Rhythmic behaviour and pattern-generating circuits in the locust: key concepts and recent updates

Motor Rhythm Dissection From the Backward Circuit in C. elegans

Motor rhythm is initiated and sustained by oscillatory neuronal activity. We recently discovered that the A-class excitatory motor neurons (MNs) (A-MNs) function as intrinsic oscillators. They drive backward locomotion by generating rhythmic postsynaptic currents (rPSCs) in body wall muscles. Molecular underpinning of the rPSCs, however, is not fully elucidated. We report here that there are three types of the rPSC patterns, namely the phasic, tonic, and long-lasting, each with distinct kinetics and channel-dependence. The Na⁺ leak channel is required for all rPSC patterns. The tonic rPSCs exhibit strong dependence on the high-voltage-gated Ca²⁺ channels. Three K⁺ channels, the BK-type Ca²⁺-activated K⁺ channel, Na⁺-activated K⁺ channel, and voltage-gated K⁺ channel (Kv4), primarily inhibit tonic and long-lasting rPSCs with varying degrees and preferences. The elaborate regulation of rPSCs by different channels, through increasing or decreasing the rPSCs frequency and/or charge, correlates with the changes in the reversal velocity for respective channel mutants. The molecular dissection of different A-MNs-rPSC components therefore reveals different mechanisms for multiplex motor rhythm.

Central pattern generating networks in insect locomotion

Central pattern generators (CPGs) are neural circuits that based on their connectivity can generate rhythmic and patterned output in the absence of rhythmic external inputs. This property makes CPGs crucial elements in the generation of many kinds of rhythmic motor behaviors in insects, such as flying, walking, swimming, or crawling. Arguably representing the most diverse group of animals, insects utilize at least one of these types of locomotion during one stage of their ontogenesis. Insects have been extensively used to study the neural basis of rhythmic motor behaviors, and particularly the structure and operation of CPGs involved in locomotion. Here, we review insect locomotion with regard to flying, walking, and crawling, and we discuss the contribution of central pattern generation to these three forms of locomotion. In each case, we compare and contrast the topology and structure of the CPGs, and we point out how these factors are involved in the generation of the respective motor pattern. We focus on the importance of sensory information for establishing a functional motor output and we indicate behavior-specific adaptations. Furthermore, we report on the mechanisms underlying coordination between different body parts. Last but not least, by reviewing the state-of-the-art knowledge concerning the role of CPGs in insect locomotion, we endeavor to create a common ground, upon which future research in the field of motor control in insects can build.

The puzzle of locust density-dependent phase polyphenism

Locust density-dependent phase polyphenism presents a quintessential example of environmentally induced plasticity. Almost a century of research has yielded ample knowledge regarding the multitude of ecological, physiological, and molecular phase-dependent characteristics. This short review highlights the considerable advances that have been made in our understanding of the locust's extreme plasticity and the highly complex nature of the phase phenomenon. Several challenges in locust research resulting from this unique complexity are also presented. It is concluded that the joint, interdisciplinary collaborative efforts, already underway, hold the promise of translating our ample knowledge into a complete solution to untangling the locust phase puzzle.

A short guide to insect oviposition: when, where and how to lay an egg

Egg-laying behavior is one of the most important aspects of female behavior, and has a profound impact on the fitness of a species. As such, it is controlled by several layers of regulation. Here, we review recent advances in our understanding of insect neural circuits that control when, where and how to lay an egg. We also outline outstanding open questions about the control of egg-laying decisions, and speculate on the possible neural underpinnings that can drive the diversification of oviposition behaviors through evolution.

Respiratory gas levels interact to control ventilatory motor patterns in isolated locust ganglia

Large insects actively ventilate their tracheal system even at rest, using abdominal pumping movements, which are controlled by a central pattern generator (CPG) in the thoracic ganglia. We studied the effects of respiratory gases on the ventilatory rhythm by isolating the thoracic ganglia and perfusing its main tracheae with various respiratory gas mixtures. Fictive ventilation activity was recorded from motor nerves controlling spiracular and abdominal ventilatory muscles. Both hypoxia and hypercapnia increased the ventilation rate, with the latter being much more potent. Sub-threshold hypoxic and hypercapnic levels were still able to modulate the rhythm as a result of interactions between the effects of the two respiratory gases. Additionally, changing the oxygen levels in the bathing saline affected ventilation rate, suggesting a modulatory role for haemolymph oxygen. Central sensing of both respiratory gases as well as interactions of their effects on the motor output of the ventilatory CPG reported here indicate convergent evolution of respiratory control among terrestrial animals of distant taxa.

