Wing hair sensilla underlying aimed hindleg scratching of the locust

The anatomy and physiology of exteroceptors on the surfaces of the wings have been described in many insects, but their roles in behaviour have been less well studied. They have often been assumed to have a role primarily in flight. We show that the wings of the locust Schistocerca gregaria possess at least three different hair types with characteristic patterns of distribution that determine the probability of eliciting targeted hindleg scratching behaviour. The different hair types are defined by their morphology and innervation. The shortest hairs (14-46 microm) are basiconic receptors containing both chemosensory and mechanosensory afferents. They are distributed widely across the dorsal surfaces of the forewings; some are located on the ventral surfaces of the hindwings, but none are found on the ventral surfaces of the forewings or the dorsal surfaces of the hindwings. Medium length hairs (73-159 microm) are found on all wing surfaces, but are restricted to the veins, principally the subcosta on the dorsal surface of the forewings. The longest hairs (316-511 microm) are found only on the postcubitus vein on the dorsal surfaces of the forewings, so that they form a pair of dorsal rows when the wings are folded at rest. Touching the dorsal surface of a forewing can elicit aimed scratching movements of a hindleg, and we show that the probability of eliciting a scratch differs for different stimulus sites and for different start positions of the hind leg. The effectiveness of different stimulus sites can be correlated with the distribution of tactile hairs on the dorsal forewing surface. Touching the long hairs provides the strongest drive to elicit a scratch, and ablating them reduces the probability to almost zero. We conclude that input from forewing tactile hairs plays an important role in eliciting hindleg scratching and encodes the spatial location required for targeting.

Inheritance of song and stridulatory peg number divergence between Chorthippus brunneus and C. jacobsi, two naturally hybridizing grasshopper species (Orthoptera: Acrididae)

Knowledge of the genetic basis of divergence in mating signal characters that contribute to reproductive isolation is critical to understanding speciation. Here, we describe a semi-automated system for characterizing grasshopper acoustic signals. We used this system to study the genetic basis of divergence in three male calling song components [echeme (EL), syllable (SL) and phrase (PL) lengths] between Chorthippus brunneus and C. jacobsi, two species of grasshoppers that hybridize in northern Spain. We also studied the number of pegs in the stridulatory file. For all characters, additive effects accounted for most of the genetic differentiation between species. However, the three song components also showed small but significant epistatic effects. No sex linkage was detected. Wright-Castle-Lande estimates of the minimum numbers of genetic factors underlying song and peg number divergence were low: peg number (n(e)=5.87+/-5.84), SL (n(e)=2.37+/-4.79) and PL (n(e)=0.87+/-0.86). On the other hand, EL appeared to be controlled by many genes. These results suggest that divergence in SL and PL might be driven by sexual selection whereas EL might not be under selection. This is consistent with experimental results on female song preference in related species. However, the fact that few factors appear to underlie the differences in peg number is surprising. Peg number is not closely related to song characteristics. It often varies between closely related grasshopper species and it has been assumed to be a neutral character. The biometrical approaches used here tend to underestimate the number of factors influencing a trait but provide valuable background for subsequent quantitative trait loci analyses.

Projection patterns of posterior dorsal unpaired median neurons of the locust subesophageal ganglion

Six neurons in a group of dorsal unpaired median (DUM) neurons with cell bodies in the posterior part-maxillary and labial neuromeres-of the subesophageal ganglion of locusts have two axons each that descend into both the left and the right halves of the ganglia of the ventral nerve cord. None of the neurons has peripheral axons, so they are interneurons. Electrophysiology shows that the axons of at least four neurons project to the terminal abdominal ganglion to which they conduct spikes at a velocity of 0.5-0.6 m. second(-1). In the somata, the spikes have a smaller amplitude and briefer duration at half height than the spikes of thoracic, efferent DUM neurons. Each neuron has bilaterally symmetrical branches within the subesophageal ganglion and in the thoracic ganglia. On the basis of the specific patterns of branches, and the neuropiles, tracts, and commissures in which they occur, three types of neurons (DUM SD 1-3) can be recognized. DUM SD 1 and 3 project to ventral regions of neuropile in the thoracic ganglia in which the efferent DUM neurons of these ganglia have no branches. DUM SD 2 projects to dorsal neuropiles. The projection patterns of these putatively octopaminergic neurons suggest that they could be the source of the octopaminergic modulation of networks underlying sensory processing and motor pattern generation within these ganglia. Within this group of posterior DUM neurons, two additional cells were stained that have axons ascending to the brain.

