated by a particular frequency of motor spikes will be greater if it is preceded by a higher frequency of spikes than if preceded by a lower frequency.

• CCAP.

  Crustacean cardioactive peptide.

• CCK.

  Cholecystokinin.

• Central complex.

  The central region of the brain where fibres pass from one hemisphere of the brain to the other and which consists of the protocerebral bridge, the upper and lower divisions of the central body, and paired noduli. A role in visual processing is implicated on the basis of pathways from the compound eyes and ocelli and it receives a large input from neurons that show serotonin-like immunoreactivity, which may indicate that its processing is highly modifiable.

• Central pattern generator.

  A group of central neurons that, by virtue of their intrinsic membrane properties and the connections that they form with each other, are able to produce the basics of a motor pattern without reference to the sensory feedback that would normally be generated by such movements. The basics of many motor patterns such as those underlying flying and breathing can be produced by the central nervous system that is isolated from all sensory feedback. The motor programme, defined as the action of particular motor neurons, muscles or joints, results from the interplay between the output of these central neurons with sensory information fed back from these movements. The idea of central pattern generators has led to the unfortunate idea that separate groups of neurons are responsible for the production of different patterns, and that they represent a distinct entity in the central nervous system separable from other neurons. The reality is that neurons participate in many computations and may even contribute to the generation of different patterns in different circumstances. See circuit, motor programme, network.

• Cerci.

  Paired appendages of the last abdominal segment that have many hair sensilla that are highly sensitive to air currents, and other receptors. These appendages can be moved.

• Chordotonal organ.

  Sensory receptor with no obvious external cuticular structure that is slung from one point on the body to another, often in such a way that they can respond to movements of a joint. It consists of three types of cell: bipolar ciliated sensory neurons, a scolopale cell which envelops the distal region of the sensory neuron by secreting the fibrous scolopale to form a scolopidium, and a support cell. In each organ there may be several to many sensory neurons that insert on a ligament moved by joints or other parts of the body, and which therefore serve a proprioceptive function (e.g. FeCO at the femoro-tibial joint of a leg). They can also act as vibration (subgenual organ) or sound receptors (tympanum).

• CI1,2,3.

  Three inhibitory motor neurons that each innervate several muscles of a leg, so that they are called common inhibitory motor neurons and numbered 1–3.

• Circuit.

  A term often used interchangeably with network to define the neurons processing particular sensory stimuli, or with central pattern generator to define those neurons contributing to the production of a particular movement. A local circuit results from interactions between neurons in a restricted region of the central nervous system, such as a ganglion. As for central pattern generator, a circuit should not be considered as a separable entity from other neurons in the central nervous system; the contribution of particular neurons to a circuit can vary in different circumstances.

• (p. 600 ) Commissure.

  Bundle of neuronal processes running between the two sides of the central nervous system. In the segmental ganglia, many of the commissures are discrete and are therefore named, and contain the processes of identified neurons.

  Commissures in segmental ganglia

  Dorsal commissures	DCI-DCVI
  Ventral commissures	VCI, VCII
  Supramedian commissure	SMC
  Posterior ventral commissure	PVC

• Common inhibitor.

  Motor neurons with an inhibitory action on muscles. Each motor neuron innervates several muscles. See CI1,2,3.

• Corollary discharge.

  A group or pattern of spikes in an interneuron or population of interneurons that corresponds to the expression of some feature of a motor response that is being generated. The interneurons showing the corollary discharge are not thought to be primarily responsible for the generation of the motor pattern. It may simply be a way of describing the response of an interneuron when its action is unclear, or it may imply that certain signals are being fed back or fed forward to other known components of the networks involved in the production of the movement.

• Corpora allata.

  Paired neurosecretory structures behind the brain that secrete juvenile hormone (JH3), which plays an important role in development and in maturation. They are each linked to the corpora cardiaca by NCAI and to the suboesophageal ganglion by NCAII (also called suboesophageal N3). Each half of the corpora allata is innervated by 17 neurons, 13 with ipsilateral cell bodies and axons, and four with bilateral axons projecting in NCCII to the corpora cardiaca and then by NCAI to the ipsilateral corpora allata.

• Corpora cardiaca.

  Paired secretory structures just behind the brain and in front of the oesophagus innervated directly by two nerves from the brain and indirectly by a third (NCCI-III). The storage lobes contain the terminals of neurosecretory cells originating in the brain. The glandular lobes consist of intrinsic neurosecretory cells that synthesise, store and release into the haemolymph peptides, most notably the adipokinetic hormones (AKH).

• Coxa.

  Section of the proximal part of the leg of a locust between the thorax and the trochanter. The articulation with the thorax is complex and allows movement in three dimensions under the control of six muscle groups arranged in three pairs. It contains two of the parts of the depressor muscle and the levator muscles that move the trochanter about the hinge joint of the coxa with the trochanter. In the hind leg, the fusion of the trochanter and femur means that the femur is also moved by the action of these muscles. See also trochantin, tibia, tarsus, unguis.

• DCMD.

  Descending contralateral movement detector. An interneuron with its cell body in the protocerebrum of the brain and an axon in the contralateral connective to the thorax that codes the approach of an object along the axis of the eye.

• Decticus albifrons.

  A tettigonid from southern Europe.

• Dendrite.

  A term derived from vertebrates which means a branch from a cell body that bears input synapses. Strictly, the term has no meaning in insects because there are no such branches. Sometimes, however, it is used to describe the fine branches in the neuropil, but a better term is neurite or simply a branch. In sensory neurons, it is a process from the cell body that, for example, inserts into the shaft of a hair or onto the apodeme of a chordotonal organ.

• Deutocerebrum.

  A posterior region of the brain, often called the antennal lobe, that is the main processing region for the primary olfactory signals. It is indented where the gut (p. 601 ) protrudes between the connectives, so that it is separated ventrally into left and right parts. Each part of the deutocerebrum consists of a dome-shaped anterior (ventral) lobe that protrudes on either side of the oesophagus, and a smaller dorsal lobe, with the antennal nerve entering laterally. Cortically arranged cell bodies of neurons are clustered into particular groups and neuropil areas are concentrated into spherical glomeruli. See protocerebrum, tritocerebrum.

• Diadic.

  A synaptic arrangement, as seen with an electron microscope, in which a presynaptic process is associated with two postsynaptic processes. This contrasts with a monodic arrangement in which there is only one postsynaptic process.

• Drosophila melanogaster.

  Fruit fly.

• DUM neuron.