The functional connectivity between the locust leg pattern generators and the subesophageal ganglion higher motor center

Higher motor centers and central pattern generators (CPGs) interact in the control of coordinated leg movements during locomotion throughout the animal kingdom. The subesophageal ganglion (SEG) is one of the insect head ganglia reported to have a role in the control of walking behavior. Here we explored the functional relations between the SEG and the thoracic leg CPGs in the desert locust. Backfill staining revealed about 300 SEG descending interneurons (DINs) altogether. Recordings from an in-vitro isolated chain of thoracic ganglia, with intact or severed connections to the SEG, during pharmacological activation were used to determine how the SEG affects the centrally generated motor output to the legs. The SEG was demonstrated to both activate leg CPGs and synchronize their bilateral activity. The role of the SEG in insect locomotion is discussed in light of these findings.

The subesophageal ganglion modulates locust inter-leg sensory-motor interactions via contralateral pathways

The neural control of insect locomotion is distributed among various body segments. Local pattern-generating circuits at the thoracic ganglia interact with incoming sensory signals and central descending commands from the head ganglia. The evidence from different insect preparations suggests that the subesophageal ganglion (SEG) may play an important role in locomotion-related tasks. In a previous study, we demonstrated that the locust SEG modulates the coupling pattern between segmental leg CPGs in the absence of sensory feedback. Here, we investigated its role in processing and transmitting sensory information to the leg motor centers and mapped the major related neural pathways. Specifically, the intra- and inter-segmental transfer of leg-feedback were studied by simultaneously monitoring motor responses and descending signals from the SEG. Our findings reveal a crucial role of the SEG in the transfer of intersegmental, but not intrasegmental, signals. Additional lesion experiments, in which the intersegmental connectives were cut at different locations, together with double nerve staining, indicated that sensory signals are mainly transferred to the SEG via the connective contralateral to the stimulated leg. We therefore suggest that, similar to data reported for vertebrates, insect leg sensory-motor loops comprise contralateral ascending pathways to the head and ipsilateral descending ones.

Precopulatory behavior and sexual conflict in the desert locust

Studies of mating and reproductive behavior have contributed much to our understanding of various animals' ecological success. The desert locust, Schistocerca gregaria, is an important agricultural pest. However, knowledge of locust courtship and precopulatory behavior is surprisingly limited. Here we provide a comprehensive study of the precopulatory behavior of both sexes of the desert locust in the gregarious phase, with particular emphasis on the conflict between the sexes. Detailed HD-video monitoring of courtship and mating of 20 locust pairs, in a controlled environment, enabled both qualitative and quantitative descriptions of the behavior. A comprehensive list of behavioral elements was used to generate an eight-step ethogram, from first encounter between the sexes to actual copulation. Further analyses included the probability of each element occurring, and a kinematic diagram based on a transitional matrix. Eleven novel behavioral elements are described in this study, and two potential points of conflict between the sexes are identified. Locust sexual interaction was characterized by the dominance of the males during the pre-mounting stage, and an overall stereotypic male courtship behavior. In contrast, females displayed no clear courtship-related behavior and an overall less organized behavioral sequence. Central elements in the sexual behavior of the females were low-amplitude hind-leg vibration, as well as rejecting males by jumping and kicking. Intricate reciprocal interactions between the sexes were evident mostly at the mounting stage. The reported findings contribute important insights to our knowledge of locust mating and reproductive behavior, and may assist in confronting this devastating agricultural pest.