Mas-allatotropin/Lom-AG-myotropin I immunostaining in the brain of the locust, Schistocerca gregaria

Mas-allatotropin (Mas-AT) and Lom-accessory gland-myotropin I (Lom-AG-MTI) are two members of a conserved family of insect neuropeptides, collectively termed allatotropins, which have diverse functions, ranging from stimulation of juvenile hormone secretion to myotropic effects on heart and hindgut. In addition, allatotropins appear to be abundant within the nervous system, suggesting neuroactive roles. To identify neurons in the insect brain suitable for a neurophysiological analysis of the roles of allatotropins, we used antisera against Mas-AT and Lom-AG-MTI to map allatotropin-immunoreactive neurons in the brain of a suitable insect, the locust Schistocerca gregaria. Both antisera revealed basically identical staining patterns throughout the locust brain with more than 12,500 immunostained interneurons per brain hemisphere. Neurosecretory cells were not labeled, and the retrocerebral complex was devoid of immunostaining. Prominent immunoreactive cell types include about 9,600 lamina monopolar neurons, medulla to lobula interneurons, local neurons of the antennal lobe, a giant interneuron of the mushroom body, projection neurons of the glomerular lobe to the mushroom body, and three systems of tangential neurons of the central complex. Several groups of neurons showed colocalization of Mas-AT- and gamma-aminobutyric acid immunostaining. Mass spectrometric analysis identified a peptide with a molecular mass identical to Lom-AG-MTI in all major parts of the locust brain but not in the retrocerebral complex. This study strongly suggests that Lom-AG-MTI is highly abundant in the locust brain, and is likely to play a neuroactive role in many brain circuits including all stages of sensory processing, learning and memory, and higher levels of motor control.

On the origin of the desert locust Schistocerca gregaria (Forskål) (Orthoptera: Acrididae: Cyrtacanthacridinae)

The locust genus Schistocerca (Stål) has a transatlantic disjunction, which has been controversial for more than a century. Among 50 species within the genus, only one species, the desert locust (S. gregaria Forskål), occurs in the Old World, and the rest occur in the New World. Earlier taxonomists suggested that the desert locust is a migrant from America, but this view was strongly challenged when a large swarm of the desert locust successfully crossed the Atlantic Ocean from West Africa to the West Indies in 1988. The currently accepted view, supported by this incident, is that the New World species are descendants of a gregaria-like ancestor, and the desert locust would be ancestral to the rest of the genus. However, there is surprisingly little evidence to support this view other than the 1988 swarm. I present the most comprehensive phylogenetic study that suggests that the desert locust originated from the New World, contrary to the accepted view. I also present a hypothesis about how the ancestral Schistocerca might have colonized the New World in the first place in light of phylogenetic relationships with other cyrtacanthacridine genera.

Differential transcription in defined parts of the insect brain: comparative study utilizing Drosophila melanogaster and Schistocerca gregaria

The brain of all higher organisms has a modular architecture. Processing of various tasks, such as learning, olfaction, or motor control is performed in specialized brain areas, characterized by morphological and molecular peculiarities. To identify those genes that are transcribed in only one region of the insect brain, we chose two different approaches, differential display PCR and DNA array hybridization, with two different insect species, the desert locust Schistocerca gregaria and the fruitfly Drosophila melanogaster. The optic lobes (centers of visual information processing), the midbrain (the region of the brain where almost all "higher" centers are localized), and the thoracic ganglia (regions required to control various peripheral organs) were compared in both types of experiments. Both, the differential display PCR screen of the different parts of the locust brain as well as the DNA array screen of the Drosophila brain revealed almost identical numbers of transcripts exclusively present in either of the three above-mentioned brain areas. Interestingly, the brain areas with the largest number of differential transcripts are the thoracic ganglia and not the midbrain.

