The locust olfactory system as a case study for modeling dynamics of neurobiological networks: from discrete time neurons to continuous time neurons

Both chaotic and periodic activities are observed in networks of the central nervous systems. We choose the locust olfactory system as a good case study to analyze the relationships between networks' structure and the types of dynamics involved in coding mechanisms. In our modeling approach, we first build a fully connected recurrent network of synchronously updated McCulloch and Pitts neurons (MC-P type). In order to measure the use of the temporal dimension in the complex spatio-temporal patterns produced by the networks, we have defined an index the Normalized Euclidian Distance NED. We find that for appropriate parameters of input and connectivity, when adding some strong connections to the initial random synaptic matrices, it was easy to get the emergence of both robust oscillations and distributed synchrony in the spatiotemporal patterns. Then, in order to validate the MC-P model as a tool for analysis for network properties, we examine the dynamic behavior of networks of continuous time model neuron (Izhikevitch Integrate and Fire model -IFI-), implementing the same network characteristics. In both models, similarly to biological PN, the activity of excitatory neurons are phase-locked to different cycles of oscillations which remind the ones of the local field potential (LFP), and nevertheless exhibit dynamic behavior complex enough to be the basis of spatio-temporal codes.

Maternal effects on phase characteristics in the desert locust, Schistocerca gregaria: a review of current understanding

Desert locusts demonstrate pronounced density-dependent polyphenism: a complex suite of traits shifts over the lifetime of an individual in response to crowding or isolation. These changes also accumulate across generations through a maternal effect. Female desert locusts alter the developmental trajectory of their offspring in response to their own experience of crowding. The mother possesses a memory of both the recency and extent of crowding and shifts the phase state of her hatchlings accordingly. Extensive experimental work has shown that offspring behaviour is controlled by a low molecular weight, polar compound (or compounds) released from the mother's accessory glands. The chemical identity of this agent is not yet known.

Dendritic projections of different types of octopaminergic unpaired median neurons in the locust metathoracic ganglion

Octopaminergic dorsal unpaired median (DUM) neurons of locust thoracic ganglia are important components of motor networks and are divided into various sub-populations. We have examined individually stained metathoracic DUM neurons, their dendritic projection patterns, and their relationship to specific architectural features of the metathoracic ganglion, such as longitudinal tracts, transverse commissures, and well-defined sensory neuropils. The detailed branching patterns of individually characterized DUM neurons of various types were analyzed in vibratome sections in which architectural features were revealed by using antibodies against tubulin and synapsin. Whereas DUM3,4,5 and DUM5 neurons (the group innervating leg and "non-wing-power" muscles) had many ventral and dorsal branches, DUM1 and DUM3,4 neurons (innervating "wing-power" muscles) branched extensively only in dorsal areas. The structure of DUM3 neurons differed markedly from that of the other DUM neurons examined in that they sent branches into dorsal areas and had differently structured side branches that mostly extended laterally. The differences between the branching patterns of these neurons were quantified by using currently available new reconstruction algorithms. These structural differences between the various classes of DUM neurons corresponded to differences in their function and biophysical properties.

Neural Correlations Increase Between Consecutive Processing Levels in the Auditory System of Locusts

Nervous systems may encode information about sensory stimuli using the temporal relations in spiking patterns between neurons. By conducting simultaneous intracellular recordings on pairs of auditory neurons we measured the strength of correlations between elements of the first three processing levels in the metathoracic auditory network of the locust. We quantified the degree of spike synchronization and rate covariations that occur among auditory neurons during acoustic stimulation. In addition to the acoustic stimulation, current pulses were injected into both neurons to study the connectivity within this network. Our findings support the view that the metathoracic auditory system is a hierarchically organized feedforward network. Strong synaptic connections were observed only between consecutive processing levels, whereas there was no indication for strong connections between elements of the same processing level. Both spike synchronization and rate covariations were increased among neurons on higher processing levels. We further investigated the consequences that correlations may have on the common estimates of neuronal variability. For example, rate covariations caused by strong synaptic coupling between two neurons may lead to an overestimation if the variability is measured trial by trial with respect to only single neurons. For the vast majority of cell pairs tested, however, no strong synaptic coupling could be demonstrated. Thus we could show that in most cases no serious errors are made if one determines variability by following the usual procedure on the basis of single-cell recordings.

