The kinematics and neural control of high-speed kicking movements in the locust

The tibiae of locust hind legs can be extended fully in a kick in 3 ms with peak angular velocities of at least 80° ms–1. If the mass of the distal part of the leg is halved, then the extension is complete in less than 1 ms with angular velocities of more than 200° ms–1. The high velocities and the associated power are generated by a preceding storage of energy and its sudden release produced by a specific motor pattern and specialisations of the femoro-tibial joints. To understand the dynamics of these rapid movements and the interrelations between joint mechanics and the motor pattern, kicks were analysed with high-speed video images coupled to simultaneous intracellular recordings from identified leg motor neurones. The first movement is a full tibial flexion followed by co-contraction of the extensor and flexor tibiae muscles for 0.3–1 s, during which the distal end of the femur is flattened dorso-ventrally and expanded laterally. The two semi-lunar processes on the distal femur are bent when the fast extensor tibiae motor neurone spikes so that their tips move ventrally by up to 0.6 mm. The inward projections of these processes into the femur form the proximal part of the hinge joint with the tibia, so that the pivot of the joint also changes and the tibia therefore moves proximally and ventrally, widening the gap between it and the femur. Extension of the tibia begins on average 34 ms after the flexor motor neurones are inhibited at the end of the co-contraction phase. The tibia then begins to extend slowly, reaching peak velocities only when it has extended by 60–70°. The semi-lunar processes do not start to unfurl until the tibia has extended by 55°, so they cannot provide the initial energy for extension. An audible click is produced when the semi-lunar processes unfurl. The peak velocity of tibial extension is correlated with the amount of bending of the semi-lunar processes and with the number of fast extensor motor spikes, but the same amount of semi-lunar bending can be produced by both short and long co-contractions. When the tibia reaches full extension, inertial forces may cause it to bend by as much as 33° at a plane of weakness in the proximal tibia, thus allowing further extension of the distal end.

Monoamines and neuropeptides in antennal lobe interneurons of the desert locust, Schistocerca gregana: an immunocytochemical study

As a first step towards unravelling some of the complexity of the signalling and modulatory mechanisms in the antennal lobe (AL) of the desert locust Schistocerca gregaria, I analysed the immunocytochemical identity of AL interneurons. Antibodies against serotonin, histamine, locustatachykinin, leucokinin and FMRFamide were used to reveal the morphology of interneurons ramifying in the AL. In addition, double-labelling experiments were performed in order to demonstrate colocalisation of GABA and locustatachykinin and to investigate the ramification patterns of immunolabelled interneurons and physiologically characterised olfactory projection neurons (PNs) injected with Lucifer yellow. Immunoreactivity to these antibodies revealed six different types of interneurons with different patterns of ramification within the glomerular neuropil: (1, 2) Centrifugal interneurons displaying serotonin immunoreactivity, which arborised extensively within the AL and extended varicose fibres into the microglomerular core where close associations with dendrites of AL PNs could be distinguished. (3) Histamine-immunoreactive centrifugal interneurons with arborisations in the protocerebrum and the dorsal non-glomerular regions of the AL and the lobus glomerulatus (LG). (4) Locustatachykinin-immunoreactive local interneurons, colocalising GABA, arborising throughout the AL and extending varicose fibres throughout the glomerular neuropil where close associations with dendrites of AL PNs could be distinguished. (5) Leucokinin-immunoreactive descending neurons connecting the protocerebrum, the AL, the LG and all ganglia of the ventral nerve cord. These neurons displayed sparse innervation of the AL and extended varicose fibres into the interglomerular space. (6) FMRF-amide-immunoreactive centrifugal interneurons, connecting the lateral protocerebrum with the AL and the LG, which arborised sparsely within these neuropils and displayed similar innervation of the microglomeruli as (1) and (2).

