STRUCTURE OF THE FOREWING STRETCH-RECEPTOR AXON IN IMMATURE AND MATURE ADULT LOCUSTS

During the first 2 weeks following imaginal ecdysis, the wingbeat freq uency of Locusta migratoria doubles, and the activity of the forewing stretch receptor (fSR), in response to wing elevation, increases. We e xamined the three-dimensional structure of the centrally projecting ax on of the fSR during adult maturation to determine if there are change s in the branching geometry. We found that changes occur in the mesoth oracic projection (IISR Meso). Here, there was a significant increase in the volume of the projection from 2.3 x 10(4) +/- 0.2 x 10(4) mu m( 3) in immature locusts to 6.0 x 10(4) +/- 1.2 x 10(4) mu m(3) in matur e locusts. There were also significant increases in the total length, the number of branch points, the number of axonal swellings, and the d iameters of first- and second-order branches of the projection. No sig nificant 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 tha t during adult maturation, growth of the fSR axon is heteromorphic bet ween different ganglionic projections and that there is a potential in crease in the connectivity of IISR Meso to other flight neurons in the mesothoracic ganglion. We suggest that this may be a mechanism for ma intaining the efficacy of afferent input to flight interneurons that a re also growing during maturation. (C) 1996 Wiley-Liss, Inc.