Hormonal control of phase-related changes in the number of antennal sensilla in the desert locust, Schistocerca gregaria: possible involvement of [His7]-corazonin

The effect of [His(7)]-corazonin on the abundance of antennal sensilla in the desert locust, Schistocerca gregaria, was investigated to test the hypothesis that injection of this neuropeptide would mimic a crowding effect. Solitarious locusts (reared in isolation) were injected with [His(7)]-corazonin at the 3rd nymphal instar and the numbers of sensilla on the 2nd, 8th and 14th antennal segments in the adult stage were compared with those for oil-injected solitarious controls or un-injected gregarious locusts (reared in group). The numbers of sensilla on these antennal segments were all reduced significantly after [His(7)]-corazonin injection compared with those for oil-injected controls, but similar to the values for gregarious individuals. Among the four major types of olfactory sensilla, coeloconic, trichoid, basiconic type A and basiconic type B, [His(7)]-corazonin injection influenced the abundance of all but the last type. The effect of [His(7)]-corazonin injection varied with the time of injection; the earlier the injection the larger the effects on the abundance of total antennal sensilla on the 8th segment, although the way in which the injection affected the abundance varied with the sensillum type. A hypothesis explaining how crowding affects the abundance of antennal sensilla and other phase-related characteristics through changes in [His(7)]-corazonin concentrations was proposed.

Phase-related morphological changes induced by [His7]-corazonin in two species of locusts, Schistocerca gregaria and Locusta migratoria (Orthoptera: Acrididae)

The effects of a neurohormone, [His(7)]-corazonin, on phase-related morphological traits (F/C and E/F ratios; F = length of the hind femur, C = maximum width of the head; E = length of fore wing) were re-examined in the desert locust, Schistocerca gregaria Forskål. The F/C ratio was significantly different between adults with five and six nymphal instars, respectively, indicating that they need to be analysed separately. Injections of the synthesized peptide (1 nmol) into individually-reared (solitary) nymphs at the second and third instars caused a shift in classical morphometric ratio towards the value typical for crowded (gregarious) individuals in both sexes. The E/F ratio, which is smaller in solitary locusts than in gregarious ones, was also influenced significantly by injections of [His(7)]-corazonin into individually-reared locusts. The effect of [His(7)]-corazonin on E/F ratios was shown more clearly when the nymphs were injected at a higher dose (2 nmol) at the beginning of the third instar. Single injections of the peptide into individually-reared nymphs at different instars revealed that the earlier the injection the larger the 'gregarizing' effects of the peptide on F/C and E/F ratios. The same tendency was also detected in Locusta migratoria Linnaeus. These results supported the hypothesis that [His(7)]-corazonin plays an important role in the control of phase polymorphism in locusts.

Immunocytochemistry of histamine in the brain of the locust Schistocerca gregaria

Histamine serves a neurotransmitter role in arthropod photoreceptor neurons, but is also present in a small number of interneurons throughout the nervous system. In search of a suitable model system for the analysis of histaminergic neurotransmission in insects, we mapped the distribution of histamine in the brain of the desert locust Schistocerca gregaria by immunocytochemistry. In the optic lobe, apparently all photoreceptor cells of the compound eye with projections to the lamina and medulla showed intense immunostaining. Photoreceptors of the dorsal rim area of the eye had particularly large fiber diameters and gave rise to uniform varicose immunostaining throughout dorsal rim areas of the lamina and medulla. In the locust midbrain 21 bilateral pairs of histamine-immunoreactive interneurons were found, and 13 of these were reconstructed in detail. While most neuropil areas contained a dense meshwork of immunoreactive processes, immunostaining in the antennal lobe and in the calyces of the mushroom body was sparse and no staining occurred in the pedunculus and lobes of the mushroom body, in the protocerebral bridge, and in the lower division of the central body. A prominent group of four immunostained neurons had large cell bodies near the median ocellar nerve root and descending axonal fibers. These neurons are probably identical to previously identified primary commissure pioneer neurons of the locust brain. The apparent lack in the desert locust of certain histamine-immunoreactive neurons which were reported in the migratory locust may be responsible for differences in the physiological role of histamine between both species.