Spatial structure and habitat variation in a grasshopper hybrid zone

A hybrid zone between the grasshoppers Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) in northern Spain has been analyzed for variation in morphology and ecology. These species are readily distinguished by the number of stridulatory pegs on the hind femur. Both sexes are fully winged and inhabit disturbed habitats throughout the study area. We develop a maximum-likelihood approach to fitting a two-dimensional cline to geographical variation in quantitative traits and for estimating associations of population mean with local habitat. This method reveals a cline in peg number approximately 30 km south of the Picos de Europa Mountains that shows substantial deviations in population mean compared with the expectations of simple tension zone models. The inclusion of variation in local vegetation in the model explains a significant proportion of the residual variation in peg number, indicating that habitat-genotype associations contribute to the observed spatial pattern. However, this association is weak, and a number of populations continue to show strong deviations in mean even after habitat is included in the final model. These outliers may be the result of long-distance colonization of sites distant from the cline center or may be due to a patchy pattern of initial contact during postglacial expansion. As well as contrasting with the smooth hybrid zones described for Chorthippus parallelus, this situation also contrasts with the mosaic hybrid zones observed in Gryllus crickets and in parts of the hybrid zone between Bombina toad species, where habitat-genotype associations account for substantial amounts of among-site variation.

Primary commissure pioneer neurons in the brain of the grasshopper Schistocerca gregaria: development, ultrastructure, and neuropeptide expression

The bilaterally paired primary commissure pioneer neurons in the median domain of the grasshopper brain are large, descending interneurons that uniquely express the TERM-1 antigen, even in the adult. After pioneering the primary interhemispheric brain commissure, these neurons extend TERM-1-immunoreactive collaterals into most parts of the brain except the mushroom bodies. In this report, the authors show that the TERM-1 antigen is located in the cell body cytoplasm of these neurons and not on the membranes. Screening with antisera to insect neuropeptides reveals that an antiserum recognizing peptides of the leucokinin family labels the cell body cytoplasm of the primary commissure neurons. Leucokinin-related peptides are known to modulate motility of visceral muscle, play a role in diuresis, and are likely to be neuromodulators in the insect nervous system. The primary commissure neurons differ ultrastructurally from median neurosecretory cells in that their cell body cytoplasm is more extensive, contains high numbers of mitochondria and extensive endoplasmic reticulum, but does not contain neurosecretory granules. In the adult, the cell somata are enveloped by multiple glia membranes and associated trophospongia. According to these ultrastructural characteristics, the primary commissure pioneers are not classical neurosecretory cells.

Central projections from contact chemoreceptors of the locust ovipositor and adjacent cuticle

Contact chemoreceptors (basiconic sensilla) located on the ovipositor and genital segments of the locust serve to control the chemical features of the substrate before and during oviposition. They occur dispersed and also crowded in fields between mechanosensory exteroceptors sensitive to touch or wind (trichoid and filiform sensilla). The central nervous projections of the four chemosensory and one mechanosensory neurons from single basiconic sensilla were stained selectively, focusing on receptors on the ovipositor valves, which usually contact the substrate during the pre-oviposition probing movements. All axons and neurites from one contact chemoreceptor usually stay close together in most of their projections. Segregation occurs mainly when single axons terminate in one neuromere while the others proceed to a different neuromere or ganglion. For projections from one chemoreceptor, there is evidence neither for functional segregation of mechanosensory from chemosensory afferent terminals nor for specific segregation between different chemosensory afferents. The projections from sensilla of dorsal cuticle tend to project rather uniformly along the midline of the terminal ganglion. Comparative staining of touch- and wind-sensitive hair receptor neurons shows mostly central projections, similar to those of neighbouring contact chemoreceptors. From the typical intersegmental projections of most primary afferents and from the lack of segregation into glomerular structures, we conclude that integration of chemosensory information from the genital segments is distributed in the terminal and the 7th abdominal ganglion.

The hind wing of the desert locust (Schistocerca gregaria Forskal). III. A finite element analysis of a deployable structure

Finite element analysis is used to model the automatic cambering of the locust hind wing during promotion: the umbrella effect. It was found that the model required a high degree of sophistication before replicating the deformations found in vivo. The model has been validated using experimental data and the deformations recorded both in vivo and ex vivo. It predicts that even slight modifications to the geometrical description used can lead to significant changes in the deformations observed in the anal fan. The model agrees with experimental data and produces deformations very close to those seen in free-flying locusts. The validated model may be used to investigate the varying geometries found in orthopteran anal fans and the stresses found throughout the wing when loaded.

