Metathoracic neurons integrating intersegmental sensory information in the locust

Aimed limb movements in a hemimetabolous insect are intrinsically compensated for allometric wing growth by developmental mechanisms

For aimed limb movements to remain functional, they must be adapted to developmental changes in body morphology and sensory-motor systems. Insects use their limbs to groom the body surface or to dislodge external stimuli, but they face the particular problem of adapting these movements to step-like changes in body morphology during metamorphosis or moulting. Locusts are hemimetabolous insects in which the imaginal moult to adulthood results in a sudden and dramatic allometric growth of the wings relative to the body and the legs. We show that, despite this, hind limb scratches aimed at mechanosensory stimuli on the wings remain targeted to appropriate locations after moulting. In juveniles, the tips of the wings extend less than halfway along the abdomen, but in adults they extend well beyond the posterior end. Kinematic analyses were used to examine the scratching responses of juveniles (fifth instars) and adults to touch of anterior (wing base) and posterior (distal abdomen) targets that develop isometrically, and to wing tip targets that are anterior in juveniles but posterior in adults. Juveniles reach the (anterior) wing tip with the distal tibia of the hind leg using anterior rotation of the thoraco-coxal and coxo-trochanteral (‘hip’) joints and flexion of the femoro-tibial (‘knee’) joint. Adults, however, reach the corresponding (but now posterior) wing tip using posterior rotation of the hip and extension of the knee, reflecting a different underlying motor pattern. This change in kinematics occurs immediately after the adult moult without learning, indicating that the switch is developmentally programmed.

Highlighted Article: A developmentally programmed change in the scratching movements of locusts permits adult animals to aim their movements at new wing tip targets, without learning.

Exposure to static electric fields leads to changes in biogenic amine levels in the brains of Drosophila

Natural and anthropogenic static electric fields are commonly found in the environment and can have both beneficial and harmful effects on many animals. Here, we asked how the fruitfly responds to these fields and what the consequences of exposure are on the levels of biogenic amines in the brain. When given a choice in a Y-tube bioassay Drosophila avoided electric fields, and the greater the field strength the more likely Drosophila were to avoid it. By comparing wild-type flies, flies with wings surgically removed and vestigial winged flies we found that the presence of intact wings was necessary to produce avoidance behaviour. We also show that Coulomb forces produced by electric fields physically lift excised wings, with the smaller wings of males being raised by lower field strengths than larger female wings. An analysis of neurochemical changes in the brains showed that a suite of changes in biogenic amine levels occurs following chronic exposure. Taken together we conclude that physical movements of the wings are used by Drosophila in generating avoidance behaviour and are accompanied by changes in the levels of amines in the brain, which in turn impact on behaviour.

Quantification of gait parameters in freely walking wild type and sensory deprived Drosophila melanogaster

Coordinated walking in vertebrates and multi-legged invertebrates [corrected] such as Drosophila melanogaster requires a complex neural network coupled to sensory feedback. An understanding of this network will benefit from systems such as Drosophila that have the ability to genetically manipulate neural activities. However, the fly's small size makes it challenging to analyze walking in this system. In order to overcome this limitation, we developed an optical method coupled with high-speed imaging that allows the tracking and quantification of gait parameters in freely walking flies with high temporal and spatial resolution. Using this method, we present a comprehensive description of many locomotion parameters, such as gait, tarsal positioning, and intersegmental and left-right coordination for wild type fruit flies. Surprisingly, we find that inactivation of sensory neurons in the fly's legs, to block proprioceptive feedback, led to deficient step precision, but interleg coordination and the ability to execute a tripod gait were unaffected.DOI:http://dx.doi.org/10.7554/eLife.00231.001.

Quantification of gait parameters in freely walking wild type and sensory deprived Drosophila melanogaster

Coordinated walking in vertebrates and multi-legged invertebrates such as Drosophila melanogaster requires a complex neural network coupled to sensory feedback. An understanding of this network will benefit from systems such as Drosophila that have the ability to genetically manipulate neural activities. However, the fly's small size makes it challenging to analyze walking in this system. In order to overcome this limitation, we developed an optical method coupled with high-speed imaging that allows the tracking and quantification of gait parameters in freely walking flies with high temporal and spatial resolution. Using this method, we present a comprehensive description of many locomotion parameters, such as gait, tarsal positioning, and intersegmental and left-right coordination for wild type fruit flies. Surprisingly, we find that inactivation of sensory neurons in the fly's legs, to block proprioceptive feedback, led to deficient step precision, but interleg coordination and the ability to execute a tripod gait were unaffected.