Rigidity and Flexibility: The Central Basis of Inter-Leg Coordination in the Locust

Many motor behaviors, and specifically locomotion, are the product of an intricate interplay between neuronal oscillators known as central pattern generators (CPGs), descending central commands, and sensory feedback loops. The relative contribution of each of these components to the final behavior determines the trade-off between fixed movements and those that are carefully adapted to the environment. Here we sought to decipher the endogenous, default, motor output of the CPG network controlling the locust legs, in the absence of any sensory or descending influences. We induced rhythmic activity in the leg CPGs in isolated nervous system preparations, using different application procedures of the muscarinic agonist pilocarpine. We found that the three thoracic ganglia, each controlling a pair of legs, have different inherent bilateral coupling. Furthermore, we found that the pharmacological activation of one ganglion is sufficient to induce activity in the other, untreated, ganglia. Each ganglion was thus capable to impart its own bilateral inherent pattern onto the other ganglia via a tight synchrony among the ipsilateral CPGs. By cutting a connective and severing the lateral-longitudinal connections, we were able to uncouple the oscillators’ activity. While the bilateral connections demonstrated a high modularity, the ipsilateral CPGs maintained a strict synchronized activity. These findings suggest that the central infrastructure behind locust walking features both rigid elements, which presumably support the generation of stereotypic orchestrated leg movements, and flexible elements, which might provide the central basis for adaptations to the environment and to higher motor commands.

Imaging fictive locomotor patterns in larval Drosophila

We have established a preparation in larval Drosophila to monitor fictive locomotion simultaneously across abdominal and thoracic segments of the isolated CNS with genetically encoded Ca²⁺ indicators. The Ca²⁺ signals closely followed spiking activity measured electrophysiologically in nerve roots. Three motor patterns are analyzed. Two comprise waves of Ca²⁺ signals that progress along the longitudinal body axis in a posterior-to-anterior or anterior-to-posterior direction. These waves had statistically indistinguishable intersegmental phase delays compared with segmental contractions during forward and backward crawling behavior, despite being ∼10 times slower. During these waves, motor neurons of the dorsal longitudinal and transverse muscles were active in the same order as the muscle groups are recruited during crawling behavior. A third fictive motor pattern exhibits a left-right asymmetry across segments and bears similarities with turning behavior in intact larvae, occurring equally frequently and involving asymmetry in the same segments. Ablation of the segments in which forward and backward waves of Ca²⁺ signals were normally initiated did not eliminate production of Ca²⁺ waves. When the brain and subesophageal ganglion (SOG) were removed, the remaining ganglia retained the ability to produce both forward and backward waves of motor activity, although the speed and frequency of waves changed. Bilateral asymmetry of activity was reduced when the brain was removed and abolished when the SOG was removed. This work paves the way to studying the neural and genetic underpinnings of segmentally coordinated motor pattern generation in Drosophila with imaging techniques.

Drosophila ovipositor extension in mating behavior and egg deposition involves distinct sets of brain interneurons

Oviposition is a female-specific behavior that directly affects fecundity, and therefore fitness. If a fertilized female encounters another male that she has evaluated to be of better quality than her previous mate, it would be beneficial for her to remate with this male rather than depositing her eggs. Females who decided not to remate exhibited rejection behavior toward a courting male and engaged in oviposition. Although recent studies of Drosophila melanogaster identified sensory neurons and putative second-order ascending interneurons that mediate uterine afferents affecting female reproductive behavior, little is known about the brain circuitry that selectively activates rejection versus oviposition behaviors. We identified the sexually dimorphic pC2l and female-specific pMN2 neurons, two distinct classes of doublesex (dsx)-expressing neurons that can initiate ovipositor extension associated with rejection and oviposition behavior, respectively. pC2l interneurons, which induce ovipositor extrusion for rejection in females, have homologues that control courtship behavior in males. Activation of these two classes of neurons appears to be mutually exclusive and each governs hierarchical control of the motor program in the VNC either for rejection or oviposition, contributing centrally to the switching on or off of the alternative motor programs.

Estimating parameters and predicting membrane voltages with conductance-based neuron models

Recent results demonstrate techniques for fully quantitative, statistical inference of the dynamics of individual neurons under the Hodgkin-Huxley framework of voltage-gated conductances. Using a variational approximation, this approach has been successfully applied to simulated data from model neurons. Here, we use this method to analyze a population of real neurons recorded in a slice preparation of the zebra finch forebrain nucleus HVC. Our results demonstrate that using only 1,500 ms of voltage recorded while injecting a complex current waveform, we can estimate the values of 12 state variables and 72 parameters in a dynamical model, such that the model accurately predicts the responses of the neuron to novel injected currents. A less complex model produced consistently worse predictions, indicating that the additional currents contribute significantly to the dynamics of these neurons. Preliminary results indicate some differences in the channel complement of the models for different classes of HVC neurons, which accords with expectations from the biology. Whereas the model for each cell is incomplete (representing only the somatic compartment, and likely to be missing classes of channels that the real neurons possess), our approach opens the possibility to investigate in modeling the plausibility of additional classes of channels the cell might possess, thus improving the models over time. These results provide an important foundational basis for building biologically realistic network models, such as the one in HVC that contributes to the process of song production and developmental vocal learning in songbirds.