The hind wing of the desert locust (Schistocerca gregaria Forskal). II. Mechanical properties and functioning of the membrane

As part of an investigation of the functional mechanics of the hind wing of the desert locust Schistocerca gregaria, the Young's modulus of the membrane was measured using a newly developed universal materials test machine capable of testing very small specimens of cuticle, down to 1 mm gauge length. Strain was measured optically. Specimens were cut from various locations around the wing and tested under controlled temperature and humidity. The modulus of the membrane was typically between 1 and 5 GPa, but both this and the membrane thickness varied around the wing, with the remigium and the anal fan showing markedly different properties. The membrane was tested for chitin using two methods: a gas pyrolysis/mass spectrometry assay, and a gold-labelled immunoassay specific to chitin. None was detected, and the membrane may consist of epicuticle alone. The wings were examined for evidence of crystalline material using standard polarising microscopy and an advanced technique that distinguishes between three components of the polarised image. Birefringence was detected in the membrane of the anterior part of the wing, but vanished when the membrane was separated from the surrounding veins, suggesting that it was due to pre-stress rather than to ultrastructure. The implications are discussed.

The hind wing of the desert locust (Schistocerca gregaria Forskal). I. Functional morphology and mode of operation

Detailed morphological investigation, mechanical testing and high-speed cinematography and stroboscopic examination of desert locusts, Schistocerca gregaria, in flight show that their hind wings are adapted to deform cyclically and automatically through the wing stroke and that the deformations are subtly dependent on the wings' structure: their shape, venation and vein design and the local properties of the membrane. The insects predominantly fly fast forwards, generating most force on the downstroke, and the hind wings generate extra lift by peeling apart at the beginning of the downstroke and by developing a cambered section during the stroke's translation phase through the ‘umbrella effect’ - an automatic consequence of the active extension of the wings' expanded posterior fan. Bending experiments indicate that most of the hind wing is more rigid to forces from below than from above and demonstrate that the membrane acts as a stressed skin to stiffen the structure.

Relationships between body mass, motor output and flight variables during free flight of juvenile and mature adult locusts, Schistocerca gregaria

Little information is available about how the adult locust flight system manages to match the aerodynamic demands that result from an increase in body mass during postmoult maturation. In Schistocerca gregaria of both sexes, flight variables, including flight speed, ascent angle and body angle, were investigated under closed-loop conditions (i.e. during free flight) as a function of adult maturation. Motor patterns were examined by telemetric electromyography in juvenile and adult mature animals of both sexes. Functional relationships between particular flight variables were investigated by additional loading of the animals and by reductions in wing area. The results indicate that an increase in flight speed as the flight system matures enables it to match the aerodynamic demands resulting from increases in body mass. Furthermore, the data suggest that this postmoult increase in flight speed is not simply a consequence of the increase in wingbeat frequency observed during maturation. The instantaneous body angle during flight is controlled mainly by aerodynamic output from the wings. In addition, the mean body angle decreases during maturation in both sexes, and this may play an important part in the directional control of the resultant flight force vector.

Tests of pleistocene speciation in montane grasshoppers (genus Melanoplus) from the sky islands of western North America

There has a been a resurgence of debate on whether the Pleistocene glaciations inhibited speciation. This study tests a model of Pleistocene speciation, estimating the phylogenetic relationships and divergence times of 10 species of montane grasshoppers, genus Melanoplus, using 1300 bp of the mitochondrial gene cytochrome oxidase I (COI). Based on average pairwise distances (corrected for multiple substitutions using Kimura's two-parameter model), all species appear to have originated within the Pleistocene. Sequence divergences between species are less than 4%, corresponding to divergence times less than 1.7 million years ago. Branching patterns among the species suggest that speciation was associated with more than one glacial-interglacial cycle. A likelihood-ratio test rejected a model of simultaneous species origins, the predicted branching pattern if species arose from the fragmentation of a widespread ancestor. These grasshoppers live in an area that was previously glaciated and, as inhabitants of the northern Rocky Mountain sky islands, underwent latitudinal and probably altitudinal shifts in distribution in response to climatic fluctuations. Given the repeated distributional shifts and range overlap of the taxa, there most likely has been ample opportunity for population mixing. However, despite periodic glacial cycles, with more than 10 major glaciations over the past million years and climatic fluctuations over as short a time scale as 10(3) to 10(4) years, the dynamic history of the Pleistocene did not preclude speciation. Although relationships among some taxa remain unresolved, these grasshopper species, even with their recent origins, exhibit genetic coherence and monophyletic or paraphyletic gene trees. The frequency of glacial cycles suggests that the speciation process must have been extremely rapid. These species of grasshoppers are morphologically very similar, differing primarily in the shape of the male genitalia. These characters are posited to be under sexual selection, may play an important role in reproductive isolation, and are known to diverge rapidly. This suggests the rapidity of evolution of reproductive isolation may determine whether species divergences occurred during the Pleistocene glaciations.