DOI:http://dx.doi.org/10.7554/eLife.00231.001

Most animals need to be able to move to survive. Animals without limbs, such as snakes, move by generating by wave-like contractions along their bodies, whereas limbed animals, such as vertebrates and arthropods, walk by coordinating the movements of multi-jointed arms and legs. Locomotion in limbed animals involves bending each joint within each arm or leg in a coordinated manner, while also ensuring that the movements of all the limbs are coordinated with each other. In bipeds such as humans, for example, it is critical that one leg is in the stance phase when the other leg is in the swing phase. The rules that govern the coordination of limbs also depend on the gait, so the rules for walking are not the same as the rules for running.

The nervous systems of bipeds and other animals that walk solve these problems by using complex neural circuits that coordinate the firing of the relevant motor neurons. Two general mechanisms are used to coordinate the firing of motor neurons. In one mechanism, local interneurons within the central nervous system coordinate motor neuron activities: in vertebrates these interneurons are found in the spinal cord. A second mechanism, termed proprioception, relies on sensory neurons that report the load and joint angles from the arms and legs back to the central nervous system, and thereby influence the firing of the motor neurons. Remarkably, both of these mechanisms, and also the types of neurons that comprise motor neuron circuits, are conserved from arthropods to vertebrates.

Mendes et al. describe a new approach that can be used to analyze how the fruit fly, D. melanogaster, walks on surfaces. They use a combination of an optical touch sensor and high-speed video imaging to follow the body of the fly as it walks, and also to record when and where it places each of its six feet on the surface as it moves. Then, using a software package called FlyWalker, they are able to extract a large of number of parameters that can be used to describe locomotion in adult fruit flies with high temporal and spatial resolution. Many of these parameters have never been measured or studied before.

Mendes et al. show that fruit flies do not display the abrupt transitions in gait that are typically observed in vertebrates. However, they do modify their neural circuits depending on their speed: indeed it appears that flies use subtly different neural circuitry for walking at slow, medium and fast speeds. Moreover, when genetic methods are used to block sensory feedback, the fly is still able to walk, albeit with reduced coordination and precision. Further, the data suggest that proprioception is less important when flies walk faster compared to when they walk more slowly. The next step in this research will be to combine this new method for analyzing locomotion in flies with the wide range of genetic tools that are available for the study of Drosophila: this will allow researchers to explore in greater detail the components of the motor neuron circuitry and their role in coordinated walking.

DOI:http://dx.doi.org/10.7554/eLife.00231.002

Wing hair sensilla underlying aimed hindleg scratching of the locust

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

Load compensation in targeted limb movements of an insect

The task of a multi-jointed limb making an aimed movement towards a target requires that the movement is regulated against external perturbations such as changing load. In particular, loading one part of a limb leads to altered static forces on all proximal segments, and to additional dynamic joint interaction forces when the limb moves. We have addressed the question of load compensation in an insect preparation in which a locust makes aimed scratching movements with a hind leg in response to tactile stimulation of a wing. We show that loading the femur or tibia with the equivalent of 8.5 times the mass of the tibia (corresponding to an increase of up to 11.6 times the rotational moment of inertia at the femur-tibia joint) does not impair the animal's ability to make well-coordinated, aimed movements of that leg towards different targets. The kinematics of the movements are the same, and animals aim the same part of their distal tibia at the target, regardless of loading. The movements are carried out with equal accuracy and at the same initial velocity under all load conditions. Because loading of the leg does not change the behavioural performance, there is no indication of a change in aiming strategy. This implies high leg joint stiffness and/or the existence of high gain proprioceptive control loops. We have previously shown that in the unloaded condition, movements elicited by stimuli to different places on the wing are driven by a single underlying movement pattern that shifts depending on stimulus location along the wing surface. Our present data show that leg proprioceptive inputs are also integrated into the leg motor networks, rendering hind limb targeting robust against large changes in moment of inertia.

Graded limb targeting in an insect is caused by the shift of a single movement pattern

Grooming responses are movements of a multi-jointed limb that are targeted toward a stimulus site on the body. To be successful, they require a continuous transformation of stimulus location into a corresponding motor pattern or selection and blending of a subset of cardinal motor patterns. Tactile stimulation of one forewing of a locust elicits characteristic grooming movements of the ipsilateral hind leg. An initial targeted trajectory that moves the tarsus toward the site of stimulation is followed by a cyclic trajectory in the region of the stimulus. We have analyzed both components of this behavior to quantify the relative effects of somatotopic stimulus position and leg start posture on three parameters: initial movement direction, accuracy, and grooming distribution. Accuracy and grooming distribution were significantly affected by the stimulus location but were not influenced by the initial leg posture. Both cues systematically shifted the initial movement direction from the onset of the response. The subsequent cyclic component of grooming movements forms a behavioral continuum with no clustering in joint angle space. We therefore conclude that forewing grooming in locusts is generated by a single movement pattern that is continuously shifted by a sensory cue signaling position on the forewing surface. Both vertebrates and invertebrates can switch between distinct movement forms to groom different parts of their bodies. Our data provide the first evidence that invertebrates, like vertebrates, also have graded control of limb targeting within the somatosensory receptive field of a single form of motor response.