On the origin of grasshopper oviposition behavior: structural homology in pregenital and genital motor systems

In female grasshoppers, oviposition is a highly specialized behavior involving a rhythm-generating neural circuit, the oviposition central pattern generator, unusual abdominal appendages, and dedicated muscles. This study of Schistocerca americana (Drury) grasshoppers was undertaken to determine whether the simpler pregenital abdominal segments, which do not contain ovipositor appendages, share common features with the genital segment, suggesting a roadmap for the genesis of oviposition behavior. Our study revealed that although 5 of the standard pregenital body wall muscles were missing in the female genital segment, homologous lateral nerves were, indeed, present and served 4 ovipositor muscles. Retrograde labeling of the corresponding pregenital nerve branches in male and female grasshoppers revealed motor neurons, dorsal unpaired median neurons, and common inhibitor neurons which appear to be structural homologues of those filled from ovipositor muscles. Some pregenital motor neurons displayed pronounced contralateral neurites; in contrast, some ovipositor motor neurons were exclusively ipsilateral. Strong evidence of structural homology was also obtained for pregenital and ovipositor skeletal muscles supplied by the identified neurons and of the pregenital and ovipositor skeletons. For example, transient embryonic segmental appendages were maintained in the female genital segments, giving rise to ovipositor valves, but were lost in pregenital abdominal segments. Significant proportional differences in sternal apodemes and plates were observed, which partially obscure the similarities between the pregenital and genital skeletons. Other changes in reorganization included genital muscles that displayed adult hypertrophy, 1 genital muscle that appeared to represent 2 fused pregenital muscles, and the insertion points of 2 ovipositor muscles that appeared to have been relocated. Together, the comparisons support the idea that the oviposition behavior of genital segments is built upon a homologous, segmentally iterated motor infrastructure located in the pregenital abdomen of male and female grasshoppers.

Octopamine modulates a central pattern generator associated with egg-laying in the locust, Locusta migratoria

Egg-laying in Locusta migratoria involves the control of a variety of complex behavioural patterns including those that regulate digging of the oviposition hole and retention of eggs during digging. These two behavioural patterns are under the control of central pattern generators (CPGs). The digging and egg-retention CPGs are coordinated and integrated with overlapping locations of neural substrate within the VIIth and VIIIth abdominal ganglia of the central nervous system (CNS). In fact, the egg-retention CPG of the VIIth abdominal ganglion is involved in both egg-retention and protraction of the abdomen during digging. The biogenic amine, octopamine, has peripheral effects on oviduct muscle, relaxing basal tension of the lateral and upper common oviduct and enabling egg passage. Here we show that octopamine also modulates the pattern of the egg-retention CPG by altering the motor pattern that controls the external ventral protractor of the VIIth abdominal segment. There is no change in the motor pattern that goes to the oviducts. Octopamine decreased the frequency of the largest amplitude action potential and decreased burst duration while leading to an increase in cycle duration and interburst interval. The effects of octopamine were greatly reduced in the presence of the α-adrenergic blocker, phentolamine, indicating that the action of octopamine was via a receptor. Thus, octopamine orchestrates events that can lead to oviposition, centrally inhibiting the digging behavior and peripherally relaxing the lateral and common oviducts to enable egg-laying.

Characterization of locomotor-related spike activity in protocerebrum of freely walking cricket

To characterize the neural elements involved in the higher-order control of spontaneous walking in insects, we recorded extracellular spike activity in the protocerebrum of freely walking crickets (Gryllus bimaculatus). Locomotor behavior was simultaneously recorded using a newly developed motion tracking system. We focused on spike units that altered their firing patterns during walking. According to their activity patterns with reference to walking bouts, these locomotor-related spike units were classified into the following four types: continuously activated unit during walking (type 1); continuously inhibited unit during walking (type 2); transiently activated unit at the onset of walking (type 3); and transiently activated unit at the termination of walking (type 4). The type 1 unit was the most dominant group (25 out of 33 units), whereas only a few units each were recorded for types 2-4. Some of the locomotor-related units tended to change firing pattern before the onset or termination of walking bouts. Spike activity in some type 1 units was found to be closely correlated with walking speed. When firing timing was compared between unit pairs, their temporal relationships (synchronization/desynchronization) altered, depending on the behavioral state (standing/walking). Mechanical stimuli applied to the body surface elicited excitatory responses in the majority of the units. Histological observations revealed that the recorded sites were concentrated near or within the mushroom body and central complex in the protocerebrum.