Regional differences in degree of resilin cross-linking in the desert locust, Schistocerca gregaria

Various cuticular regions from the desert locust, Schistocerca gregaria, were quantitatively analyzed for two cross-linking amino acids, dityrosine and trityrosine, characteristic constituents of the rubberlike cuticular protein, resilin. These amino acids were found in all regions of cuticle investigated, but in widely varying amounts. In fully mature adult locusts the largest amounts of di- and trityrosine were obtained from the prealar arms and wing-hinges, structures possessing long-range elasticity and being involved in energy storage in the flight system. In structures where deformations tend to occur more slowly, such as the clypeo-labral springs and tracheae, di- and trityrosine are less abundant. In sclerotized cuticle from femur and tibia, as well as in cornea and in the highly stretchable intersegmental membranes of mature females, they are only found in trace amounts and are probably unrelated to elasticity. The trityrosine-to-dityrosine ratio in the various cuticular regions vary from nearly equal amounts of the two amino acids to about ten times more dityrosine than trityrosine, indicating that the regions differ in degree of cross-linking; the tracheal wall is the material with the highest trityrosine-to-dityrosine ratio. In some cuticular regions the ratio increases during maturation from newly moulted (teneral) adults to reproductively active locusts; the most pronounced increase was observed for the wing-hinges, and only a small increase was observed for the abdominal tergal plates. In most cuticular regions in fifth instar locust nymphs the contents of di- and trityrosine corresponded to the contents measured for the adult cuticular regions, but only trace amounts of the two amino acids were obtained from the region of the nymphal wing base which corresponds to the wing-hinge containing cuticular region in adult locusts.

The effect of manipulating ecdysteroid signaling on embryonic eye development in the locust Schistocerca americana

Adult body plan differentiation in holometabolous insects depends on global induction and control by ecdysteroid hormones during the final phase of postembryogenesis. Studies in Drosophila melanogaster and Manduca sexta have shown that this pertains also to the development of the compound eye retina. It is unclear whether the hormonal control of postembryonic eye development in holometabolous insects represents evolutionary novelty or heritage from hemimetabolous insects, which develop compound eyes during embryogenesis. We therefore investigated the effect of manipulating ecdysteroid signaling in cultured embryonic eye primordia of the American desert locust Schistocerca americana, in which ecdysteroid level changes are known to induce three rounds of embryonic molt. Although at a considerably reduced rate compared to in vivo development, early differentiation and terminal maturation of the embryonic retina was observed in culture even if challenged with the ecdysteroid antagonist cucurbitacin B. Supplementing cultures with 20-hydroxyecdysone (20E) accelerated differentiation and maturation, and enhanced cell proliferation. Considering these results, and the relation between retina differentiation and ecdysteroid level changes during locust embryogenesis, we conclude that ecdysteroids are not an essential but possibly a modulatory component of embryonic retina development in S. americana. We furthermore found evidence that 20E initiated precocious epithelial morphogenesis of the posterior retinal margin indicating a more general role of ecdysteroids in insect embryogenesis.

Wings and legs are production sites for the desert locust courtship-inhibition pheromone, phenylacetonitrile

Mature gregarious male desert locusts, Schistocerca gregaria, emit the courtship inhibition pheromone phenylacetonitrile. Wings and legs, in particular the fore wings, have been identified as the main releasing sites. Abdomen and head emit only trace amounts of this pheromone. In contrast veratrole, another typical component of male volatiles, is emitted by all body parts. Epidermal gland cells in the identified phenylacetonitrile releasing appendages are the putative sites of its biosynthesis. Incubation of these body parts in the presence of (14)C-phenylalanine results in the production of (14)C-phenylacetonitrile. Some of the phenylacetonitrile appears to be degraded to HCN and benzaldehyde presumably enhancing the repellent character of phenylacetonitrile. HCN is only detectable in volatiles of mature gregarious male desert locusts. Possible advantages of the observed distribution of the phenylacetonitrile release sites and of the cyanogenesis in relation to mating behaviour are discussed.