  Dorsal unpaired median neuron with a cell body in a group at the dorsal midline of the segmental ganglia and which is probably the progeny of the median neuroblast (with possibly a few neurons formed by other neuroblasts). There are at least three general classes.

  1. 1. Efferent DUM neurons have axons in particular nerves on both sides of a ganglion and run to muscles and other effectors on both sides of the body. Most, if not all, contain octopamine.

  2. 2. Local DUM neurons have bilateral processes that are contained within one neuromere, and may function as typical interneurons with GABA as their possible transmitter.

  3. 3. DUM interneurons have axons that project to other parts of the nervous system.

• DUMETi.

  An efferent, dorsal unpaired median neuron (DUM neuron) that supplies the extensor tibiae muscles of a segmental pair of legs. It modulates the contractions caused by the excitatory motor neurons (see SETi, FETi) by releasing octopamine.

• Equalising time constant.

  Describes the passive distribution of charge to non-isopotential regions of a neuron, and is usually much smaller than the membrane time constant.

• Exteroceptor.

  A receptor on the surface of the body and appendages that responds to signals in the environment of a particular modality. Many of these receptors signal stimuli at a distance (e.g. vision, smell and sound) while others signal contact of the limbs or body with an external object, or with other parts of its own body (e.g. touch, mechanoreception). Some of these mechanoreceptors can thus act as proprioceptors, especially those that are placed at joints or at particular points of strain or stress in the cuticle.

• Fascicle.

  A bundle of fibres that group together and run parallel to each other in peripheral nerves or in tracts and commissures of the developing central nervous system. Many fascicles contribute to each nerve, tract or commissure and newly developing neurons join a particular fascicle, indicating that its constituent fibres have unique labels.

• Femur.

  Section of the leg between the trochanter and the tibia. It contains the extensor and flexor tibiae muscles that operate the hinge joint with the tibia, and part of the retractor unguis muscle. In the hind leg, the femur is greatly enlarged to contain the large extensor muscle involved in jumping and kicking, has a hexagonal profile, and is marked externally by a herring-bone pattern formed by the insertion points of the extensor muscle. See also coxa, tarsus, unguis.

• FETi.

  Fast motor neuron innervating the extensor tibiae muscle of a leg. See also SETi, DUMETi.

• Fibrillar muscle.

  Muscle that is used to move the wings of insects such as flies and bees that have high wing beat frequencies when flying. The muscles do not act directly on the wings but instead distort the shape of the thorax. Contractions result from a muscle being stretched and not directly from its activation by motor spikes. Consequently, the pattern of wing movements is not a direct reflection of the pattern of spikes in the motor neurons. The motor spikes alter the power output. Locusts have conventional muscles to (p. 602 ) move their wings so that the wing movements are a direct reflection of the pattern of spikes in the motor neurons.

• Fictive.

  A term that has come to describe a motor pattern produced by a nervous system from which sensory feedback has been eliminated. The pattern is thus produced without any of the proprioceptive or other sensory cues that would occur during normal behaviour. To understand what the fictive pattern might represent it is necessary to define the features of a normal motor pattern and then compare them with those of the fictive pattern. The fictive patterns expressed in motor neurons innervating muscles that move the wings seem to represent flight, but those in motor neurons innervating leg muscles could represent several movements, of which walking is only one. Even if the relationships seem clear, there may still remain doubts about whether the pattern is being produced in the way that it normally would be when sensory information is present.

• Filopodium.

  An extension from the growth cone of a developing neuron that samples its immediate environment and at successive choice points makes decisions about the direction in which to extend. Each growth cone can have a number of filopodia with diameters of less than 0.5 pm extending up to 100 μm. A particular filopodium advances at rates of 1−2 pm.h⁻¹ and can either persist for more than 10 min or can be retracted within 1 min. Internal second messenger signals are transmitted from the tip of a filopodium to the actomyosin filaments at the base to amplify small differences in extrinsic clues. This ensures that the growth cone does not simply advance in a way that is determined by the sum of the adhesive forces acting on all its filopodia.

• Flight balance.

  A device used to support a locust during tethered flight and to measure the forces that it is producing. Such balances vary in the sophistication of their design with some merely allowing lift to be measured and others rotational torques. Some allow the phase relationships between selected motor neurons to be fed back and control horizontal turning torque in response to horizontal movement of the visual world. The contribution of the flight balance used to the motor pattern that is observed must be recognised, and will impose differences from the motor pattern that is used during free flight where all feedback loops are closed.

• Frontal ganglion.

  A ganglion of some 100 neurons on the dorsal surface of the pharynx anterior to the brain, linked to the tritocerebrum by paired frontal connectives, and by a recurrent nerve to the hypocerebral ganglion. It is one of the stomatogastric ganglia.

• GABA.

  γ-Aminobutyric acid, a putative transmitter of a few inhibitory motor neurons, and of many other neurons in the central nervous system. It is associated with an inhibitory effect on most neurons mediated by receptors with properties different from vertebrate GABA_A and GABA_B receptors. The action is usually through an increase in chloride conductance that can be blocked by picrotoxin.

• Gait.

  The sequence in the which the six legs are moved during walking. Typically, the legs are moved in an alternating tripod gait, with three of the legs on the ground while the other three legs are moved forwards.

• Ganglion mother cell.

  Progeny of the asymmetric division of a neuroblast in the embryonic nervous system. Each ganglion mother cell divides symmetrically just once to form cells that differentiate as neurons.

• GI.

  Abbreviation for giant interneuron.

• Giant interneuron.

  An interneuron that is substantially larger than other neurons in the nervous system. In insects, this relative term is applied only to a group of intersegmental interneurons that receive inputs from wind-sensitive hairs on the cerci. The term refers to the diameter of their axons in the abdominal nervous system.

• (p. 603 ) Glioblast.

  A precursor cell in an embryo with properties similar to a neuroblast except that its progeny are glial cells instead of neurons.

• Glomerulus.

  Spherical neuropil area characteristic of olfactory processing in the deutocerebrum. Primary sensory neurons project directly to these regions and make synaptic contacts with local and projection interneurons.

• Gregarious phase.

  Locusts with a particular morphological appearance and behaviour that results from them living in crowded conditions, as opposed to those living in isolated conditions (see solitary phase). The larvae are black and yellow and develop more quickly than solitary ones. The adults have a broader head and larger compound eyes than solitary ones, and fly readily during the day rather than at night. The phase differences are under hormonal control.

• Gryllus campestfis.

  Field cricket.

• Guidepost cell.

  A cell in an embryo that may provide one of the clues enabling a pioneer neuron and its followers to establish the route of a peripheral nerve or the tracts and commissures within the developing central nervous system.