Flight and walking in locusts-cholinergic co-activation, temporal coupling and its modulation by biogenic amines

Walking and flying in locusts are exemplary rhythmical behaviors generated by central pattern generators (CPG) that are tuned in intact animals by phasic sensory inputs. Although these two behaviors are mutually exclusive and controlled by independent CPGs, leg movements during flight can be coupled to the flight rhythm. To investigate potential central coupling between the underlying CPGs, we used the muscarinic agonist pilocarpine and the amines octopamine and tyramine to initiate fictive flight and walking in deafferented locust preparations. Our data illustrate that fictive walking is readily evoked by comparatively lower concentrations of pilocarpine, whereas higher concentrations are required to elicit fictive flight. Interestingly, fictive flight did not suppress fictive walking so that the two patterns were produced simultaneously. Frequently, leg motor units were temporally coupled to the flight rhythm, so that each spike in a step cycle volley occurred synchronously with wing motor units firing at flight rhythm frequency. Similarly, tyramine also induced fictive walking and flight, but mostly without any coupling between the two rhythms. Octopamine in contrast readily evoked fictive flight but generally failed to elicit fictive walking. Despite this, numerous leg motor units were recruited, whereby each was temporarily coupled to the flight rhythm. Our results support the notion that the CPGs for walking and flight are largely independent, but that coupling can be entrained by aminergic modulation. We speculate that octopamine biases the whole motor machinery of a locust to flight whereas tyramine primarily promotes walking.

Self body-size perception in an insect

Animals negotiating complex environments encounter a wide range of obstacles of different shapes and sizes. It is greatly beneficial for the animal to react to such obstacles in a precise, context-specific manner, in order to avoid harm or even simply to minimize energy expenditure. An essential key challenge is, therefore, an estimation of the animal's own physical characteristics, such as body size. A further important aspect of self body-size perception (or SBSP) is the need to update it in accordance with changes in the animal's size and proportions. Despite the major role of SBSP in functional behavior, little is known about if and how it is mediated. Here, we demonstrate that insects are also capable of self perception of body size and that this is a vital factor in allowing them to adjust their behavior following the sudden and dramatic growth associated with periodic molting. We reveal that locusts' SBSP is strongly correlated with their body size. However, we show that the dramatic change in size accompanying adult emergence is not sufficient to create a new and updated SBSP. Rather, this is created and then consolidated only following the individuals' experience and interaction with the physical environment. Behavioral or pharmacological manipulations can both result in maintenance of the old larval SBSP. Our results emphasize the importance of learning and memory-related processes in the development and update of SBSP, and highlight the advantage of insects as good models for a detailed study on the neurobiological and molecular aspects of SBSP.

Transformation of context-dependent sensory dynamics into motor behavior

The intrinsic dynamics of sensory networks play an important role in the sensory-motor transformation. In this paper we use conductance based models and electrophysiological recordings to address the study of the dual role of a sensory network to organize two behavioral context-dependent motor programs in the mollusk Clione limacina. We show that: (i) a winner take-all dynamics in the gravimetric sensory network model drives the typical repetitive rhythm in the wing central pattern generator (CPG) during routine swimming; (ii) the winnerless competition dynamics of the same sensory network organizes the irregular pattern observed in the wing CPG during hunting behavior. Our model also shows that although the timing of the activity is irregular, the sequence of the switching among the sensory cells is preserved whenever the same set of neurons are activated in a given time window. These activation phase locks in the sensory signals are transformed into specific events in the motor activity. The activation phase locks can play an important role in motor coordination driven by the intrinsic dynamics of a multifunctional sensory organ.