Correlated evolution of colour pattern and body size in polymorphic pygmy grasshoppers, Tetrix undulata

Theory posits that selection on functionally interrelated characters will promote physical and genetic integration resulting in evolution of favourable trait-value combinations. The pygmy grasshopper Tetrix undulata (Orthoptera: Tetrigidae) displays a genetically encoded polymorphism for colour pattern. Colour morphs differ in several traits, including behaviours, thermal biology and body size. To examine if these size differences may reflect phenotypic plasticity of growth and development in response to temperature we used a split brood-design and reared hatchlings from mothers belonging to different morphs in different thermal environments (warm or cold) until maturity. We found that time to maturity was longer in the cold compared with the warm treatment. In the warm (but not in the cold) treatment time to maturity also varied among individuals born to mothers belonging to different colour morphs. Although low temperature and long development time are normally accompanied by increased body size in ectotherms, our results revealed no difference in size at maturity between individuals reared in the two temperature treatments. There was also an increase (not a decrease) in adult body size with shortened time to maturity across families within each treatment. Taken together, this suggests that body size is canalized against environmental perturbations, and that early maturation does not necessarily trade off against a size-mediated decrease in fecundity. Heritability of body size was moderate in magnitude. Moreover, body size at maturity varied among individuals belonging to different morphs and was influenced also by maternal colour morph, suggesting that a genetic correlation exists between colour pattern and body size. These findings suggest that different characters have evolved in concert and that the various colour morphs represent different evolutionary strategies, i.e., alternative peaks in a multi-modal adaptive landscape.

FMRFamide-related peptides in the gut of Locusta migratoria L.: a comprehensive map and developmental profile

The gut tissues and associated nervous system of the African migratory locust, Locusta migratoria, were found to contain FMRFamide-like immunoreactive (FLI) material throughout the five larval instars and 2 weeks into the adult stage in both males and females. FMRFamide-like immunoreactivity associated with the locust gut was described using camera lucida techniques. FMRFamide-like immunoreactivity is observed in the frontal connectives, recurrent nerve, and oesophageal nerves; projections from the ingluvial ganglion onto the anterior midgut, and from the proctodeal nerve onto the hindgut and posterior midgut; in the neuropils of the frontal ganglion, hypocerebral ganglion and ingluvial ganglia; 30 cell bodies in the frontal ganglion; multipolar sensory cells on the foregut; and endocrine-like cells in the gastric caecae and midgut. Radioimmunoassay (RIA) was used to determine the quantities of FLI material in foreguts, gastric caecae, anterior and posterior midguts, and hindgut of first-fifth instar larvae, 1-3- and 14-17-day male and female adult locusts. As expected, as the tissue size (assessed by total protein content) increases, so does the amount of FLI material in each tissue. Normalizing for tissue size reveals significant differences in FLI content among the stages for each tissue tested. Reversed phase-high pressure liquid chromatography (RP-HPLC) followed by RIA has identified four groups of FLI fractions present in the gut, and different members of these groups are present in the various gut tissues.

Nanos plays a conserved role in axial patterning outside of the Diptera

Axial patterning is a fundamental event in early development, and molecules involved in determining the body axes provide a coordinate system for subsequent patterning. While orthologs of Drosophila bicoid and nanos play a conserved role in anteroposterior (AP) patterning within at least a subset of Diptera, conservation of this process has not yet been demonstrated outside of the flies. Indeed, it has been argued that bicoid, an instrumental "anterior" factor in Drosophila melanogaster, acquired this role during the evolution of more-derived dipterans. Interestingly, the interaction of Drosophila maternal nanos and maternal hunchback provides a system for patterning the AP axis that is partially redundant to the anterior system. Previous studies in grasshoppers suggest that hunchback may play a conserved role in axial patterning in this insect, but this function may be supplied solely by the zygotic component of hunchback expression. Here we provide evidence that the early pattern of zygotic grasshopper Hunchback expression is achieved through translational repression that may be mediated through the action of grasshopper nanos. This is consistent with the notion that an anterior gradient system is not necessary in all insects and that the posterior pole "probably conveys longitudinal polarity on the ensuing germ anlage".