Habitat use and phenology of the large insular endemic grasshopper Acrostira euphorbiae (Orthoptera: Pamphagidae)

The habitat use and the phenology of the large grasshopper Acrostira euphorbiae García and Oromí endemic to La Palma (Canary Islands) are studied. This grasshopper is entirely dependent on the Canarian endemic shrub Euphorbia lamarckii both for food and to avoid predation. Adults stay on subapical branches during the day, probably to reduce the risk of predation, and climb up to the apex at night to feed. While females seem to ensure the genetic diversity of offspring by waiting for visits by different males, the latter have to move to guarantee their reproductive success. Monophagy in this species may be related to the year-round presence of tender shoots, and to the predator-repellent toxic latex found in the host plant. Unlike related species from continental areas, adults and nymphs of A. euphorbiae are present almost all year round, probably in adaptation to the particular climate of the islands. Nymphs are more abundant in winter, when Euphorbia leaves are most available. However, adults are more abundant than nymphs in spring, summer and autumn. Males develop more quickly than females, an apparent reproductive strategy based on achieving sexual maturity to coincide with females undergoing imaginal moulting. Matings start immediately after adult females emerge. Densities oscillated between 73 and 193 individuals ha(-1), which can be considered a low value compared with other continental pamphagid species.

Maplike representation of celestial E-vector orientations in the brain of an insect

For many insects, the polarization pattern of the blue sky serves as a compass cue for spatial navigation. E-vector orientations are detected by photoreceptors in a dorsal rim area of the eye. Polarized-light signals from both eyes are finally integrated in the central complex, a brain area consisting of two subunits, the protocerebral bridge and the central body. Here we show that a topographic representation of zenithal E-vector orientations underlies the columnar organization of the protocerebral bridge in a locust. The maplike arrangement is highly suited to signal head orientation under the open sky.

Effects of B chromosomes and supernumerary segments on morphometric traits and adult fitness components in the grasshopper, Dichroplus elongatus (Acrididae)

The South American species, Dichroplus elongatus, is polymorphic for B chromosomes and supernumerary segments in chromosome pairs M6 (SS6), S9 (SS9) and S10 (SS10). Both forms of supernumerary heterochromatin shape chiasma frequency and distribution and B chromosomes also affect male fertility. Here, we analysed the effects of these polymorphisms on morphometric traits (total, 3rd femur, 3rd tibia, thorax and tegmen lengths) and several adult fitness components, including male and female mating success, and female reproductive potential. B chromosomes tend to decrease, and SS6 segments to increase the body size of carriers. The analysis of reproductive potential suggested that B chromosome carrying females have higher numbers of embryos per clutch and ovarioles per ovary. The uni- and multivariate analysis of mating success revealed that sexual selection favours larger individuals of both sexes and males with standard karyotype. B chromosomes may have accumulation mechanisms, which involve preferential transmission of B chromosomes to germ cells or functional gametes. The maintenance of Bs might be explained by interactions among accumulation mechanisms and trade-offs between detrimental and favourable effects on different fitness components.

Sex-based divergence in head shape and diet in the Eastern lubber grasshopper (Romalea microptera)

Biologists are becoming increasingly aware of sex-based differences in ecologically important traits (locomotor performance, feeding morphology, thermoregulatory behavior). Yet, the overwhelming majority of these studies have been performed on vertebrates, despite often striking examples of sexual size dimorphism among the insects. Here, I tested whether adult male and female Eastern lubber grasshoppers differ in size/shape and feeding ecology. The morphological data clearly showed sex-based differences in several aspects of head and body size, with females being significantly larger in all aspects. Moreover, the sexes also differed in two aspects of size-adjusted morphology: head width and pronotum width, with females having relatively wider heads and thoraxes than males. Multiple regression analysis indicated that width of consumed foliage was positively related to pronotum width and foliage thickness was positively related to pronotum width, head width, and size-adjusted head width. Females consumed wider and thicker foliage (primarily smooth cord grass, Spartina alterniflora) than males, which most frequently consumed the narrow-leafed forb hemp sesbania (Sesbania macrocarpa). Therefore, the sex-based differences in size and shape in this grasshopper are correlated with differences in consumed foliage shape.