How sensory information is transformed into effective motor action is one of the most fundamental questions in neuroscience. As this question is difficult to assess experimentally, biophysical models of sensory, central and motor systems contribute to understand the information processing mechanisms involved in this transformation. Biophysical models can be informed by electrophysiological data in those situations where it is possible to record neural activity at all stages of sensory-motor processing. In this paper we use this approach to describe the dual dynamics of a multifunctional sensory organ in the mollusk Clione limacina and its transformation into two different motor programs. Our experimental and modeling results indicate that the sensory signals are modified to fit the changing behavioral context, and they are readily interpreted by the rest of the nervous system to produce the correct motor output.

The neural and peptidergic control of gut contraction in Locusta migratoria: the effect of an FGLa/AST

The regulation of insect gut physiology is complex and involves the interactions of a number of mechanisms, including the neural regulation of gut contraction by altering neural input and the modulation of gut contractions by neuropeptides directly affecting the muscle. The FGLa-type allatostatins (FGLa/ASTs) are known brain/gut peptides with numerous physiological roles, including modulation of gut contraction and neural input. To further investigate the pleiotropic roles of FGLa/AST peptides in Locusta migratoria, we have examined the role of a locust FGLa/AST (Scg-AST-6) in the gut. Proctolin and Scg-AST-6 have opposing effects on gut contraction, where proctolin dose-dependently increases gut muscle tension, while Scg-AST-6 inhibits both muscle tension and spontaneous and neurogenic contractions in a dose-dependent manner. Results from neurophysiological recordings indicate that there may be a central pattern generator (CPG) within the ventricular ganglia regulated by descending inhibition, and the addition of Scg-AST-6 dose-dependently modulates this ventricular ganglion CPG. This work provides a comprehensive picture of how FGLa/ASTs may modulate and coordinate each region of the locust gut, and shows that FGLa/ASTs have both central effects, on the ventricular ganglion CPG, and peripheral effects on the gut muscle. Overall, this work shows how FGLa/ASTs contribute to the complex regulation and fine tuning of gut contraction.

Rhythmic movements are larger and faster but with the same frequency on removal of visual feedback

The brain controls rhythmic movement through neural circuits combining visual information with proprioceptive information from the limbs. Although rhythmic movements are fundamental to everyday activities the specific details of the responsible control mechanisms remain elusive. We tested 39 young adults who performed flexion/extension movements of the forearm. We provided them with explicit knowledge of the amplitude and the speed of their movements, whereas frequency information was only implicitly available. In a series of 3 experiments, we demonstrate a tighter control of frequency compared with amplitude or speed. We found that in the absence of visual feedback, movements had larger amplitude and higher peak speed while maintaining the same frequency as when visual feedback was available; this was the case even when participants were aware of performing overly large and fast movements. Finally, when participants were asked to modulate continuously movement frequency, but not amplitude, we found the local coefficient of variability of movement frequency to be lower than that of amplitude. We suggest that a misperception of the generated amplitude in the absence of visual feedback, coupled with a highly accurate perception of generated frequency, leads to the performance of larger and faster movements with the same frequency when visual feedback is not available. Relatively low local coefficient of variability of frequency in a task that calls for continuous change in movement frequency suggests that we tend to operate at a constant frequency at the expense of variation in amplitude and peak speed.

Respiratory pattern transitions in three species of Glossina (Diptera, Glossinidae)

Glossina exhibit cyclic ((CYC)GE) or continuous gas exchange ((CON)GE) patterns at rest. However, the factors influencing the transition from one pattern to another are not well understood for these or other insect species. This study examines which factors could aid in predicting the presence or absence of (CYC)GE in adults of three Glossina species: G. palpalis, G. brevipalpis and G. austeni. We report the results of temperature effects on VCO(2), pattern type and the proportion of a population showing (CYC)GE, and the prediction of (CYC)GE versus (CON)GE in Glossina. First, we investigated the influence of temperature on VCO(2) and found significant elevation in resting metabolic rate (RMR) with higher temperature in all three species (P<0.001). Temperature-induced increases in VCO(2) were modulated by increased burst volume and by cycle frequency, except in G. brevipalpis which only appeared to modulate burst volume. These results are largely in keeping with VCO(2) modulation reported for other Glossina species previously. Second, elevating temperature resulted in significantly reduced numbers of individuals showing (CYC)GE (P<0.001 for all three species) contrary to previous reports for other Glossing species. Finally, we examined a range of variables as potential predictors of presence or absence of (CYC)GE in these three species. Using an information theoretic approach (Akaike weights) to select the best explanatory combination of variables which predicts likelihood of (CYC)GE, we found that results varied among species. When species were pooled, the simplest, best-fit model (ΔAIC<2 from the best model, 44.4% probability of being the best model) for predicting pattern type variation was RMR. Overall these results suggest that RMR is a key variable driving pattern type and that elevated temperature reduces the number of individuals showing cyclic patterns through elevation of RMR in these species. This study supports the idea that an interaction between cellular metabolic demand, morphological features of the gas exchange system (e.g. tracheal and spiracular conductances), and CO(2) buffer capacity likely determine gas exchange pattern variation over short time-scales.