• Hair row.

  An array of hairs that are shorter and stiffer than other trichoid sensilla (tactile hairs). Probably serve a proprioceptive function signalling changes in joint movements and positions.

• Haemolymph.

  Fluid (with a volume of about 200 μl in an adult locust) bathing the tissues and which carries neuromodulators and neurohormones to their targets. It plays little part in gaseous exchange as this is carried out by the network of tracheae and tracheoles. The circulation is largely open through various sinuses but is propelled by the rhythmic beating of the tubular heart. Flow to the hind legs may be aided by a myogenic rhythmicity of a small bundle of fibres in the extensor tibiae muscles, and to the wings and antennae by pulsatile organs.

• Hair plates.

  Concentrations of short trichoid sensilla in small patches close to the articulations of the joints. They are stimulated when the joint is close to one of its extremes and thus act as proprioceptors.

• Hemimetabolous.

  Hemimetabolous insects, such as flies and moths, emerge from the egg as larvae that are of quite different appearance to an adult. After a period as freely moving larvae, they pupate. Adults eventually emerge from the pupae. See holometabolous.

• HEPES.

  N-2-Hydroxyethylpiperazine-N′-2-ethanesulphonic acid, used as a buffer in some Ringer solutions.

• Holometabolous.

  Holometablous insects, such as the locust, that undergo a period of embryonic growth and development within the egg before hatching as an insect that has most of the characteristics of an adult, but is much smaller. There then follows a postembryonic period of growth and moulting through five larval stages, or instars, of progressively increasing size before a final moult to the adult. See hemimetabolous.

• Homeotic genes.

  The loss of one of these genes results in the transformation of a segment or part of a segment into the pattern characteristic of another. They do not appear to affect segment boundaries. Many of these genes occur in two clusters.

  1. 1. Bithorax cluster containing, for example, Ultrabithorax and bithoraxoid.

  2. 2. Antennapedia complex containing, for example, Antennapedia, Deformed and Sex combs reduced.

  These genes are expressed both in the embryonic nervous system and body, with particular ones expressed most strongly in a particular region consisting of the posterior part of one segment and the anterior part of the next posterior segment, a parasegment. See also segmentation genes.

• (p. 604 ) Homosynaptic depression.

  Depression of the effects of a particular neuron on its postsynaptic neurons as a result of its preceding spikes. This effect contrasts with heterosynaptic depression caused by spikes in other neurons.

• Hopper.

  See larva.

• Hypocerebral ganglion.

  A small unpaired ganglion, sometimes called the occipital ganglion, behind the brain, dorsal to the pharynx and ventral to the corpora cardiaca, linked anteriorly by the nervus recurrens to the frontal ganglion, dorsally to the corpora cardiaca, and posteriorly by the paired oesophageal nerves. Its neurons are undescribed.

• Hysteresis.

  Nonlinearity in the force generated by a muscle, or in the membrane potential or spike frequency of a neuron as a result of the history of action of that muscle, or of other neurons. In muscle, it is most readily expressed as a catch-like property.

• Immunoreactivity.

  Staining with antibodies raised against specific antigens that is circumspectly described as antigen-like immunoreactivity. The caution is necessary because of the uncertainty of just what the antibody is binding to.

• Ingluvial ganglion.

  One of a pair of ganglia on either side of the crop, each containing perhaps 60 neurons that arise from the oesophageal nerves from the hypocerebral ganglion. The ganglia innervate the anterior portion of the gut and are part of the stomatogastric ganglia.

• Instar.

  See larva.

• Integrating segment.

  A term loosely applied to the expanded section of the primary neurite of a neuron as it courses through the neuropil and from which many of the fine neurites or branches arise. It is a misleading functional term, for there is little evidence that this branch performs more integration than any of the other branches. In those neurons that spike, the spike-initiating zone is generally thought to be somewhere along its length. The neutral term neuropilar segment is to be preferred.

• Intersegmental interneuron.

  An interneuron (sometimes called a projection interneuron or a principal interneuron) with a cell body in one ganglion, or one part of the brain, and an axon that projects to another ganglion or to another distinct region of the brain. These neurons contrast with local interneurons that have all their branches restricted to one part of the nervous system (often to a single segmental ganglion).

• Junction(al) potential.

  The potential change evoked in a muscle fibre by the release of transmitter from an excitatory or inhibitory motor neuron. The term is synonymous with synaptic potential used to describe the potential change in a neuron resulting from the release of transmitter from another neuron.

• Juvenile hormone.

  Hormone secreted by the corpora allata that prevents moulting in larvae and the maturation of the ovaries in adults. Its action is antagonised by precocene.

• Kenyon cell.

  Small interneurons in the mushroom bodies of the protocerebrum named after their discoverer. They are thought to be involved in olfactory processing and learning.

• Lamina.

  First (distal) neuropil region of an optic lobe that receives inputs in the form of graded depolarising signals from the axons of the retinula cells and generates graded hyperpolarising output signals in lamina monopolar neurons. The lamina consists of an array of cartridges that equal the number of ommatidia. Within each cartridge, synaptic interactions occur between the input neurons (the retinula cells), intrinsic neurons that run between adjacent cartridges, feedback neurons from the medulla, and the output neurons (the lamina monopolar neurons). Each cartridge may be electrically isolated from its neighbours by glial cell wrappings.

• Larva.

  One of a number of stages in the postembryonic development of holometabolous insects such as the locust. A locust has five larval stages or instars which progressively increase in size at each moult. They all resemble the adult locust but are sexually immature and lack (p. 605 ) functional wings and particular arrays of sensory neurons. Also called nymph, or hopper after their characteristic mode of progression. In hemimetabolous insects, the larva is the postembryonic stage that emerges from the egg and that then pupates.

• Lobula.

  The third proximal region of an optic lobe where convergence starts to occur so that the number of repeating arrays of neurons is less than the number of ommatidia. Many of the neurons are large and have outputs in the central regions of the brain or have axons that extend to the other optic lobe. Neurons here either have wide receptive fields and provide information that is necessary for the stabilisation of the visual world, or small receptive fields and provide information about the movement of small objects in the visual world.

• Local circuit.

  See central pattern generator, circuit, network, pathway.

• Local interneuron.

  A neuron that may lack an axon or has only a short axon and whose branches are restricted to one neuromere. In the segmental chain of ganglia, it can be applied strictly to mean a neuron whose processes are restricted to one ganglion. In the brain, the term becomes less useful because distinctions between regions may not be so anatomically clear and it may then define an interneuron that projects from one region to another.