Histological description of the midgut and the pyloric valve of Tropidacris collaris (Stoll, 1813) (Orthopetera: Romaleidae)

The present research describes the histology of the midgut, gastric caeca, and pyloric valve of Tropidacris collaris (Stoll, 1813), (Orthopetera: Romaleidae). We used light microscopy, staining (Gomori's trichrome and periodic acid-Schiff (PAS)), and a routine histological analysis method (hematoxilin-eosin). The insects were obtained from, and also bred in, the Laboratory of Entomology, Department of Biology, of the Rural Federal University of Pernambuco (UFRPE). The collected material was fixed in alcoholic Boüin and embedded in paraplast. The results demonstrated that the midgut wall is composed of an inner epithelial layer and two outer layers of striate muscles: one internal (circular) and the other external (longitudinal), with connective tissue between the muscle fibers. The epithelium is single-layered, with two cell types: regenerative and elongated columnar. The gastric caeca presents muscle layers similar to those of the midgut. Simple columnar epithelium lines the gastric caeca, which presents villi and projects towards the lumen. The pyloric valve is of striate muscle tissue, covered by a single epithelial-cell layer.

Neuromodulation of the locust frontal ganglion during the moult: a novel role for insect ecdysis peptides

In insects, continuous growth requires the periodic replacement of the exoskeleton during the moult. A moulting insect displays a stereotypical set of behaviours that culminate in the shedding of the old cuticle at ecdysis. Moulting is an intricate process requiring tightly regulated physiological changes and behaviours to allow integration of environmental cues and to ensure the proper timing and sequence of its components. This is under complex hormonal regulation, and is an important point of interaction between endocrine and neural control. Here, we focus on the locust frontal ganglion (FG), an important player in moulting behaviour, as a previously unexplored target for ecdysis peptides. We show that application of 10(-7) mol l(-1) ecdysis-triggering hormone (ETH) or 10(-7) mol l(-1) and 10(-6) mol l(-1) Pre-ecdysis-triggering hormone (PETH) to an isolated FG preparation caused an increase in bursting frequency in the FG, whereas application of 10(-6) mol l(-1) eclosion hormone (EH) caused an instantaneous, though temporary, total inhibition of all FG rhythmic activity. Crustacean cardioactive peptide (CCAP), an important peptide believed to turn on ecdysis behaviour, caused a dose-dependent increase of FG burst frequency. Our results imply a novel role for this peptide in generating air-swallowing behaviour during the early stages of ecdysis. Furthermore, we show that the modulatory effects of CCAP on the FG motor circuits are dependent on behavioural state and physiological context. Thus, we report that pre-treatment with ETH caused CCAP-induced effects similar to those induced by CCAP alone during pre-ecdysis. Thus, the action of CCAP seems to depend on pre-exposure to ETH, which is thought to be released before CCAP in vivo.

Surgical lesion of the anterior optic tract abolishes polarotaxis in tethered flying locusts, Schistocerca gregaria

Many insects can detect the polarization pattern of the blue sky and rely on polarization vision for sky compass orientation. In laboratory experiments, tethered flying locusts perform periodic changes in flight behavior under a slowly rotating polarizer even if one eye is painted black. Anatomical tracing studies and intracellular recordings have suggested that the polarization vision pathway in the locust brain involves the anterior optic tract and tubercle, the lateral accessory lobe, and the central complex of the brain. To investigate whether visual pathways through the anterior optic tract mediate polarotaxis in the desert locust, we transected the tract on one side and tested polarotaxis (1) with both eyes unoccluded and (2) with the eye of the intact hemisphere painted black. In the second group of animals, but not in the first group, polarotaxis was abolished. Sham operations did not impair polarotaxis. The experiments show that the anterior optic tract is an indispensable part of visual pathways mediating polarotaxis in the desert locust.

