During the first 2 weeks following imaginal ecdysis, the wingbeat frequency of Locusta migratoria doubles, and the activity of the forewing stretch receptor (fSR), in response to wing elevation, increases. We examined the three-dimensional structure of the centrally projecting axon of the fSR during adult maturation to determine if there are changes in the branching geometry.
We found that changes occur in the mesothoracic projection (IISR Meso). Here, there was a significant increase in the volume of the projection from 2.3 x lo4 2 0.2 x lo4 pm3 in immature locusts to 6.0 x lo4 ? 1.2 x lo4 p,m3 in mature locusts. There were also significant increases in the total length, the number of branch points, the number of axonal swellings, and the diameters of first-and second-order branches of the projection. No significant changes were observed in the prothoracic projection (IISR Pro), and the only significant change observed in IISR Meta was negative allometric growth relative to IISR Meso.
These results demonstrate that during adult maturation, growth of the fSR axon is heteromorphic between different ganglionic projections and that there is a potential increase in the connectivity of IISR Meso to other flight neurons in the mesothoracic ganglion. We suggest that this may be a mechanism for maintaining the efficacy of afferent input to flight interneurons that are also growing during maturation.