• Locusta migratoria migratoriodes.

  Migratory locust from Africa.

• M neuron.

  An intersegmental interneuron with its cell body in the metathoracic ganglion and given the functional name multimodal neuron because it responds to more than one modality of sensory stimulus. Many interneurons are, however, affected by stimuli of different modalities. It is thought to be involved in controlling the kicking and jumping movements of the hind legs.

• Malpighian tubules.

  A series of about 200 long, narrow, blind-ending tubules that arise close to the junction of the midgut and hind gut and lie in the haemolymph of the abdominal cavity. Their secretory epithelium is involved in the transport of fluids and the production of urine, and thereby the regulation of the fluid and ionic balance of the haemolymph.

• Manduca sexta.

  Tobacco hornworm moth from North America.

• Median nerve.

  An unpaired nerve arising from the dorsal midline of the thoracic and abdominal ganglia. It contains the axons of motor neurons that branch to supply spiracular muscles on the left and right sides of the body and the axons of neurons containing peptides that end in neurohaemal swellings of this nerve. In the prothoracic and abdominal ganglia there are both anterior and posterior median nerves which anastomose in the abdomen to form a chain.

• Medulla.

  The second neuropil region of an optic lobe, in which the repeating arrays of neurons still match the number of ommatidia. The number of neuron types is greatly increased over those present in the lamina (first distal neuropil layer). Some of these neurons signal aspects of movement in the visual world and the orientation of polarised light.

• Melanoplus sanguinipes.

  Rocky mountain locust from North America.

• Membrane time constant.

  Describes the time taken for a voltage to decay to 63% of its original amplitude when the membrane is in a passive state. (See equalising time constant.)

• Midline precursor cell.

  One of a small group of precursor cells at the midline of the developing nervous system. Some are recognised as ganglion mother cells of the median neuroblast. Others are distinct from the neuroblasts in that they arise from different ectodermal cells and divide symmetrically.

• Monodic.

  A synaptic arrangement, as seen with an electron microscope, in which a presynaptic process is associated with a single postsynaptic process. This contrasts with a diadic arrangement of two postsynaptic neurons.

• (p. 606 ) Motor pool.

  Set of motor neurons that innervate a particular muscle. Each motor neuron in a pool may innervate a particular part of the muscle, or more commonly the innervation overlaps so that a muscle fibre is innervated by more than one excitatory motor neuron.

• Motor programme.

  The pattern of spikes in motor neurons (and other efferent neurons), muscle contractions, or joint movements that underlie a particular movement. In many of these movements a central pattern generator may supply a basic rhythmicity whose frequency and patterning can be substantially altered to produce the sequence of motor spikes that is observed as the motor programme.

• Moult.

  A periodic shedding of the cuticle that enables a locust to increase in size through five larval instars, and which is under hormonal control. The movements that cause the old cuticle to split and the locust to emerge and inflate its soft body are aided by the abdominal muscles pumping air around the tracheae.

• Multipolar receptor.

  A receptor cell at the joints that acts in parallel to the strand receptors and chordotonal organs. Usually occurs in small groups of 1−3 but the information it might provide in addition to that from the more numerous cells of the chordotonal organs is not understood.

• Muscle receptor organ.

  A sensory receptor associated with a muscle whose contractions can potentially set the sensitivity of the receptor cells. One such receptor in the coxa of a leg (coxo-trochanteral muscle receptor organ, CxTrMRO) consists of a single sensory cell associated with a bundle of three muscle fibres innervated by a single motor neuron. The dendrites of the sensory neuron insert in series only on the collagenous insertion of the receptor muscle, so that they should be affected by both force and length changes in the receptor muscle. It is the only receptor in the leg for which specific efferent control has so far been demonstrated. See also tension receptor.

• Mushroom bodies.

  Region of the anterior protocerebrum that derives its name from a superficial resemblance to an inverted lamellated mushroom. Involved in the processing of olfactory and probably other modalities of sensory information.

• Myogenic.

  A contraction of a muscle that results from intrinsic properties of its fibres and/or interactions between those fibres. The contractions, which are often rhythmical, do not depend on spikes in the motor neurons that innervate the muscle, although these spikes may alter the contractions. Only a few skeletal muscles, such as a small part of the extensor tibiae muscle of a hind leg, show such contractions, but they are common in muscles of the gut and oviducts. See also neurogenic.

• Myogram.

  An electrical recording of the activity of a muscle usually recorded with pairs of small diameter wires. The activity reflects the response of the muscle fibres to spikes in motor neurons, although sometimes the motor spikes themselves can be recorded. The use of thin and light wires allows recordings to be made during unimpeded natural movements. See neurogram.

• Network.

  See central pattern generator and circuit.

• Neurite.

  Branch of a neuron within the neuropil, variously called a process, an arborisation, or simply a branch, but sometimes erroneously called a dendrite. The single process emerging from the cell body of a neuron is called the primary neurite, and this may expand to form the so-called integrating segment, from which many smaller branches or neurites emerge.

• Neuroblast.

  A cell that emerges from the sheet of ectoderm of an embryo and enlarges before embarking on a series of asymmetric divisions to form ganglion mother cells. These then divide symmetrically to form two cells that differentiate into neurons. Each segment contains a fixed number of neuroblasts which form distinct lineages of neurons. Only a (p. 607 ) few neuroblasts in holometabolous insects such as the locust remain mitotic in postembryonic development. See also glioblast.

• Neurogenic.

  A contraction of a muscle produced as a result of the action of spikes in the motor neurons that innervate it. Skeletal muscles typically contract in this way, although a few can also contract myogenically. See also neurogenic genes.

• Neurogenic genes.

  The spatial and temporal sequence of neuroblast formation is controlled by the neurogenic genes such as Notch. Loss of a neurogenic gene results in all cells in the ventral strip of ectoderm becoming neuroblasts, whereas loss of the proneural achaete-scute complex can result in embryos that lack only a single neuroblast but which then show much neuronal degeneration.

• Neurogram.

  An electrical recording of potential changes in many neurons in the central nervous system usually made with electrodes larger than those used for intracellular recording or for extracellular recording of spikes. Sometimes, small wires have been inserted into the tracheae that invade the central areas of the CNS, but this technique has yet to be exploited fully. See myogram.

• Neurohaemal organ.

  Typically a swollen enlargement of a median nerve containing the terminals of central neurons and the cell bodies of other neurons that release a variety of neurosecretory or neuromodulatory substances. The release is probably into the haemolymph.

• Neuromere.