Intraspecific variation in tracheal volume in the American locust,Schistocerca americana, measured by a new inert gas method

The volume of a tracheal system influences breath-holding capacity and provides an index of an insect's investment in its respiratory system. Here,we describe a new, generally applicable method to measure tracheal volume that enables repeatable determinations on live animals. Animals are isolated in a closed chamber of a known volume and equilibrated with a helium:oxygen gas mixture. The chamber is then rapidly flushed with a nitrogen:oxygen gas mixture to eliminate the helium surrounding the animal, and sealed. After a period of time sufficient to allow equilibration of helium between tracheal system and chamber air, a gas sample is taken from the chamber, and tracheal volumes are calculated from the helium content of the sample, using a gas chromatograph. We show that relative investment in the tracheal system increases with age/size in the grasshopper; tracheal volume scales with mass to the power 1.3. This increased proportional investment in the tracheal system provides a mechanistic basis for the enhanced respiratory capacity of older grasshoppers. Tracheal volumes decrease strongly as grasshoppers grow within an instar stage, explaining reduced safety margins for oxygen delivery. Finally, tracheal volumes are smaller in gravid females than males, probably due to compression of air sacs by eggs.

Ronderosia ommexechoides: a new species of Brazilian dichroplini (Orthoptera: Acrididae, Melanoplinae)

A new species of the genus Ronderosia Cigliano 1997 is described. Color illustrations of the female and male, drawings of female and male external genitalia as well as drawings of its phallic structures and photographs of the chromosomes are included.

Adaptive significance of death feigning posture as a specialized inducible defence against gape-limited predators

Death feigning is fairly common in a number of taxa, but the adaptive significance of this behaviour is still unclear and has seldom been tested. To date, all proposed hypotheses have assumed that prey manage to escape predation by sending a death-mimicking signal, although death-feigning postures are markedly different from those of dead animals. Moreover, the efficacy of this technique may largely depend on the foraging mode of the predator; death feigning seldom works with sit-and-wait predators that make the decision to attack and consume prey within a very brief time. We examined whether death feigning in the pygmy grasshopper Criotettix japonicus Haan was an inducible defence behaviour against the frog Rana nigromaculata, a sit-and-wait, gape-limited predator. The characteristic posture assumed by the grasshopper during death feigning enlarges its functional body size by stretching each of three body parts (pronotum, hind legs and lateral spines) in three different directions, thereby making it difficult for the predator to swallow the prey. Our result is the first consistent explanation for why death-mimicking animals do not always mimic the posture of dead animals.

Common and specific inhibitory motor neurons innervate the intersegmental muscles in the locust thorax

The inhibitory innervation of the intersegmental (body wall) muscles between the first and the second thoracic segment of the migratory locust, Locusta migratoria, was investigated using neuroanatomical, immunocytochemical and electrophysiological techniques. Three neurons located in the prothoracic ganglion show GABA-like immunoreactivity and project into the intersegmental nerve. Two are common inhibitors. One of those innervates the oblique intersegmental muscle M59 and two dorsal longitudinal muscles (M81 and M82). The second common inhibitor also innervates M59 and the ventral longitudinal muscle M60. The third neuron innervates M60 exclusively and, for that reason, has to be regarded as the first specific inhibitor ever observed in insect neuromuscular assemblies. According to their innervation pattern, we term these neurons CI(59/60), CI(59/81/82), and SI(60). CI(59/81/82) and CI(59/60) appear to be segmentally homologous to CI(a) and CI(b) neurons, respectively, in the other body segments.

The regenerative nidi of the locust midgut as a model to study epithelial cell differentiation from stem cells

