Motor activity and trajectory control during escape jumping in the locust Locusta migratoria

Abstract

We investigated the escape jumps that locusts produce in response to approaching objects. Hindleg muscular activity during an escape jump is similar to that during a defensive kick. Locusts can direct their escape jumps up to 50 degrees either side of the direction of their long axis at the time of hindleg flexion, allowing them to consistently jump away from the side towards which an object is approaching. Variation in jump trajectory is achieved by rolling and yawing movements of the body that are controlled by the fore- and mesothoracic legs. During hindleg flexion, a locust flexes the foreleg ipsilateral to its eventual jump trajectory and then extends the contralateral foreleg. These foreleg movements continue throughout co-contraction of the hindleg tibial muscles, pivoting the locust's long axis towards its eventual jump trajectory. However, there are no bilateral differences in the motor programs of the left and right hindlegs that correlate with jump trajectory. Foreleg movements enable a locust to control its jump trajectory independent of the hindleg motor program, allowing a decision on jump trajectory to be made after the hindlegs have been cocked in preparation for a jump.

High-speed video analysis of jumping in Locusta migratoria

“…The fact 315 locusts that with only one hindleg left or intact locust but one hindleg slipping during takeoff 316 can still have the possibility to jump successfully shows that asynchronous extension of the 317 two hindlegs or lack of one hindleg do not affect the trajectory of natural jumps. This is 318 consistent with previous observations that asynchronous leg extension or even the complete 319 absence of one hindleg can jump successfully also but cause little difficulty in jumping 320 (Santer et al, 2005;Bennet-Clark, 1975). Because elevation is determined by the initial 321 position of hindleg, and remains constant throughout the jump, asynchrony of the forces 322 between the left and right hindlegs does not lead to a catastrophic failure of elevation control.…”

“…During cocking and co-contraction phase, locusts 65 store energy in extensor and semi-lunar process which is similar as a torsion spring by the 66 coordinated movement of femur and tibiae (Brown, 1967). Through triggering phase, by rapid 67 release of stored energy, locusts realize jumping while middle legs and fore legs play an 68 important role mainly in adjusting take-off angle and elevation direction (Heitler &Burrows, 69 1977;Santer et al, 2005).…”

“…Takeoff velocity is decided by the amount of energy stored inside the 76 extensor muscles and semi-lunar process of knee joints of hindlegs during the contraction 77 phase (Bennet-Clark, 1975). Azimuth angle is controlled by changing the direction of the body 78 with the front and middle legs (Santer et al, 2005). Forelegs movements help locusts control 79 their azimuth angle independent of the hindlegs position which contributes to the takeoff 80 velocity and elevation angle, which makes it possible to change the azimuth angle after the 81 hindlegs are cocked and makes the movement of locusts more agile.…”

Preparing for escape: anti-predator posture and fast-start performance in gobies

“…On the other hand, postures that prepare for escape have been observed in certain legged animals: locusts, for example, prepare for escape by positioning their legs so that elastic energy is stored for a subsequent jump. Hindlegs are flexed into a 'cocked' position, which may or may not be followed by an escape jump (Bennet-Clark, 1975;Heitler and Burrows, 1977;Santer et al, 2005). Escape 'preparation' has also been observed in Drosophila flies which, when stimulated, execute postural adjustments so that leg extension will push them away from the stimulus, whether escape will eventually be performed or not (Card and Dickinson, 2008).…”

Relationship between the Phases of Sensory and Motor Activity during a Looming-Evoked Multistage Escape Behavior

“…The jump is triggered when the activity of the flexors ceases, allowing the extensors to develop their movement and the stored energy to be released. Moreover, the front legs are known to influence the direction of the jump (Santer et al, 2005).…”