Connections between thoraco-coxal proprioceptive afferents and motor neurons in the locust

The position of the coxal segment of the locust hind leg relative to the thorax is monitored by a variety of proprioceptors, including three chordotonal organs and a myochordotonal organ. The sensory neurons of two of these proprioceptors, the posterior joint chordotonal organ (pjCO) and the myochordotonal organ (MCO), have axons in the purely sensory metathoracic nerve 2C (N2C). The connections made by these afferents with metathoracic motor neurons innervating thoraco-coxal and wing muscles were investigated by electrical stimulation of N2C and by matching postsynaptic potentials in motor neurons with afferent spikes in N2C. Stretch applied to the anterior rotator muscle of the coxa (M121), with which the MCO is associated, evoked sensory spikes in N2C. Some of the MCO afferent neurons make direct excitatory chemical synaptic connections with motor neurons innervating the thoraco-coxal muscles M121, M126 and M125. Parallel polysynaptic pathways via unidentified interneurons also exist between MCO afferents and these motor neurons. Connections with the common inhibitor 1 neuron and motor neurons innervating the thoraco-coxal muscles M123/4 and wing muscles M113 and M127 are polysynaptic. Afferents of the pjCO also make polysynaptic connections with motor neurons innervating thoraco-coxal and wing muscles, but no evidence for monosynaptic pathways was found.

Cytochemical evidence that acetylcholine is a neurotransmitter of neurons that make excitatory and inhibitory outputs in the locust ocellar visual system

Three different cytochemical methods were used to detect acetylcholine in large, second-order neurons of locust ocelli (L-neurons). The first method used polyclonal antibodies raised against choline cleaved from acetylcholine and then conjugated with native protein, and this revealed strong staining for acetylcholine in axons whose number, size, and location indicated that they were of L-neurons. A corresponding staining pattern was found using the second method with a polyclonal antiserum against choline acetyltransferase (ChAT). The third method was the histochemical detection at the electron microscope level of acetylcholinesterase, the enzyme responsible for the breakdown of acetylcholine. We found that this enzyme is located in synaptic clefts of L-neurons in both of the brain regions where L-neurons are known to make excitatory and inhibitory output synapses. Acetylcholinesterase was confined to synaptic sites, which is consistent with a role in synaptic transmission at these synapses. Taken together, the findings suggest that L-neurons use acetylcholine as a neurotransmitter.

Neurogenesis in the median domain of the embryonic brain of the grasshopper Schistocerca gregaria

Embryonic development in the median domain of the brain of the grasshopper Schistocerca gregaria was investigated with immunohistochemical, histological, and intracellular dye injection techniques. The early head midline is divisible into a dorsal median domain and a ventral median domain based on the orientation of cell somata in each region. At 25% of embryogenesis, a single large midline precursor differentiates in the dorsal median domain and produces a lineage of six neuronal progeny before degenerating. No further precursors arise. In addition, the primary commissure pioneers and a pair of lateral neurons differentiate directly from the ectoderm in this region. Lucifer yellow dye injected into the midline precursor stains only this cell and its progeny. Similarly, there is no dye coupling from the primary commissure pioneers to the midline lineage or to neuroblasts of the brain hemispheres. Neurogenesis in the dorsal median domain therefore proceeds separately within each subset of cells, and is not related to development in the brain hemispheres. Beginning at 42% of embryogenesis, the primary commissure pioneers undergo a morphological transformation and concomittantly express the Term-1 antigen. Expression continues throughout embryogenesis and into the adult, where the midline primary commissure pioneer cells are the only ones labeled by Term-1 in the entire brain. The cellular organization of the dorsal median domain therefore remains remarkably conserved throughout embryogenesis, even as the brain undergoes extensive morphological transformation.