Effects of starvation and parasitism on foregut contraction in larval Manduca sexta

Larvae of Manduca sexta are parasitised by the braconid wasp, Cotesia congregata. In this study we examined whether contraction activity of the semi-isolated foregut was affected by parasitism. Parasitised larvae fed significantly less compared with unparasitised control larvae, therefore starved unparasitised animals were used as controls. Rate and force of foregut contraction in control caterpillars significantly increased with days of starvation. However, only contraction force in foreguts of parasitised larvae increased over time following infection. The presence of food in the foregut of caterpillars starved 7 days suggested that food moved anteriorly from the midgut and that contraction became antiperistaltic, but only normal peristalsis occurred in parasitised caterpillars. Rate and force of gut contractions may be controlled independently and starvation did not truly mimic the effects of the parasitoids. Dissection of caterpillars with emerged wasps indicated that 47% had a single wasp larva wedged between the brain and foregut. Removal of this wasp caused an increased rate of foregut contraction of the caterpillar. Brain removal resulted in an increased rate of foregut contraction only for unparasitised insects. Sectioning of the recurrent nerve temporarily eliminated foregut contraction, but the contraction began again in 250 s in parasitised caterpillars prior to wasp emergence, compared with over 500 s for unparasitised controls and parasitised caterpillars following wasp emergence.

Neuroanatomy and neurophysiology of the locust hypocerebral ganglion

The insect stomatogastric ganglia control foregut movements. Most previous work on the system has concentrated on the frontal ganglion (FG), including research into the role of the FG in feeding as well as molting-related behavior, mostly in locusts, but also in other insect species. The stomatogastric system exerts its physiological actions by way of careful interaction and coordination between its different neural centers and pattern-generating circuits. One such hitherto unstudied neural center is the hypocerebral ganglion (HG), which is connected to the FG via the recurrent nerve. It sends two pairs of nerves along the esophagus and to the posterior region of the crop, terminating in the paired ingluvial ganglia. Very little is known about the neuroanatomy and neurophysiology of the insect HG. Here we investigate, for the first time, the neuronal composition of the locust HG, as well as its motor output. We identify rhythmic patterns endogenous to the isolated HG, demonstrating the presence of a central pattern-generating network. Our findings suggest interactions between the HG and FG rhythm-generating circuits leading to complex physiological actions of both ganglia. This work will serve as a basis for future investigation into the physiology of the HG and its role in insect behavior.

Neural substrate and allatostatin-like innervation of the gut of Locusta migratoria

Allatostatin-like immunoreactivity (ALI) is widely distributed in processes and varicosities on the fore-, mid-, and hindgut of the locust, and within midgut open-type endocrine-like cells. ALI is also observed in cells and processes in all ganglia of the central nervous system (CNS) and the stomatogastric nervous system (SNS). Ventral unpaired median neurons (VUMs) contained ALI within abdominal ganglia IV-VII. Neurobiotin retrograde fills of the branches of the 11th sternal nerve that innervate the hindgut revealed 2-4 VUMs in abdominal ganglia IV-VIIth, which also contain ALI. The VIIIth abdominal ganglion contained three ventral medial groups of neurons that filled with neurobiotin and contained ALI. The co-localization of ALI in the identified neurons suggests that these cells are the source of ALI on the hindgut. A retrograde fill of the nerves of the ingluvial ganglia that innervate the foregut revealed numerous neurons within the frontal ganglion and an extensive neuropile in the hypocerebral ganglion, but there seems to be no apparent co-localization of neurobiotin and ALI in these neurons, indicating the source of ALI on the foregut comes via the brain, through the SNS.