  Part of the central nervous system belonging to one segment of the body. For much of the body, the organisation of the nervous system reflects its segmentation, with one segmental ganglion representing the neuromere of a particular segment. However, in the brain, suboesophageal, metathoracic and last abdominal ganglion, several neuromeres are fused. In the brain of the adult, the segmental origins of the different regions are not obvious.

• Neuropil (neuropile).

  The region in the central nervous system where most of the synaptic interactions between neurons take place. It is a fibrous region inside the cortex of cell bodies and contains no cell bodies itself. It is penetrated by longitudinal and vertical tracts and by commissures. Prominent regions of neuropil in segmental ganglia are named.

  Neuropil regions in segmental ganglia

  Ventral neuropils	VAC
  Ventral association centre	aVAC
  anterior	aVAC
  lateral	lVAC
  medial	mVAC (aRT)
  ventralmost	vVAC
  Dorsal neuropils
  Lateral association centre	LAC
  anterior	aLAC
  posterior	pLAC

• Neuropilar segment.

  The expanded primary neurite of a neuron as it courses through the neuropil and gives rise to many fine neurites or branches. Sometimes, the functional term integrating segment is also used but the implication behind this term may be misleading.

• Nonspiking interneuron.

  An interneuron that does not produce action potentials even during the performance of apparently normal behaviour and which cannot be made to spike by experimentally activating a particular input pathway, or by the intracellular injection of depolarising current. Under some extreme and unphysiological experimental conditions (p. 608 ) these neurons can generate small spikes, but there is no evidence that these are normally used for intracellular or intercellular communication. All the nonspiking interneurons that are known are local interneurons with their branches restricted to one ganglion, or one region of the central nervous system. The term nonspiking is thus a pragmatic one, based on what is normally observed. The important feature of these neurons is their ability to communicate with other neurons with graded signals and without the intervention of spikes.

• Nymph.

  See larva.

• Occipital ganglion.

  See hypocerebral ganglion.

• Octopamine.

  p-Hydroxyphenylethanolamine, a neuromodulatory substance first isolated from an octopus, and contained in a number of central neurons of the locust, most notably the efferent DUM neurons. There are two isomers of octopamine: D-octopamine the natural isomer occurs as the minus form, D(-)octopamine; L-octopamine occurs as the plus form, L(+)octopamine. DL-octopamine is thus a mixture of the two isomers. Sometimes, when the isomers present are not known, octopamine is called p-octopamine to indicate that the hydroxyl group is parallel to the chain.

• Ommatidium (ommatidia).

  One of the several thousand repeating optical and neural units in the compound eye. Each ommatidium consists of a hexagonally shaped corneal lens and eight retinula cells that contain the visual pigment, and may be screened from neighbouring ommatidia by pigment-containing cells.

• Optic lobe.

  Region of the protocerebrum below the compound eye forming a prominent lateral lobe. It is chiefly concerned with the processing of visual signals and its most distal two regions, the lamina and medulla contain as many repeating elements as there are ommatidia in the eye. Convergence occurs in the third and most proximal region, the lobula.

• Optomotor response.

  A movement of the head, or of both the head and the whole body, in response to a movement of the visual world, that attempts to stabilise the visual image on the retina. Experimentally, it is usually induced by the rotation of a pattern of black and white stripes around the animal.

• Orthodromic.

  The direction in which a spike is normally conducted in a neuron away from the spike-initiating zone. It contrasts with antidromic spikes that can be induced experimentally, or which describe spikes in neurons with distinct spike-initiating zones.

• Paramedian nerve.

  A ladder-like arrangement of paired nerves running parallel to the main nerve cord in the abdomen and formed by anastomoses of the unpaired median nerve with lateral nerves of the abdominal ganglia.

• Parasegment.

  A unit of the nervous system (primarily in the embryo) consisting of the posterior part or compartment of one segment and the anterior compartment of the next posterior segment.

• Paraventricular ganglion.

  Largely undescribed ganglion associated with the foregut that is part of the stomatogastric ganglia.

• Pathway.

  A general term used to describe the route taken by signals within the nervous system. The pathway from sensory to motor neurons could imply that the sensory neurons make direct connections with the motor neurons, or that several layers of interneurons intervene. See central pattern generator, circuit, network.

• PDH.

  Pigment dispersing hormone.

• Perineurium.

  Thin specialised layer of glia beneath the connective tissue sheath of the central nervous system that provides an ionic barrier, essentially acting like a blood-brain barrier, so that the ionic concentrations at the surface of the neurons are quite different from those in the haemolymph.

• (p. 609 ) Periplaneta americana.

  American cockroach.

• Pheromone.

  A chemical released by an insect that affects the behaviour of another insect, usually of the same species, over a distance. Many pheromones are used as sexual attractants.

• Photoinactivation.

  A method for killing a whole neuron or selected parts of an individual neuron by the injection of a photoactive dye and its subsequent activation by a bright light of the appropriate wavelength. Thus, if the dye Lucifer Yellow is injected into a neuron, the neuron will be killed if the whole preparation is illuminated with a bright blue light, but if a laser is used to focus the light to a specific region then a neurite or an axon can be severed from the rest of the neuron.

• Picrotoxin.

  A substance that is used to block the action of GABA by its action on chloride channels.

• Pilocarpine.

  A cholinergic, muscarinic agonist, most notable for its ability to induce rhythmic motor activity in isolated thoracic ganglia.

• Pioneer neuron.

  A neuron in an embryo that is the first to chart the route of a peripheral nerve to the central nervous system, or to establish the path of a tract or commissure within the central nervous system.

• Plateau potential.

  A prolonged depolarisation in a neuron caused by a brief depolarisation above a certain voltage threshold. It results from the activation of a voltage-dependent inward current that in some neurons may be carried by Ca²⁺ and which imparts a region of negative slope in the current–voltage relationship. In some neurons, these effects can be generated by synaptic inputs alone, but in others they occur only in the presence of neuromodulators. This intrinsic membrane property means that a neuron will respond in a nonlinear way to the synaptic inputs that it receives depending on whether the voltage changes are sufficient to activate or deactivate these currents. A neuron can thus exist at two stable states; one at resting potential, the other at a depolarised plateau. Such neurons are thus often said to have bistable membrane properties.

• Pleomorphic.

  Synaptic vesicles with an irregular, rather flattened or ovoid shape. They are often associated with neurons that also show GABA-like immunoreactivity, but the existence of one of these features does not necessarily herald the presence of the other.

• Polyneuronal.

  The innervation of an individual muscle fibre by more than one motor neuron.

• Precocene.