Proprioceptors monitoring forces in a locust hind leg during kicking form negative feedback loops with flexor tibiae motor neurons

In preparation for jumping and kicking, a locust slowly generates large forces in the femoral muscles of its hind legs and stores them in elastic distortions of the tendons and femoral cuticle. At the femoro-tibial joints, the semi-lunar processes are bent, the cuticle of the dorsal distal femur is crumpled, and the femur is expanded in a mediolateral direction. We have analysed whether these distortions are monitored by sense organs and whether the information they provide is used to limit the forces generated and thus prevent structural damage to the joint. The two sensory neurons comprising the lump receptor lie in a groove in the ventral part of the distal femur. The sensory neurons spike if force is applied to the flexor tendon when the joint is fully flexed, but not when it is extended. They also spike as the tendon of the flexor muscle slides into the ventral femoral groove when the tibia is fully flexed during the co-contraction phase of kicking. Their spike frequency correlates with the extent of bending of a semi-lunar process that provides a quantifiable measure of the joint distortions. If the tibia is not fully flexed, however, then muscle contractions still cause distortions of the joint but these are not signalled by sensory spikes from the lump receptor. The lump receptor, therefore, does not respond primarily to the joint distortions but to the movements or force in the flexor tendon. Contractions of the flexor tibiae muscle caused by spikes in individual flexor motor neurons can evoke spikes in sensory neurons from the lump receptor when the joint is fully flexed. In turn, the sensory neurons cause a hyperpolarisation in particular flexor motor neurons in a polysynaptic negative feedback loop. The lump receptor could, therefore, regulate the output of the flexor motor neurons and, thus, limit the amount of force generated during co-contraction. It may also contribute to the inhibition of the flexors at the end of co-contraction that allows rapid kicking movements to occur.

Development of phenotypic differences in sensillum populations on the antennae of a grasshopper, Schistocerca americana

The development of diet-induced phenotypic differences in numbers of sensilla on the antennae of the grasshopper Schistocerca americana was studied using the exuviae produced at each molt. This made it possible to follow changes within an individual insect. In the first instar, insects had similar numbers of four sensillum types: uniporous trichoid sensilla, coeloconic sensilla, and large and small multiporous basiconic sensilla. Rearing on lettuce resulted in sixth instars with greater numbers of three sensillum types than siblings reared on an artificial diet. The first statistically significant differences between treatments in numbers of trichoid sensilla and large basiconic sensilla occurred in the third and fourth instars, respectively. No major reductions in sensillum numbers occurred at any time and the phenotypic differences resulted from differences in the numbers added at each molt.

Divergence and reproductive isolation in the early stages of speciation

To understand speciation we need to identify the factors causing divergence between natural populations. The traditional approach to gaining such insights has been to focus on a particular theory and ask whether observed patterns of reproductive isolation between populations or species are consistent with the hypothesis in question. However, such studies are few and they do not allow us to compare between hypotheses, so often we cannot determine the relative contribution to divergence of different potential factors. Here, I describe a study of patterns of phenotypic divergence and premating and postmating isolation between populations of the grasshopper Chorthippus parallelus. Information on the phylogeographic relationships of the populations means that a priori predictions from existing hypotheses for the evolution of reproductive isolation can be compared with observations. I assess the relative contributions to premating isolation, postmating isolation and phenotypic divergence of long periods of allopatry, adaptation to different environments and processes associated with colonisation (such as population bottlenecks). Likelihood analysis reveals that long periods of allopatry in glacial refugia are associated with postmating reproductive isolation, but not premating isolation, which is more strongly associated with colonisation. Neither premating nor postmating isolation is higher between populations differing in potential environmental selection pressures. There are only weak correlations between patterns of genetic divergence and phenotypic divergence and no correlation between premating and postmating isolation. This suggests that the potential of a taxon to exercise mate choice may affect the types of factor that promote speciation in that group. I discuss the advantages and disadvantages of the general approach of simultaneously testing competing hypotheses for the evolution of reproductive isolation.

Jumping and kicking in the false stick insectProsarthria teretrirostris: kinematics and motor control

The false stick insect Prosarthria teretrirostris looks and behaves like a real stick insect but can jump and kick rapidly and powerfully like a locust, to which it is more closely related. It has an elongated body with slender hind legs that are some 2.5 times longer than the front and middle legs. A male with a body 67 mm long and weighing 0.28 g can jump 90 cm with a take-off angle of 40° and velocity of 2.5 ms-1,requiring an energy expenditure of 850 μJ. The body is accelerated at 165 ms-2 for only 30 ms. The larger and heavier females (mean body length 104 mm and weighing 1.5 g) can jump on average a distance of 49 cm.

