Neural correlates of flight loss in a Mexican grasshopper, Barytettix psolus. I. Motor and sensory cells

Abstract

The nervous systems of locusts (Schistocerca gregaria) and flightless grasshoppers (Barytet tix psolus) are compared to evaluate modifications to neurons which are associated with flight loss. Locusts are well known for their powerful flight capability. Barytettix never fly. They lack hindwings, have immobile vestiges of forewings, and are devoid of skeletal specializations for wing movement. Their pterothoracic musculature is similar to that of locusts, except for the absence of those muscles that, in locusts, have the primary function of moving the wings. Individually identified leg motorneurons, the extensors of the tibia, were compared between locusts and Barytettix and were found to have very similar morphologies. Nerve roots which correspond to those supplying wing muscles of locusts were stained by cobalt backfilling in Barytettix to test for presence of counterparts to wing muscle motorneurons. Cobalt backfills of metathoracic nerve 1 reveal the presence in Barytet tix of neurons corresponding to locust dorsal longitudinal motorneurons--neurons which persist in adult Barytettix in the complete absence of peripheral targets. These cells occupy characteristic positions within the CNS but their soma sizes are greatly reduced by comparison to their locust counterparts. Locust metathoracic ganglia bear large flight motorneurons on their ventral anterolateral margin. Viewed in toluidine blue-stained wholemounts, Barytettix ganglia show considerably smaller neuron somata in the corresponding region. In locusts, comparisons of the fast extensor tibiae (FETi) motorneuron soma profile areas with those of the largest anterior cell showed no significant difference between the two, while in Barytettix, the largest anterior cell is 51% smaller than the FETi. A counterpart to the locust wing hinge stretch receptor (SR) was revealed by backfilling metathoracic nerve 1 in Barytettix. Despite its lack of function as a wing movement detector, the central projection of Barytettix SR differs from its locust counterpart only in reduced spread of specific central branches.