A single allatostatin-immunoreactive neuron innervates skeletal muscles of several segments in the locust

In the nervous system of embryos and adult Locusta migratoria, somata, neurites within the ganglia, and axons leaving the thoracic ganglia show allatostatin immunoreactivity. The immunoreactive efferent axons divide to follow different nerve branches and form varicose terminals on skeletal muscles. In the adult locust, one pair of motor neurons is particularly prominent among the allatostatin-immunoreactive neurons. The somata are located symmetrically in a lateral position in the first abdominal neuromere of the fused metathoracic ganglion. Each neuron gives rise to five axon branches projecting into ipsilateral nerves. Three axons project posteriorly and exit through the dorsal nerves of the abdominal neuromeres A1, A2, and A3. One axon extends into the metathoracic neuromere and exits through metathoracic nerve 1 (N1). The fifth axon extends anteriorly through the connective into the mesothoracic ganglion, where it leaves through the mesothoracic N1. The targets of this neuron, among them the mesothoracic and metathoracic muscles M87, M88, M116 and the dorsal longitudinal muscles M81 and M112, are located in five different segments. In addition to supplying skeletal muscles, the neuron forms neurohaemal-like structures in the sheath of nerve branches. The authors call this neuron the common lateral neuron (CLN). The innervation of several muscles by Diploptera allatostatin 7-immunoreactive axon branches with a common cellular origin and the anatomy of one of the corresponding motor neurons in adults, the CLN, suggest that allatostatin acts as a modulator of neuromuscular parameters in insects by multisegmental direct innervation of skeletal muscles.

Axonal degeneration with tympanal nerve of Schistocerca gregaria

This study describes time course and ultrastructural changes during axonal degeneration of different neurones within the tympanal nerve of the locust Schistocerca gregaria. The tympanal nerve innervates the tergit and pleurit of the first abdominal segment and contains the axons of both sensory and motor neurones. The majority of axons (approx. 97%) belong to several types of sensory neurones: mechano- and chemosensitive hair sensilla, multipolar neurones, campaniform sensilla and sensory cells of a scolopidial organ, the auditory organ. Axons of campaniform sensilla, of auditory sensory cells and of motor neurones are wrapped by glial cell processes. In contrast, the very small and numerous axons (diameter <1 microm) of multipolar neurones and hair sensilla are not separated individually by glia sheets. Distal parts of sensory and motor axons show different reactions to axotomy: 1 week after separation from their somata, distal parts of motor axons are invaded by glial cell processes. This results in fascicles of small axon bundles. In contrast, distal parts of most sensory axons degenerate rapidly after being lesioned. The time to onset of degeneration depends on distance from the lesion site and on the type of sensory neurone. In axons of auditory sensory neurones, ultrastructural signs of degeneration can be found as soon as 2 days after lesion. After complete lysis of distal parts of axons, glial cell processes invade the space formerly occupied by sensory axons. The rapid degeneration of distal auditory axon parts allows it to be excluded that they provide a structure that leads regenerating axons to their targets. Proximal parts of severed axons do not degenerate.

Mechanosensory pegs constitute stridulatory files in grasshoppers

Stridulatory files on the inner face of hindleg femora were shown to consist of mechanosensory pegs in males and females of Syrbula montezuma (Saussure) and in males of Chorthippus biguttulus (L.). Females of Chorthippus had stiff protuberances on their stridulatory files, with an innervated tubercle instead of pegs. Pegs and tubercles of adult grasshoppers were shown to develop from innervated tubercular hairs present from the first instar onward in Chorthippus. In adults of Chorthippus, two sensory cells innervated each peg of males and each tubercle of females. Central projections of these afferents from the stridulatory files were very similar to those of the neighboring tactile hairs on the femur. The afferents from pegs in Syrbula responded to deflection and pressure introduced via the widened cuticular cap. In both species, selective stimulation of femoral cuticular receptors elicited antagonistic reflex responses in a coxal retractor muscle: pegs inhibited and neighboring hairs raised the efferent tonic discharges. Apparently, in these two distantly related grasshopper species, stridulatory files function as both sound-producing and proprioceptive organs.