During jumping, the tibiae of the hind legs are extended in 30 ms with maximum rotational velocities of 11.5° per ms, but during kicking, when there is no body weight to support, extension is complete in 7 ms with rotational velocities as high as 48° per ms. The short time available to accelerate the body indicates that the movements are not powered by direct muscle contractions and that there must be storage of elastic energy in advance. The motor patterns responsible for generating the necessary forces in the hind legs for jumping and kicking are similar and consist of three phases;an initial flexion of the tibia is followed by a co-contraction of the small flexor and large extensor tibiae muscles lasting several hundred milliseconds while the tibia remains fully flexed. Finally, the flexor motor neurons stop spiking so that the tibia is able to extend rapidly. The small semi-lunar processes at the femoro-tibial joints are not distorted, so that they cannot act as energy stores. Some 7% of the energy is stored transiently by bending the thin tibiae during the initial acceleration phase of a jump and releasing it just before take-off.

The jumping and kicking mechanisms of Prosarthria teretrirostrishave features in common with those used by locusts but also have their own characteristics. The evolution of jumping in Orthoptera is discussed in this context.

The locust tegula: kinematic parameters and activity pattern during the wing stroke

The tegula is a complex, knob-shaped sense organ associated with the base of the locust wing. Despite a detailed knowledge of its role in flight motor control, little is known about the relationship between the stroke parameters of the wing, movement of the tegula organ and the pattern of tegula activity. In this study, therefore, the kinematic parameters of the fore- and hindwings were investigated with respect to the tegula activity pattern during tethered flight. The following results were obtained. (i) The tegula moves through a complex three-dimensional trajectory during the wing stroke, involving inclination and rotation about its longitudinal axis. (ii) The kinematic parameters of tegula movement are phase-locked to the wing stroke and vary in conjunction with wing stroke parameters such as amplitude and cycle period.(iii) In accordance with these phase-locked kinematics, both the onset of tegula activity with respect to the downstroke (latency) and the discharge of the organ (burst duration and amplitude) vary in conjunction with downstroke movement and cycle period, resulting in an (almost) constant phase of tegula activation during the stroke cycle. (iv) The pattern of tegula activity during flight is largely independent of stroke amplitude. (v) The latency, burst duration and amplitude of tegula activity are strongly related to the angular velocity of the wing during the downstroke, with latency reaching a steady minimum value at higher angular velocities. The data suggest that the tegula encodes the timing and velocity of the downstroke and that it may be involved in the control of the stroke's angular velocity.

Metathoracic neurons integrating intersegmental sensory information in the locust

This paper describes the morphology and physiology of five types of local interneurons and three types of ascending intersegmental interneurons in the locust metathoracic ganglion that are points of convergence of sensory information from the wings. Four types of spiking local interneurons are members of a population with somata at the ventral midline. They are depolarised by stimulation of a metathoracic wing nerve, suggesting that they encode a sensory representation of this appendage. Some are also depolarised with short latencies following stimulation of a mesothoracic wing nerve, indicating that they collate intersegmental as well as local information. All the local interneurons have branches in the anterior ventral association centre or around the roots of the nerve that carries wing sensory neurons. This distinguishes them from other interneurons in the population. A fifth type of local interneuron that has unusual bilateral branching and is not a member of this population is described for the first time. The ascending interneurons are members of three populations. Neurons of each population have a characteristic pattern of responses to stimulation of the mesothoracic or metathoracic wing nerves, and some respond to tactile stimulation or movements of a hind leg. These latter interneurons thus collate information from both wings and legs. All three types of intersegmental interneurons have branches in the anterior ventral association centre or around the roots of the wing nerve. The responses of the interneurons described here shed new light on both local and intersegmental network function in this model system.

