AbstractInsect spermatozoa are characterized by having a set of accessory tubules that surrounds the microtubular doublets of the axoneme and that are formed from the B‐subtubules of the doublets. In trichopteran species, the accessory tubules have an unusually large diameter. Those of one species,Odontocerum albicorne, were seen to have a number of protofilaments that is 19 in the main part of the axoneme, but gradually decreasing to 18, 17, and 16 near the distal tip. The accessory tubule of the trichopteran axoneme has an asymmetrical shape and a skewed orientation, which makes it easy to distinguish a tubule that is viewed from its plus‐end from one viewed from the minus‐end. The shape of a cross‐sectioned protofilament in the trichopteran accessory tubules differs from that of microtubules in general, including accessory tubules of other insects, by being polygonal with the most acute angle pointing centripetally. © 1993 Wiley