Colocalisation of taurine- with transmitter-immunoreactivities in the nervous system of the migratory locust

The expression of taurine immunoreactivity (TAU-IR) by neurones immunoreactive for octopamine (OA-IR), gamma-aminobutyric acid (GABA-IR), and the C-terminal peptide sequence arginine-phenylalanine (RFamide-IR) was investigated in the migratory locust (Locusta migratoria). TAU-IR is colocalised with OA-IR in the dorsal unpaired median neurones, which are efferent neuroparacrine cells. TAU-IR is not, however, expressed by OA-IR interneurones in the thoracic ganglia and brain. The only other TAU-IR somata found with peripheral axons are the medial neurosecretory cells in abdominal ganglia that project to the neurohaemal organs. These cells exhibit RFamide-IR. The majority of TAU-IR somata in the thoracic abdominal nervous system exhibit GABA-IR. These cells correspond to populations of identified local and intersegmentally projecting inhibitory interneurones. TAU-IR is not, however, exhibited by the well-known GABAergic common inhibitor neurones, which have peripherally projecting axons. This differential distribution of TAU-IR in basically two, functionally different, neuronal subsets (efferent neurosecretory and neuroparacrine cells, inhibitory interneurones) conforms with the concept of taurine acing as a depressive agent to limit excitation during stressful conditions.

Developing grasshopper neurons show variable levels of guanylyl cyclase activity on arrival at their targets

The ability of certain grasshopper neurons to respond to exogenously applied donors of nitric oxide (NO) by producing cyclic GMP (cGMP) depends on their developmental state. ODQ, a selective blocker of NO-sensitive guanylyl cyclase, blocks cGMP production at 10(-5) M, thus confirming the nature of the response. Experiments in which the distal axon is separated from its proximal stump before application of an NO donor show that guanylyl cyclase is distributed uniformly throughout the neuron. In the locust abdomen, where segments are formed sequentially, the pattern of guanylyl cyclase up-regulation is predictable and sequential from anterior to posterior. There are two patterns of innervation by cGMP-expressing motor neurons. In the first, typified by muscle 187, an innervating neuron begins to be NO responsive on arrival at its muscle and continues to be so over most of the remainder of embryonic development, including the formation of motor end plates. In the second, typified by a neuron innervating muscle 191, the neuron extends well along the muscle, apparently laying down a number of sites of contact with it, before it becomes NO responsive. In both patterns, however, NO responsiveness marks the neuron's transition from growth cone elongation to the production of lateral branches. Individual muscles receive innervation from multiple motor neurons, some of which express transient NO sensitivity during development and others which do not. With the exception of the leg motor neuron SETi, the first motor neuron to reach any muscle is usually not NO responsive. We suggest that cGMP plays a role in, or reflects, the early stages of communication between a target and specific innervating neurons.

Tracing of a neuronal network in the locust by pressure injection of markers into a synaptic neuropil

Central neuronal circuits of vertebrates have often been investigated using injection of markers into synaptic neuropils, whereas similar techniques have rarely been applied in invertebrates. In this study we tested several neuroanatomical tracers for their ability to mark central neuronal circuits in insects, using the well described auditory network of the locust, Locusta migratoria. After physiological localization of an auditory neuropil various tracers were pressure injected. Horseradish peroxidase, dextrans (3 and 10 kDa) and especially biocytin and neurobiotin were effectively incorporated by auditory interneurons, which resulted in their extensive labeling. Postsynaptic regions turned out to be the major, if not exclusive sites of uptake of injected markers, which is deduced from two lines of evidence: (i) for labeling of identified auditory neurons it was necessary to apply the tracer to postsynaptic sites of the neuron; (ii) only a few non-auditory neurons were labeled (probably by lesioning axons during electrode impalement). No evidence could be found for an activity dependent uptake. We conclude that pressure injection of certain tracers into synaptic areas can be used to identify central nervous circuits in insects.

