Developmental Plasticity in Aimed Scratching Movements of a Locust

Developmental changes may affect aimed limb movements by altering limb mass, muscle strength and musculo-skeletal resistance or changing the position of a target on the body relative to the responding limb. Scratching movements following stimulation of wings by 5th instars and adult Schistocerca gregaria were compared before and after the imaginal moult, during which the wings increase in length and rotate, presenting different wing surfaces to external contact. The arrangement of mechanosensory hairs on the wings of 5th instars and adults was determined and associated with the development of scratching behaviour. The neuronal projections of mechanosensory hairs on the hind wing projected intersegmentally to anterior ventral association centre (aVAC) of the metathoracic ganglion and continued intersegmentally to the mesothoracic aVAC. Scratching movements accommodated developmental changes that occurred between the 5th instar and adulthood of S. gregaria. There was no change in movement characteristics between 5th instars and adults, indicating that developmental changes in the muscle strength, limb mass and musculo-skeletal resistance were compensated. Movements were appropriately aimed to accommodate the increase in wing size, and were associated with stimulation of tactile hairs on different wing surfaces in 5th instars and adults, implying different synaptic connections onto post-synaptic interneurones. Therefore, changes in limb mass, muscle strength and musculo-skeletal resistance due to growth are be compensated for, possible mechanism are proprioceptive reflexes and dynamic joint stiffness. Changes in target position caused by body growth may be facilitated by the development of new mechanosensory hairs and different synaptic connections onto postsynaptic interneurones. Wing rotation is likely to be accommodated by the intersegmental projections of wing hair afferents, and the convergence of hind wing and fore wing sensory signals.