Malcolm Burrows

The Neurobiology of an Insect Brain

Preface 1. Biology of locusts and grasshoppers 2. Anatomy of the nervous system 3. Components of the nervous system 4. Development of the nervous system 5. Neurotransmitters, neuromodulators and neurohormones 6. Actions of neuromodulators 7. Controlling local movements of the legs 8. Walking 9. Jumping 10. Escape movements 11. Flying 12. Breathing Glossary References

Serotonin Mediates Behavioral Gregarization Underlying Swarm Formation in Desert Locusts

Desert locusts, Schistocerca gregaria, show extreme phenotypic plasticity, transforming between a little-seen solitarious phase and the notorious swarming gregarious phase depending on population density. An essential tipping point in the process of swarm formation is the initial switch from strong mutual aversion in solitarious locusts to coherent group formation and greater activity in gregarious locusts. We show here that serotonin, an evolutionarily conserved mediator of neuronal plasticity, is responsible for this behavioral transformation, being both necessary if behavioral gregarization is to occur and sufficient to induce it. Our data demonstrate a neurochemical mechanism linking interactions between individuals to large-scale changes in population structure and the onset of mass migration.

Substantial changes in central nervous system neurotransmitters and neuromodulators accompany phase change in the locust.

SUMMARY Desert locusts (Schistocerca gregaria) can undergo a profound transformation between solitarious and gregarious forms, which involves widespread changes in behaviour, physiology and morphology. This phase change is triggered by the presence or absence of other locusts and occurs over a timescale ranging from hours, for some behaviours to change, to generations, for full morphological transformation. The neuro-hormonal mechanisms that drive and accompany phase change in either direction remain unknown. We have used high-performance liquid chromatography (HPLC) to compare amounts of 13 different potential neurotransmitters and/or neuromodulators in the central nervous systems of final instar locust nymphs undergoing phase transition and between long-term solitarious and gregarious adults. Long-term gregarious and solitarious locust nymphs differed in 11 of the 13 substances analysed: eight increased in both the brain and thoracic nerve cord (including glutamate, GABA, dopamine and serotonin), whereas three decreased (acetylcholine, tyramine and citrulline). Adult locusts of both extreme phases were similarly different. Isolating larval gregarious locusts led to rapid changes in seven chemicals equal to or even exceeding the differences seen between long-term solitarious and gregarious animals. Crowding larval solitarious locusts led to rapid changes in six chemicals towards gregarious values within the first 4 h (by which time gregarious behaviours are already being expressed), before returning to nearer long-term solitarious values 24 h later. Serotonin in the thoracic ganglia, however, did not follow this trend, but showed a ninefold increase after a 4 h period of crowding. After crowding solitarious nymphs for a whole larval stadium, the amounts of all chemicals, except octopamine, were similar to those of long-term gregarious locusts. Our data show that changes in levels of neuroactive substances are widespread in the central nervous system and reflect the time course of behavioural and physiological phase change.

Local circuits for the control of leg movements in an insect

To produce behaviour that is adaptive, local circuits in the CNS must transform mechanosensory signals from receptors on the body into changes in movement. Substantial insights into the mechanisms underlying these transformations can be obtained by analysing the local circuits of animals from which intracellular recordings can be made from identified neurones during behavior, thus allowing the complete pathways between inputs and outputs to be followed. In the locust (Schistocerca gregaria) these circuits contain both non-spiking and spiking local neurones so that it is possible to elucidate two basic issues of neuronal integration: (1) the operation of the reflex circuitry that must adjust locomotion, and (2) the integrative role of local circuits that use graded interactions in complex neuropil, perhaps even involving compartmentalized neurones.

Input and output synapses on identified motor neurones of a locust revealed by the intracellular injection of horseradish peroxidase

SummaryPhysiologically characterised motor neurones in the thoracic ganglia of the locust were injected with horseradish peroxidase in order that the spatial relationship between their input and output synapses could be observed with the electron microscope. A modification in the development procedure for the peroxidase ensured that the internal fine structure of the stained neurones was not obscured by the diaminobenzidine reaction product. Input and output synapses may occur within 1 μm of each other on the neuropilar processes of the motor neurones. This supports physiological evidence that motor neurones may be involved in local circuit interactions within the thoracic ganglia.