  A natural plant product extracted from Ageratum houstonianum (commonly used as a summer bedding plant) that has an anti-juvenile hormone action. Application to a larval stage results in precocious moulting to the next stage. Two compounds, precocene 1 and 2, are known. (See azadirachtin.)

• Primary neurite.

  The single process that emerges from the cell body. Within the neuropil this process often expands in diameter, and is then sometimes called the integrating segment. From the primary neurite arise the many fine branches or neurites which ramify in the neuropil.

• Principal interneuron.

  See intersegmental interneuron.

• Proctodeum.

  The hind gut, which like the foregut or stomodeum is formed from ectoderm and is thus lined by a layer of cuticle that is shed at each moult. It consists of the ileum and rectum. The midgut is, however, formed from endoderm.

• Projection interneuron.

  See intersegmental interneuron.

• Promotor.

  The name used to describe the anterior tergocoxal muscles (muscle 62 of the prothorax, 89 of the mesothorax and 118 of the metathorax) and their supposed action in lifting the anterior edge of the coxa upwards. These muscles in the meso- and metathorax have been considered to be bifunctional in that they are active in elevating (p. 610 ) the wing during flying and moving the coxa during walking. Evidence that the metathoracic muscle 118 is used in walking is, however, lacking. See remotor.

• Pronation.

  Twisting movement of a wing during flight in which the anterior edge is depressed and the posterior part is elevated about the long axis of the wing. The effect is a reduction in the amount of lift that the wing produces. The subalar muscle normally causes pronation accompanying depression, and this can be opposed by contraction of the pleuroaxillary muscle, thus restoring the lift generating capability of the wing. See supination.

• Pronotum.

  Rigid, highly sclerotised region of the exoskeleton on the dorsal and lateral surfaces of the body behind the head (neck).

• Proprioceptor.

  A receptor (usually internal) that may signal the movements of a joint, the force generated by a muscle, and the distortions of the cuticle that result from changes in the loading on the body, or from the contractions of the muscles. These effects are thus usually generated by the actions of the locust itself, but the same receptors will also signal externally imposed movements of the joint or deformations of the exoskeleton. Some proprioceptors may thus at times act like exteroceptors.

• Protocerebrum.

  The anterior part of the brain that consists of prominent areas such as the optic lobes, mushroom bodies and central complex. Generally considered the region for higher order processing where signals from different modalities are brought together and where learning occurs. See deutocerebrum, tritocerebrum.

• Protraction.

  See swing phase.

• PTTH.

  Prothoracicotropic hormone.

• Quisqualate.

  A potent agonist of a particular glutamate receptor characterised on muscle membrane that gates cations and causes a depolarisation. It is extracted with difficulty from the seeds of Quisqualis indica, the Rangoon creeper, a semi-evergreen climbing plant.

• Range fractionation.

  Subdivision of the coding of a sensory stimulus among a group of sensory neurons. For example, in the femoral chordotonal organ of a leg, some sensory neurons respond to flexion and others to extension, and within the groups that respond to one direction, individual sensory neurons respond to different velocities of joint movements and/or to different ranges of joint angles.

• Receptive field.

  The volume of space, or the area on the surface of the body, that leads to the excitation (or inhibition) of a neuron by a particular modality of sensory stimulus. For example, the DCMD interneuron is excited by a moving stimulus anywhere in the visual field of the compound eye contralateral to its axons. A spiking local interneuron of the midline group in a thoracic ganglion receives direct synaptic inputs from sensory neurons of mechanoreceptors on specific regions of a leg and these define its receptive field. Receptive fields of multimodal neurons are composed of several regions and, by definition, of more than one modality.

• Remotor.

  The name used to describe the first and second posterior tergocoxal muscles (63, 64 of the prothorax; 90, 91 of the mesothorax; and 119, 120 of the metathorax) and their supposed action in lifting the posterior edge of the coxa upwards. These muscles in the meso- and metathorax have been considered to be bifunctional in that they are active in elevating the wing during flying and moving the coxa during walking. In the metathorax, it seems that different sets of motor neurons that innervate muscle 120 are active in the two movements, and that 119 is active only in flying. See promotor.

• Resistance reflex.

  A postural adjustment in which an imposed movement is met by increased force in muscles generating the opposing movements. For example, an imposed flexion of the tibia of a leg is met by increased excitation of excitatory extensor tibiae motor (p. 611 ) neurons and inhibition of excitatory flexor tibiae motor neurons so that greater extension force is developed. The effect may spread to other joints of the leg so that stability is enhanced. The gain of the motor response to the same stimulus can change in different circumstances and may even reverse in sign to give an assistance reflex, particularly during locomotion.

• Retinula cell.

  A receptor cell in the compound eyes that contains visual pigment and is responsible for the conversion of light into a graded depolarising electrical signal that is conveyed by its axon to the lamina of an optic lobe.

• Retraction.

  See stance phase.

• Retrocerebral complex.

  Group of paired and unpaired ganglia close to the brain and suboesophageal ganglion. The paired ganglia are the corpora cardiaca and corpora allata and have a secretory function; the unpaired ganglia are the stomatogastric ganglia.

• Ringer solutions.

  The following Ringer solutions have been used for analyses of the central nervous system and for peripheral nerves and muscles.

  Author(s)	Tissue	NaCl	KCl	CaCl2	MgCl2	Buffer		Sucrose	pH
  					in mmol.l−1 (mM)
  Holye (1953)	nerve	140	10	2	2	NaHCO3	4	–	6.8
  					NaH2PO4	6
  Usherwood and Grundfest (1965)	skeletal muscle (used subsequently by many others for recording from intact CNS)	140	10	4		NaHCO3	4		6.8
  						NaH2PO4	6
  Clements and May (1974)	skeletal muscle	140	10	2		NaH2PO4	4	90	6.8
  						Na2HPO4	6
  Cuthbert and Evans (1989)	heart muscle	150	–	4		KHCO3	4	90	6.8
  						KH2PO4	6
  Dubas (1991)	desheathed ganglion	155	3	4		Hepes	10	25	6.8
  Wolf and Laurent (1994)	intact CNS and isolated sheathed ganglia	150	5	5	1	NaHCO3	4	–	7.1
  						Hepes	6.3
  Robertson and Pearson (1982)	recording from intact CNS, particularly during flight	146	10	3		MOPS	5		6.8

• Sarcoplasmic reticulum.

  An internal series of membranes in muscle fibres that are contacted by the T tubule invaginations of the plasma membrane. Ca²⁺ may be stored and released from it to activate the contractile machinery.

• Satellite nervous system.

