‘Giant’ Fibres in Dragonfly Nymphs

Abstract

A PRELIMINARY study of transverse and horizontal sections through the ventral nerve cord of late instar nymphs of Anax imperator (Anisoptera) has revealed several notable features. Accurate counts of the number of fibres within the commissures have shown that these range continuously from 0 to 16 µ, as set out in the accompanying histogram (Fig. 1). The total number of fibres is approximately the same for the two commissures. Measurements of axon diameters show that some of the larger fibres (12–16 µ) are comparable with the ‘giant’ fibres of the cockroach¹ and locust². As in the nerves to the stellate ganglia of Sepia³, these fibres form part of a continuous size spectrum and do not appear to differ in nature from the smaller ones. The ‘giant’ fibres are almost certainly the same as fibres a, b and c described by Zawarzin⁴ in his classical studies of Aeschna nymphs using a methylene blue technique. The ‘giant’ fibres pass through all the abdominal ganglia and send off collaterals into the neuropile of each. Their course through the closely associated metathoracic and first abdominal ganglia to the other thoracic ganglia is more complicated, and has not been fully traced as yet.