Mechanosensory-induced behavioural gregarization in the desert locust Schistocerca gregaria

SUMMARY Desert locusts show an extreme form of phenotypic plasticity, changing between a cryptic solitarious phase and a swarming gregarious phase that differ in many aspects of behaviour, physiology and appearance. Solitarious locusts show rapid behavioural phase change in response to tactile stimulation directed to the hind femora. Repeatedly touching as little as one quarter of the anterior (outer) surface area of a hind femur produced full behavioural gregarization within 4 h. Solitarious locusts have approximately 30% more mechanosensory trichoid sensilla on the hind femora than do gregarious locusts but have similar or fewer numbers of sensilla elsewhere on the legs. Tactile stimulation of a hind femur in solitarious locusts that had been restrained so that they could not move their legs failed to induce any behavioural gregarization. Patterned electrical stimulation of metathoracic nerve 5, which innervates the hind leg, however, produced full gregarization in restrained locusts. Our data show for the first time that the gregarizing signal combines both exteroceptive and proprioceptive components, which travel in both nerves 5B1 and 5B2, and provides us with a powerful experimental method with which to elicit and study neuronal plasticity in this system. Acetic acid odour, a strong chemosensory stimulus that activates the same local processing pathways as exteroceptive stimuli, failed to elicit behavioural gregarization, suggesting an early segregation in the central nervous system of the mechanosensory signals that leads to gregarization.

Jumping performance of froghopper insects.

SUMMARY The kinematics of jumping in froghopper insects were analysed from high speed sequences of images captured at rates up to 8000 s-1. In a jump, the attitude of the body is set by the front and middle legs, and the propulsion is delivered by rapid and synchronous movements of the hind legs that are 1.5 times longer than the other legs, but are only about half the length of the body and represent just 2% of the body mass. The wings are not moved and the front and middle legs may be raised off the ground before take-off. The hind legs are first cocked by a slow levation of the trochantera about the coxae so that the femora are pressed against the ventral, indented wall of the thorax, with the femoro-tibial joints tucked between the middle legs and body. Only the tips of the hind tarsi are in contact with the ground. In this position, the hind legs stay motionless for 1-2 s. Both trochantera are then synchronously and rapidly depressed about the coxae at rotational velocities of 75 500 deg. s-1 and the tibiae extended, to launch a jump that in Philaenus reaches a height of 700 mm, or 115 body lengths. In the best jumps by Philaenus, take-off occurs within 0.875 ms of the start of movements of the hind legs at a peak velocity of 4.7 m s-1 and involves an acceleration of 5400 m s-2, equivalent to 550 times gravity. This jumping performance requires an energy output of 136 μJ, a power output of 155 mW and exerts a force of 66 mN.

A presynaptic gain control mechanism among sensory neurons of a locust leg proprioceptor

The chordotonal organ at the femorotibial joint of a locust hind leg monitors extension and flexion movements of the tibia. During evoked or imposed movements of this joint the central terminals of afferent neurons from the chordotonal organ receive depolarizing, inhibitory synaptic inputs. The afferent spikes are therefore superimposed on these depolarizing IPSPs, which are generated indirectly by other afferents from the same organ that respond to the same movement. Each afferent spikes preferentially to particular features of a joint movement, and its synaptic input is typically greatest at the joint position or during the movement that generates its best response. Afferents that respond to only one direction of movement receive synaptic inputs either during movements in both directions, or only during movements in their preferred direction. Phasic velocity- sensitive afferents receive either phasic inputs during movements, or tonic inputs at new sustained joint positions, or both. The spikes of tonic position-sensitive afferents are superimposed on synaptic inputs that are dependent on joint position. The synaptic inputs sum but do not themselves evoke antidromic spikes in the afferent terminals. They reduce the amplitude of orthodromic afferent spikes by 12–28%, and this is accompanied by a reduction of up to 50% in the amplitude of monosynaptic EPSPs evoked by an afferent in postsynaptic leg motor neurons. These interactions suggest that a local gain control mechanism operates between the afferents of this proprioceptor. Thus, the effectiveness of the output synapses of an individual afferent is regulated by the network action of other chordotonal afferents that respond to the same movement.