A better knowledge of the regulatory mechanisms involved in stem cell proliferation and/or differentiation could reveal new methods for the treatment of some diseases. Most previous studies in the field of stem cell biology have been carried out on cultured isolated cells. In the case of adult tissue stem cells, mesenchymal bone marrow derived cells have been most widely studied, while the undifferentiated stem cells present in the epithelial tissues are less known. In order to advance further our understanding of epithelial tissue stem cells, new in vivo models are required. The present study focuses on the dynamics of a new and simple model of intestinal epithelial regeneration found in the midgut of the migratory locust, Locusta migratoria (Linnaeus 1758). The locust midgut consists of three cell types: columnar cells, endocrine cells and undifferentiated regenerative clustered cells. The undifferentiated epithelial midgut cells give rise to two other cell types and are located in a nest of regenerative cells known as regenerative niche. We have performed single and continuous bromodeoxyuridine (BrdU) administration experiments to study regeneration niches and their cellular dynamics. Immunocytochemistry and immunofluorescence techniques were used to detect the incorporation of BrdU into regenerative niches and the presence of FMRFamide-like immunoreactivity, as a marker for endocrine cell differentiation. Some isolated label retaining cells (LRC) were observed at the niche base 10-15 days after the final BrdU administration. We propose that these cells are the stem cells of this epithelial tissue. We also calculated the length of the cell cycle phases for a subpopulation of transit amplifying cells within the regenerative niche: G1, 2.5+/-0.5 h; S, 5.5+/-0.5 h; G2, 0.75+/-0.25 h; M, 2.5+/-0.5 h. These amplifying cells will give rise to the columnar epithelial non-endocrine lineage. The differentiation of an endocrine cell from a niche stem cell occurs less frequently and thus leads to a lower proportion of endocrine cells as compared with epithelial columnar digestive cells (up to three endocrine cells per niche). Endocrine cell commitment seems to occur very early in the differentiation process within the niche, as double-labelled BrdU and FMRF endocrine cells have never been found. The only exception is the endocrine cells located in the ampullar region of the midgut, some of which show double immunostaining after long-term chronic BrdU injection. In summary, we have characterized a new and simple animal model of epithelial stem cell regeneration that may be useful for understanding the complex biological process that drives tissue renewal from undifferentiated and uncommitted progenitor cells.

Nonlinear time-periodic models of the longitudinal flight dynamics of desert locusts Schistocerca gregaria

Previous studies of insect flight control have been statistical in approach, simply correlating wing kinematics with body kinematics or force production. Kinematics and forces are linked by Newtonian mechanics, so adopting a dynamics-based approach is necessary if we are to place the study of insect flight on its proper physical footing. Here we develop semi-empirical models of the longitudinal flight dynamics of desert locusts Schistocerca gregaria. We use instantaneous force-moment measurements from individual locusts to parametrize the nonlinear rigid body equations of motion. Since the instantaneous forces are approximately periodic, we represent them using Fourier series, which are embedded in the equations of motion to give a nonlinear time-periodic (NLTP) model. This is a proper mathematical generalization of an earlier linear-time invariant (LTI) model of locust flight dynamics, developed using previously published time-averaged versions of the instantaneous force recordings. We perform various numerical simulations, within the fitted range of the model, and across the range of body angles used by free-flying locusts, to explore the likely behaviour of the locusts upon release from the tether. Solutions of the NLTP models are compared with solutions of the nonlinear time-invariant (NLTI) models to which they reduce when the periodic terms are dropped. Both sets of models are unstable and therefore fail to explain locust flight stability fully. Nevertheless, whereas the measured forces include statistically significant harmonic content up to about the eighth harmonic, the simulated flight trajectories display no harmonic content above the fundamental forcing frequency. Hence, manoeuvre control in locusts will not directly reflect subtle changes in the higher harmonics of the wing beat, but must operate on a coarser time-scale. A state-space analysis of the NLTP models reveals orbital trajectories that are impossible to capture in the LTI and NLTI models, and inspires the hypothesis that asymptotic orbital stability is the proper definition of stability in flapping flight. Manoeuvre control on the scale of more than one wing beat would then consist in exciting transients from one asymptotically stable orbit to another. We summarize these hypotheses by proposing a limit-cycle analogy for flapping flight control and suggest experiments for verification of the limit-cycle control analogy hypothesis.