Signal processing in a simple visual system: the locust ocellar system and its synapses

The neurons with the widest axons that carry information into a locust brain belong to L-neurons, the large, second-order neurons of the ocelli. L-neurons play roles in flight control and boosting visual sensitivity. Their morphology is simple, and their axons convey graded potentials from the ocellus with little decrement to the brain, which makes them good subjects in which to study transmission of graded potentials. L-neurons are very sensitive to changes in light, due to an abnormally high gain in the sign inverting synapses they receive from photoreceptors. Adaptation ensures that L-neurons signal contrast in a light signal when average light intensity changes, and that their responses depend on the speed of change in light. Neurons L1-3 make excitatory output synapses with third-order neurons and with L4-5. These synapses transmit tonically, but are unable to convey hyperpolarising signals about large increases in light. Graded rebound spikes enhance depolarising responses. L1-3 also make reciprocal inhibitory synapses with each other and transmission at these decrements so rapidly that it normally requires a presynaptic spike. The resolution with which graded potentials can be transferred has been studied at the inhibitory synapses, and is limited by intrinsic variability in the mechanism that determines neurotransmitter release. Electron microscopy has shown that each excitatory connection made from an L-neuron to a postsynaptic partner consists of thousands of discrete synaptic contacts, in which individual dense-staining bars in the presynaptic neuron are associated with clouds of vesicles. Acetylcholine is likely to be a neurotransmitter released by L-neurons.

Developmental changes in the structure and function of the central olfactory system in gregarious and solitary desert locusts

Desert locusts are guided by olfactory cues in different behavioural contexts. In order to understand the basis for the variable olfactory guided behaviour displayed by different developmental stages and by solitary and gregarious locusts, we investigated their central olfactory system with neuroanatomical and neurophysiological methods. The primary olfactory centre of the brain, the antennal lobe (AL), increases in size during development due to an increased number and size of glomeruli. These glomeruli are innervated by a constant number of projection neurons that display increased dendritic arborizations during the development of the locust. The anatomical parameters do not differ between gregarious and solitary locusts. In parallel with the observed neuroanatomical changes, neurophysiological changes in response spectra and response specificity of AL neurons were found. During development, the percentage of neurons responding specifically to aggregation pheromone components decreases, whereas an increase in both pheromone-generalists and plant-pheromone generalist neurons is observed. The percentage of neurons responding to green leaf volatiles, however, remains constant. A decrease in the number of nymph blend-specific neurons was also observed. Our data show that anatomical and physiological properties of the AL and its neurons to a large extent reflect the changes in olfactory guided behaviour during development and between phases. The majority of our results are also in accordance with findings that the number of olfactory receptor neurons increases during development, resulting in increasing convergence on AL neurons.

Motion detectors in the locust visual system: From biology to robot sensors

Motion detectors in the locust optic lobe and brain fall into two categories: neurones that respond selectively to approaching vs. receding objects and neurones that respond selectively to a particular pattern of image motion over a substantial part of the eye, generated by the locust's own movements through its environment. Neurones from the two categories can be differentiated on the basis of their response to motion at a constant velocity at a fixed distance from the locust: neurones of the first category respond equally well to motion in any direction whereas neurones in the second category respond selectively to one preferred direction of motion. Several of the motion detectors of the first category, responding to approaching objects, share the same input organisation, suggesting that it is important in generating a tuning for approaching objects. Anatomical, physiological, and modelling studies have revealed how the selectivity of the response is generated. The selectivity arises as a result of a critical race between excitation, generated when image edges move out over the eye and delayed inhibition, generated by the same edge movements. For excitation to build up, the velocity and extent of edge motion over the eye must increase rapidly. The ultrastructure of the afferent inputs onto the dendrites of collision sensitive neurones reveals a possible substrate for the interaction between excitation and inhibition. This interpretation is supported by both physiological and immunocytochemical evidence. The input organisation of these neurones has been incorporated into the control structure of a small mobile robot, which successfully avoids collisions with looming objects. The ecological role of motion detectors of the second category that respond to image motion over a substantial part of the visual field, is discussed as is the input organisation that generates this selective response. The broad tuning of these neurones, particularly at low velocities (<0.02 degree/s), suggests they may have a role in navigation during migratory flights at altitude. By contrast, their optimum tuning to high-image velocities suggests these motion detectors are adapted for use in a fast flying insect, which does not spend significant time hovering.