  A series of superficial neurohaemal release sites formed by the terminals of seven pairs of neurons in the suboesophageal ganglion along particular nerves of this ganglion. These neurons contain 5-HT.

• Schistocerca gregaria.

  African desert locust.

• (p. 612 ) Sclerite.

  A small piece of cuticle that may articulate with a main segment of a limb and which is often embedded in the joint membrane. Muscle fibres attach to it. Sclerites are most obvious at the articulation of a wing with the thorax.

• Scolopidium.

  An internal mechanoreceptor sensillum with 1–3 associated bipolar sensory neurons that have ciliated dendrites. It is enveloped by a scolopale cell that secretes a fibrous, barrel-shaped sleeve called the scolopale around the cilia. The whole is suspended from the cuticle by an attachment cell. These structures linked in various numbers are characteristic of chordotonal organs.

• Segmentation genes.

  These genes interact to establish the segmentation of the embryo and to control the identity of the cells within each segment. They can be divided into three groups based on the defects seen in mutants lacking a particular gene.

  1. 1. Zygotic gap genes produce a deletion between contiguous segments

  2. 2. Pair-rule genes such as fushi tarazu and even-skipped loci produce homologous defects in alternate segments.

  3. 3. Segment polarity genes such as engrailed are needed for pattern formation within each segment.

     See also homeotic genes.

• Semilunar process.

  A black, hard and brittle region of cuticle at the femoro-tibial joint of a hind leg of a locust that is bowed during the contraction of the extensor and flexor tibiae muscles that precedes jumping and kicking. The distortion of the cuticle stores energy which, when released suddenly, contributes to the rapid extension of the tibia.

• Sensillum (sensilla).

  A sensory structure (receptor) protruding from, or embedded in, the cuticle consisting of four cell types; the trichogen, or hair-forming cell, the tormogen, or socket-forming cell, a neuron, and a neurilemma cell. See trichoid, basiconic, and campaniform sensilla.

• Serotonin.

  See 5-HT.

• SETi.

  Slow motor neuron innervating the extensor tibiae muscle of a leg. See also FETi, DUMETi.

• SN1, 2.

  Salivary neurons 1 and 2. SN1 has its cell body in the labial neuromere of the suboesophageal ganglion, contains dopamine and has an axon ipsilateral to its soma. SN2 has its cell body in the mandibular neuromere of the suboesophageal ganglion, contains 5-HT and GABA and has an axon contralateral to its soma.

• Solitary phase.

  These are locusts with a particular morphological appearance and behaviour that results from them living in isolated conditions, as opposed to those living in crowded conditions (see gregarious phase). The larvae are green or yellow-green, and develop more slowly than gregarious ones. Solitary adults fly only at night but lay more eggs than gregarious adults. The phase differences are under hormonal control.

• Soma (somata).

  The cell body of a neuron. Most sensory neurons have cell bodies in the periphery associated with particular sensory structures, the exception being strand receptors. A few neurosecretory neurons have cell bodies associated with peripheral nerves. All other neurons have cell bodies in the cortex of the brain or a ganglion. They have no synapses, even though receptors for various potential neurotransmitters and neuromodulators may be present. Synaptic potentials and spikes are therefore reflected into a cell body from sites in the neuropil. They are usually electrically inexcitable (but not DUM neurons). Each cell body is enveloped by glial cells which form many invaginations close to the exit of the single primary neurite. Groups of cell bodies are bundled together by further glial wrappings and this organisation may reflect an origin from a common neuroblast(s).

• (p. 613 ) Spike-initiating zone.

  The region of membrane in a spiking neuron where the integrated synaptic input initiates a spike that is then transmitted orthodromically along the axon. It is generally thought that the zone can shift along a small length of membrane and is not a discrete site.

• Spiracle.

  An opening in the cuticle connecting with tracheae and through which air can be exchanged. The aperture is controlled by the action of muscles. The spiracles are paired structures on most segments and open and close with the ventilatory rhythm.

• Stance phase.

  Part of the step cycle of an individual leg during walking that is often called retraction, the support phase, or the power stroke, in which the foot is in contact with the ground and propels the body forwards (or backwards). The duration of this phase varies with the speed of walking. The stance phase alternates with the swing phase (protraction).

• Stomatogastric ganglia.

  A group of small aggregations of neurons into ganglia that are closely associated with the brain, suboesophageal ganglion, corpora cardiaca and corpora allata in the retrocerebral complex. They consist of the frontal, hypocerebral (occipital) and ingluvial (ventricular), and paraventricular ganglia. They would appear to be responsible for controlling some of the movements of the gut, but their actions are not known in detail.

• Stomodeum.

  The foregut, which like the hindgut or proctodeum is formed from ectoderm and thus is lined by a layer of cuticle that is shed at each moult. It consists of the pharynx, oesophagus and crop. The midgut, however, is formed from endoderm.

• Strand receptor.

  Sensory cell associated with a strand of connective tissue in the periphery that responds to movements of a joint. These sensory cells are set apart from all other sensory neurons in that their cell bodies (somata) are in the central nervous system and not at the receptor itself.

• Stretch receptor.

  Sensory structure often consisting of a single neuron associated with a strand of connective tissue (e.g. at a wing hinge), or with a receptor muscle (e.g. coxa or abdomen). Gives information about length changes and hence about the movements of a joint.

• Subgenual organ.

  A sense organ consisting of a group of chordotonal sensilla in the tibia that respond to vibrations transmitted through the ground. In cockroaches they may also respond to sound. In locusts, their function is largely unexplored.

• Supination.

  Movement of the wing during flight in which the anterior edge is elevated and the posterior part is depressed about the long axis of the wing. The effect will be to increase the amount of lift that a wing can produce. See pronation.

• Swarm.

  Vast aggregation of adult locusts (gregarious phase) either in flight or feeding.

• Swing phase.

  Part of the step cycle of an individual leg during walking that is often called protraction, the recovery phase, or the return stroke, in which the leg is swung forwards and the foot is off the ground. The duration of this phase varies little with the speed of walking. The swing phase alternates with the stance phase (retraction).

• Synergist.

  A description of a muscle that is working with another muscle, in contrast to one that is opposing the action of another muscle and therefore acting as an antagonist. Synergy in the actions of muscles is common at complex joints where a movement results only from the balance of actions of a set of muscles.

• T tubules.

  Invaginations of the plasma membrane of muscle fibres that allow ionic changes associated with excitation and inhibition to be carried deep within a fibre. Contacts are made with an internal system of membranes called the sarcoplasmic reticulum from which Ca²⁺ may be released to activate the contractile machinery.

• (p. 614 ) Tarsus.