Resilin and chitinous cuticle form a composite structure for energy storage in jumping by froghopper insects

BackgroundMany insects jump by storing and releasing energy in elastic structures within their bodies. This allows them to release large amounts of energy in a very short time to jump at very high speeds. The fastest of the insect jumpers, the froghopper, uses a catapult-like elastic mechanism to achieve their jumping prowess in which energy, generated by the slow contraction of muscles, is released suddenly to power rapid and synchronous movements of the hind legs. How is this energy stored?ResultsThe hind coxae of the froghopper are linked to the hinges of the ipsilateral hind wings by pleural arches, complex bow-shaped internal skeletal structures. They are built of chitinous cuticle and the rubber-like protein, resilin, which fluoresces bright blue when illuminated with ultra-violet light. The ventral and posterior end of this fluorescent region forms the thoracic part of the pivot with a hind coxa. No other structures in the thorax or hind legs show this blue fluorescence and it is not found in larvae which do not jump. Stimulating one trochanteral depressor muscle in a pattern that simulates its normal action, results in a distortion and forward movement of the posterior part of a pleural arch by 40 μm, but in natural jumping, the movement is at least 100 μm.ConclusionCalculations showed that the resilin itself could only store 1% to 2% of the energy required for jumping. The stiffer cuticular parts of the pleural arches could, however, easily meet all the energy storage needs. The composite structure therefore, combines the stiffness of the chitinous cuticle with the elasticity of resilin. Muscle contractions bend the chitinous cuticle with little deformation and therefore, store the energy needed for jumping, while the resilin rapidly returns its stored energy and thus restores the body to its original shape after a jump and allows repeated jumping.

Jumping and kicking in bush crickets

SUMMARY Bush crickets have long, thin hind legs but jump and kick rapidly. The mechanisms underlying these fast movements were analysed by correlating the activity of femoral muscles in a hind leg with the movements of the legs and body captured in high-speed images. A female Pholidoptera griseoaptera weighing 600 mg can jump a horizontal distance of 300 mm from a takeoff angle of 34° and at a velocity of 2.1 m s-1, gaining 1350μJ of kinetic energy. The body is accelerated at up to 114 m s-2, and the tibiae of the hind legs extend fully within 30 ms at maximal rotational velocities of 13 500 deg. s-1. Such performance requires a minimal power output of 40 mW. Ruddering movements of the hind legs may contribute to the stability of the body once the insect is airborne. During kicking, a hind tibia is extended completely within 10 ms with rotational velocities three times higher at 41 800 deg. s-1. Before a kick, high-speed images show no distortions of the hind femoro-tibial joints or of the small semi-lunar groove in the distal femur. Both kicks and jumps can be generated without full flexion of the hind tibiae. Some kicks involve a brief, 40-90 ms, period of co-contraction between the extensor and flexor tibiae muscles, but others can be generated by contraction of the extensor without a preceding co-contraction with the flexor. In the latter kicks, the initial flexion of the tibia is generated by a burst of flexor spikes, which then stop before spikes occur in the fast extensor tibiae (FETi) motor neuron. The rapid extension of the tibia can follow directly upon these spikes or can be delayed by as long as 40 ms. The velocity of tibial movement is positively correlated with the number of FETi spikes. The hind legs are 1.5 times longer than the body and more than four times longer than the front legs. The mechanical advantage of the hind leg flexor muscle over the extensor is greater at flexed joint angles and is enhanced by a pad of tissue on its tendon that slides over a protuberance in the ventral wall of the distal femur. The balance of forces in the extensor and flexor muscles, coupled with their changing lever ratio at different joint positions, appears to determine the point of tibial release and to enable rapid movements without an obligatory co-contraction of the two muscles.