Levels of genetic polymorphism: marker loci versus quantitative traits

Species are the units used to measure ecological diversity and alleles are the units of genetic diversity. Genetic variation within and among species has been documented most extensively using allozyme electrophoresis. This reveals wide differences in genetic variability within, and genetic distances among, species, demonstrating that species are not equivalent units of diversity. The extent to which the pattern observed for allozymes can be used to infer patterns of genetic variation in quantitative traits depends on the forces generating and maintaining variability. Allozyme variation is probably not strictly neutral but, nevertheless, heterozygosity is expected to be influenced by population size and genetic distance will be affected by time since divergence. The same is true for quantitative traits influenced by many genes and under weak stabilizing selection. However, the limited data available suggest that allozyme variability is a poor predictor of genetic variation in quantitative traits within populations. It is a better predictor of general phenotypic divergence and of postzygotic isolation between populations or species, but is only weakly correlated with prezygotic isolation. Studies of grasshopper and planthopper mating signal variation and assortative mating illustrate how these characters evolve independently of general genetic and morphological variation. The role of such traits in prezygotic isolation, and hence speciation, means that they will contribute significantly to the diversity of levels of genetic variation within and among species.

Engrailed protein is expressed in interneurons but not motor neurons of the dorsal unpaired median group in the adult grasshopper

We report that the homeodomain protein Engrailed (En) is differentially expressed by neuronal type. Expression was examined within identified midline neurons in T3, A1, and A2 neuromeres of the adult grasshopper by using immunohistochemistry. All save a few neurons in the adult dorsal unpaired median (DUM) group arise embryonically from a single precursor, the median neuroblast. DUM neurons are efferent neurons, local interneurons, or intersegmental interneurons, recognizable as such by their distinct morphologies and neurotransmitter phenotypes. We show that interneurons are En-positive, whereas efferents are En-negative. In the T3 DUM group, the 70 or so interneurons contained cytoplasmic immunoreactivity for gamma-aminobutyric acid (GABA) and glutamate decarboxylase. In double-labeling experiments, all GABA-immunoreactive neurons were also En-positive, and all En-positive neurons contained GABA immunoreactivity. In complementary experiments, the 20 or so efferents in the T3 DUM group, which are octopaminergic, were selectively labeled with a histological marker and then processed to reveal En immunoreactivity. No efferents in the group were En-positive. The abdominal DUM groups contain fewer neurons, but the same dichotomy of labeling was found. The En pattern is established during embryogenesis, with the type-specific pattern apparent by stage 90% of development, the earliest stage examined here. The differential expression of En in the embryo and its continued expression in the adult nervous system suggest a role in the development and maintenance of neuronal phenotype. Morphological differences between efferents and interneurons are discussed in light of a hypothesis that En mediates differential expression of cell adhesion or cell-affinity molecules.

Neuroarchitecture of the lower division of the central body in the brain of the locust (Schistocerca gregaria)

We have investigated the anatomical organization of the lower division of the central body in the brain of the locust Schistocerca gregaria. Bodian preparations, Golgi impregnations, and intracellular filling with Lucifer yellow have revealed that the lower division of the central body is organized into six horizontal layers and sixteen vertical columns. Neurons of the lower division of the central body have been classified into five types of tangential neuron (TL1-TL5) and two types of columnar neuron (CL1, CL2). TL1-TL4 neurons ramify in specific layers in the lower division of the central body and in the lateral triangle (TL1, TL2 neurons), the median olive (TL3 neurons), or the dorsal shell (TL4 neurons) of the lateral accessory lobe. TL5 neurons ramify in the protocerebral bridge, in the lateral accessory lobe, and in all layers of the lower division of the central body. The two types of columnar neurons have arborizations in the protocerebral bridge and in the lower division of the central body and project to the lateral triangle of the lateral accessory lobe (CL1 neurons) or to the lower subunit of the nodulus (CL2 neurons). Possible functional implications for the processing of neuronal information in the central complex are discussed.