  Section of the distal part of the leg between the tibia and the unguis, that forms the greater part of the foot. It is moved by muscles in the tibia but contains no muscles itself. The ventral surface has several pads packed with sensory receptors. See also coxa, trochanter, femur.

• TCD.

  Tritocerebral dwarf neuron with its cell body in the tritocerebrum of the brain and an axon that passes along the tritocerebral commissure before joining the contralateral connective.

• TCG.

  Tritocerebral giant neuron with its cell body in the tritocerebrum of the brain and an axon that passes along the tritocerebral commissure before joining the contralateral connective.

• Tegula.

  A sensory structure associated with a wing that consists of exteroceptive hairs and an internal chordotonal organ. Both signal the downstroke of a wing and provide an important synaptic input to wing elevator motor neurons.

• Tension receptor.

  Single multipolar sensory neuron embedded in muscles that responds primarily to the isometric force generated by a particular muscle rather than the movement of a joint or an appendage that may result. So far known only in the flexor tibiae muscles of the middle and hind legs.

• Tethered (walking or flying).

  A description of an experimental arrangement to study walking or flying, in which the movements of the locust are restricted to allow the actions of neurons and muscles to be analysed. The term is not specific and describes a number of different experimental arrangements. In tethered walking, the locust may be glued to a rigid holder and allowed to grasp a light ball which it can then spin as it walks, or it may be attached to a balance so that it has to support its body weight as it turns a servo-controlled ball, or walks on a tread-wheel. In some experiments on stick insects, the left and right legs walk on separate tread-wheels whose speed can be varied independently. In other arrangements the orientation of the insect may vary, so that it may walk upside-down inside a tread-wheel against which it has to push by an amount equivalent to its own body weight. In tethered flying, the support may again be either rigid or a flight balance, which typically requires the insect to lift its own body weight. More sophisticated, but highly unstable balances allow more degrees of freedom of movement.

• Tibia.

  Section of the leg between the femur and the tarsus. It is light and tubular in construction, containing two parts of the retractor unguis muscle proximally, and the levator and depressor tarsi muscles distally that operate the hinge joint with the tarsus. The dorsal surface of the tibia of a hind leg has two rows of spines; the most distal two pairs of spines can be moved passively, but the remainder are fixed. By contrast, the front and middle legs have passively moveable spines on the ventral surface. See also coxa, trochanter, unguis.

• Tormogen.

  The cell associated with a sensory sensillum that forms the socket of the receptor. See Trichogen.

• Trachea(e).

  A tube, often reinforced by helical cuticular strips, that is part of a complex network conveying gases from the spiracles to the tissues. In the body, it normally has a silvery appearance because it is filled with air. The tracheae are connected to air sacs that inflate with each cycle of the ventilatory rhythm. Within the tissues, tracheae branch into fine blind-ending tubes called tracheoles.

• Tracheole.

  Fine, fluid-filled and blind-ending branches of the tracheae which are responsible for gaseous exchange with tissues.

• Tract.

  A bundle of axons that are aggregated together as they pass through a particular region of the central nervous system. The connectives as they pass through the thoracic and abdominal ganglia split up into nine prominent longitudinal tracts. Smaller tracts mark (p. 615 ) the entry of the axons from the peripheral nerves and the accumulation of small groups of axons and processes that run vertically or obliquely.

  Tracts in segmental ganglia

  Longitudinal tracts
  Dorsal intermediate tract	DIT
  Dorsal median tract	DMT
  Median dorsal tract	MDT
  Median ventral tract	MVT
  Lateral dorsal tract	LDT
  Lateral ventral tract	LVT
  Ventral intermediate tract	VIT
  Ventral lateral tract	VLT
  Ventral median tract	VMT
  Vertical and oblique tracts
  C tract	CT
  I tract	IT
  Oblique tract	OT
  Perpendicular tract	PT
  Ring tract	RT
  T tract	TT
  Deep DUM tract	DDT
  Superficial DUM tract	SDT

• Trichogen.

  The cell associated with a sensory sensillum that forms the hair of the receptor. See Tormogen.

• Trichoid sensilla.

  Sensory receptors with a shaft of variable length that protrudes from the cuticle. They are set in a membranous socket, and are usually innervated by one (or sometimes two) sensory neurons. The stiffness and thickness of the shaft, and its mounting in its socket, determine whether its sensory neuron responds to air movements or to tactile stimuli. See also basiconic sensilla.

• Tritocerebrum.

  Middle region of the brain between the protocerebrum anteriorly and the deutocerebrum posteriorly.

• Trochanter.

  Section of the proximal part of the leg between the coxa and the femur. In the hind leg it is fused to the femur, so that no movement about the joint is possible, and no muscles are present that could move the femur about it. In the front and middle legs, some movement is possible and a single muscle is present (the reductor femora). See also trochantin, tibia, tarsus, unguis.

• Trochantin.

  Small sclerite embedded in the arthrodial membrane at the coxa on which the coxal promotor muscle inserts. See also trochanter, femur, tibia, tarsus, unguis.

• Trophospongium.

  An association between glial cells and a neuron, typically at the point where the primary neurite emerges from the cell body. At this point, the glial cells form numerous invaginations into the neuron and the neuron may send processes into the glia so that there are reciprocal interdigitations. Based on the Greek word Tropbos meaning ‘one who feeds’.

• Tympanum (tympana).

  Auditory receptor consisting of a thin cuticular membrane, chordotonal organ, and closely associated tracheae. In locusts, the tympana are on the abdomen just behind the hind legs, but in crickets they are in the tibia of the front legs. The number of sensilla varies from two in some moths to more than 1500 in some cicadas.

• (p. 616 ) Unguis.

  Terminal section of the leg, often called the claw. It is moved by a retractor unguis muscle that has three parts, one in the femur and two in the proximal tibia that all attach to the same apodeme (tendon) that runs through the femur, tibia and tarsus to insert on the unguis. The action of this muscle will curl the unguis and should increase traction when the tarsus is placed on the ground or during climbing. See also coxa, trochanter.

• Ventral diaphragm.

  A horizontal septum in the abdomen just dorsal to the nerve cord separating the sinus (perineurial) around the nerve cord from that (perivisceral) around the viscera. It consists of a thin sheet of muscle that may contract in time with the inspiratory cycle of breathing.

• Wind tunnel.

  A device for inducing sustained flight in a locust that is tethered. The airflow is arranged to be warm and laminar and the locust is presented with a visual environment that may consist simply of a horizon or may be more structured. The locust may be rigidly mounted or may be attached to a flight balance that can measure various forces and torques.