Material structure, stiffness, and adhesion: why attachment pads of the grasshopper (Tettigonia viridissima) adhere more strongly than those of the locust (Locusta migratoria) (Insecta: Orthoptera)

The morphology, ultrastructure, effective elastic modulus, and adhesive properties of two different smooth-type attachment pads were studied in two orthopteran species. Tettigonia viridissima (Ensifera) and Locusta migratoria (Caelifera) have a similar structural organization of their attachment pads. They both possess a flexible exocuticle, where the cuticular fibrils are fused into relatively large rods oriented at an angle to the surface. The compliant material of the pad contributes to the contact formation with the substrate. However, the pad material structure was found to be different in these two species. L. migratoria pads bear a thick sub-superficial layer, as well as a higher density of rods. The indentation experiments showed a higher effective elastic modulus and a lower work of adhesion for L. migratoria pads. When the indentations were made at different depths, a higher effective elastic modulus was revealed at lower indentation depths in both species. This effect is explained by the higher stiffness of the superficial pad layer. The obtained results demonstrate a clear correlation between density of the fibres, thickness of the superficial layer, compliance of the pad, and its adhesive properties. Such material structures and properties may be dependent on the preferred environment of each species.

Patterns of male sterility in a grasshopper hybrid zone imply accumulation of hybrid incompatibilities without selection

It is now widely accepted that post-zygotic reproductive isolation is the result of negative epistatic interactions between derived alleles fixed independently at different loci in diverging populations (the Dobzhansky-Muller model). What is less clear is the nature of the loci involved and whether the derived alleles increase in frequency through genetic drift, or as a result of natural or sexual selection. If incompatible alleles are fixed by selection, transient polymorphisms will be rare and clines for these alleles will be steep where divergent populations meet. If they evolve by drift, populations are expected to harbour substantial genetic variation in compatibility and alleles will introgress across hybrid zones once they recombine onto a genetic background with which they are compatible. Here we show that variation in male sterility in a naturally occurring Chorthippus parallelus grasshopper hybrid zone conforms to the neutral expectations. Asymmetrical clines for male sterility have long tails of introgression and populations distant from the zone centre show significant genetic variation for compatibility. Our data contrast with recent observations on 'speciation genes' that have diverged as a result of strong natural selection.

The Temperature‐Size Rule in Ectotherms: May a General Explanation Exist after All?

The majority of ectotherms mature at a larger size at lower rearing temperatures. Although this temperature-size rule is well established, a general explanation for this phenomenon has remained elusive. In this article, we address the problem by exploring the proximate and ultimate reasons for why a temperate grasshopper, Chorthippus brunneus, is an exception to the temperature-size rule. Using a complete set of life-history data to parameterize an established life-history model, we show that it is optimal for this species to mature at a larger size at higher temperatures. We also show that plasticity in adult size is determined by the relative difference between the minimum temperature thresholds for growth and development rates. The mechanism relates to aspects of the biophysical model of van der Have and de Jong. Ectotherms that obey the temperature-size rule are identified as having a higher temperature threshold for development rate than for growth rate; exceptions are identified as having a lower temperature threshold for development rate than for growth rate. The latter scenario may arise broadly in two ways. These are discussed in reference to the thermal biology of temperate grasshoppers and ectotherms in general.

External sensilla of the locust abdomen provide the central nervous system with an interganglionic network

External mechanoreceptors and contact chemoreceptors on the cuticle of the sixth abdominal segment of locusts have divergent primary projections of their sensory neurons that form arbours in the segmental and anterior abdominal ganglia. Homologous interganglionic projections from adjacent segments converge in the neuropile of each abdominal ganglion. Of the contributing types of sensilla, three were previously unknown for locust pregenital segments: tactile mechanosensory hairs with dual innervation, external proprioceptors of the hairplate type covered by intersegmental membranes and single campaniform sensilla that monitor cuticular strain in sternites and tergites. In general, interdependence of motor coordination in the abdominal segments is based on a neural network that relies heavily on intersegmental primary afferents that cooperate to identify the location, parameters and strength of external stimuli.

Ion currents and spiking properties of identified subtypes of locust octopaminergic dorsal unpaired median neurons