Datura stramonium lectin staining of glial associated extracellular material in insect brains

To investigate how glial cells structure the neuropile of olfactory pathways in the brains of honeybees and locusts, we used a lectin as a carbohydrate specific molecular label. On frozen sections, Datura stramonium lectin (DSL) stained extracellular material which is mainly associated with glial cells. Preadsorption of the DSL with the carbohydrate N, N'-diacetylchitobiose blocked the staining. The location of glial cells was detected by an antiserum against the glial-specific nuclear repo-protein. Lectin-staining surrounded the neuropile of the antennal lobe, axonal projections of olfactory relay neurons, and the mushroom body neuropile. Within the mushroom body neuropile of the bee, DSL-staining was especially intense at the branching sites of the Kenyon cell axons and in the ventral part of the alpha-lobe. The dissection of the various cellular contributions to the lectin-staining in dissociated cell cultures suggested that certain glial cells, but also neuronal somata of the antennal lobe and Kenyon cells of the mushroom bodies express the label. The expression of lectin-staining matures during the pupal development of the bee, whereas in larval stages of the hemimetabolous locust, the staining pattern appears already completed. Since carbohydrate recognition is thought to play an important role in the formation of neuronal networks, the glial derived extracellular material may contribute to the morphogenesis and structural integrity of the olfactory neuropiles.

Morphology of locust neurosecretory cells projecting into the Nervus corporis allati II of the suboesophageal ganglion

The morphology of neurosecretory cells that project from the suboesophageal ganglion into the retrocerebral complex via the Nervus corporis allati II (NCA II) was studied in the migratory locust, Locusta migratoria, using backfilling techniques and intracellular staining. There are two populations of cells located ventrally in the ganglion: an anterior group of four larger cells, and a posterior group of up to 22 smaller cells. Apart from cell body size and position, members of both cell groups have almost all features in common. They show long-lasting soma spikes with large amplitudes typical for arthropod neurosecretory cells. Their dendritic arborisations are found in the same regions of the neuropile. Both types project into the corpora cardiaca and an additional putative neurohaemal region associated with posterior pharyngeal dilator muscles. The axons of the cells bypass the corpora allata, but frequently form putative release sites on the surface of nerve branches in the vicinity of these glands. Finally, using double-labelling techniques, both anterior and posterior cells are shown to be identical with immunoreactive suboesophageal ganglion cells detected in previous studies using antisera directed against either bovine pancreatic polypeptide (BPP) or locustamyotropin II (Lom-MT-II).

GABAergic synapses in the antennal lobe and mushroom body of the locust olfactory system

To help elucidate the role of inhibitory feedback in the genesis of odour-evoked synchronization of neural activity, we investigated the distribution of gamma-aminobutyric acid (GABA)ergic synaptic terminals in the antennal lobes (AL) and mushroom bodies (MB) of the locust olfactory system. Electron-microscopy, intracellular horseradish peroxidase labelling, and immunocytochemistry were combined to assess the distribution of GABAergic synapses, using established methods (Leitch and Laurent [1993] J. Comp. Neurol. 337:461-470). In the AL, GABA-immunoreactive presynaptic terminals contacted both immunoreactive and immunonegative profiles. Conversely, GABA-immunoreactive profiles received direct input from both reactive and negative terminals. The tract containing the axons of the projection neurons that run from the AL to the MB contained about 830 axons of fairly uniform size, none of which was immunoreactive for GABA. In the calyx of the MB, large immunoreactive terminals contacted very-small-diameter profiles thought to belong to the Kenyon cells (KCs). This was confirmed by combining immunocytochemistry with intracellular HRP-labelling of KCs. KCs were not immunoreactive for GABA. Although some GABAergic contacts were made onto the spiny profiles of KCs, others were made onto their dendritic shafts. Large GABA-immunoreactive profiles were also found to contact large negative profiles that were presynaptic to KC terminals. This suggests that KC dendrites can be both pre- and post-synaptically inhibited in the calyx. The MB pedunculus contained ca. 50,000 tightly packed KC axons, showing conspicuous en passant and often reciprocal synaptic contacts between neighbouring axons. KC axons were immunonegative, but received direct input from, and contacted directly, large immunoreactive profiles running across or along the KC axons. In the alpha- and beta-lobes of the MB, connections similar to those in the pedunculus were seen with two main differences: (1) The density of synaptic profiles was higher, giving on occasion numerous serially connected profiles in a single section; (2) large immunonegative profiles with dense-core vesicles were abundant and were frequently presynaptic to GABAergic processes and to very-small-diameter profiles which possibly belong to KCs. These results are discussed in the context of the known physiological data on olfactory processing in these complex circuits.