Efferent dorsal unpaired median (DUM) neurons are key elements of an insect neuromodulatory system. In locusts, subpopulations of DUM neurons mediate octopaminergic modulation at specific targets depending on their activity during different behaviours. This study investigates whether in addition to synaptic inputs, activity in DUM neurons depends on intrinsic membrane properties. Intracellular in situ recordings and whole-cell patch-clamp recordings from freshly isolated somata characterize somatic voltage signals and the underlying ion currents of individual subtypes of DUM neurons identified beforehand by a vital retrograde tracing technique. Na(+), Ca(2+), K(+) currents and a hyperpolarization-activated (I(h)) current are described in detail for their (in-)activation properties and subtype-specific current densities. In addition, a Ca(2+)-dependent K(+) current is demonstrated by its sensitivity to cadmium and charybdotoxin. This complex current composition determines somatic excitability similar in all subtypes of DUM neurons. Both Na(+) and Ca(2+) currents generate overshooting somatic action potentials. Repolarizing K(+) currents, in particular transient, subthreshold-activating A-currents, regulate the firing frequency and cause delayed excitation by shunting depolarizing input. An opposing hyperpolarization-activated (I(h)) current contributes to the resting membrane potential and induces rebound activity after prolonged inhibition phases. A quantitative analysis reveals subtype-specific differences in current densities with more inhibitory I(K) but less depolarizing I(Na) and I(h) - at least in DUM3 neurons promoting a reliable suppression of their activity as observed during behaviour. In contrast, DUM neurons that are easily activated during behaviour (DUM3,4,5 and DUMETi) express less I(K) and a pronounced depolarizing I(h) promoting excitability.

Ontogeny of tracheal dimensions and gas exchange capacities in the grasshopper, Schistocerca americana

How does body size affect the structure and gas exchange capacities of insect tracheae? Do insects become more oxygen-limited as they grow? We addressed these questions by measuring the dimensions of two transverse tracheae within the abdomen of American locusts of different ages, and evaluating the potential for diffusion or convection to provide adequate gas exchange. The grasshopper abdomen has longitudinal tracheae that run along the midgut, heart, nerve cord, and lateral body wall. Transverse tracheae run from each spiracle to the longitudinal tracheae. Dorsal air sacs attach near each spiracle. In both transverse tracheae studied, diffusive capacities increased more slowly than metabolic rates with age, and calculated oxygen gradients necessary to supply oxygen by diffusion increased exponentially with age. However, surgical studies demonstrated that transport of gas through these transverse tracheae occurred by convection, at least in adults. Convective capacities paralleled metabolic rates with age, and the calculated pressure gradients required to sustain oxygen consumption rates by convection were independent of age. Thus, in growing grasshoppers, tracheal capacities matched tissue oxygen needs. Our morphological and physiological data together suggest that use of convection allows older grasshoppers to overcome potential limitations on size imposed by diffusion through tracheal systems.

The evolution of the phenotypic covariance matrix: evidence for selection and drift in Melanoplus

Phenotypic variation in trait means is a common observation for geographically separated populations. Such variation is typically retained under common garden conditions, indicating that there has been evolutionary change in the populations, as a result of selection and/or drift. Much less frequently studied is variation in the phenotypic covariance matrix (hereafter, P matrix), although this is an important component of evolutionary change. In this paper, we examine variation in the phenotypic means and P matrices in two species of grasshopper, Melanoplus sanguinipes and M. devastator. Using the P matrices estimated for 14 populations of M. sanguinipes and three populations of M. devastator we find that (1) significant differences between the sexes can be attributed to scaling effects; (2) there is no significant difference between the two species; (3) there are highly significant differences among populations that cannot be accounted for by scaling effects; (4) these differences are a consequence of statistically significant patterns of covariation with geographic and environmental factors, phenotypic variances and covariances increasing with increased temperature but decreasing with increased latitude and altitude. This covariation suggests that selection has been important in the evolution of the P matrix in these populations Finally, we find a significant positive correlation between the average difference between matrices and the genetic distance between the populations, indicating that drift has caused some of the variation in the P matrices.

Conspicuous and aposematic spines in the animal kingdom

Spines serve as a common physical defence mechanism in both the plant and animal kingdoms. Here we argue that as in plants, defensive animal spines are often conspicuous (shape and colour) and should be considered aposematic. Conspicuous spines may evolve as signals or serve as a cue for potential predators. Spine conspicuousness in animals has evolved independently across and within phyla occupying aquatic and terrestrial ecosystems, indicating that this convergent phenomenon is highly adaptive. Still, many spines are cryptic, suggesting that conspicuity is not simply constrained by developmental factors such as differences in the chemical composition of the integument. Aposematism does not preclude the signalling role of conspicuous spines in the sexual arena.