Systematics and bionomics of the species of the subgenus schadonophasma chaoborus chaoboridae diptera

Borkent A.

Study of structure, intraspecific variation, life cycles and bionomic features suggests the presence of 3 spp. of the subgenus Schadonophasma. Chaoborus trivittatus (Loew) and C. cooki Saether are restricted to North America; C. nyblaei (Zetterstedt) is known only from Fennoscandia. C. brunskilli Saether and C. knabi (Dyar) are conspecific with C. trivittatus. All stages of each species were studied except 1st, 2nd and 3rd larval instars of C. nyblaei, which are unknown. Eggs of C. cooki and C. nyblaei, unlike those of C. trivittatus, exhibit a thickened exochorion. C. cooki eggs are laid in a spherical mass with little gelatinous matrix; those of C. trivittatus are laid in a spiral arrangement in a disc of gelatinous matrix. First instar larvae of C. cooki possess a more pronounced egg burster than do those of C. trivittatus. A compound character index is provided for separation of 4th instar larvae of C. trivittatus and C. cooki and was used to test possibilities of conspecificity of these 2 spp. Male adults can be identified by the shape of the penis valves and a ratio of the length of 2 wing veins. The thickened exochorion of eggs of C. cooki and probably C. nyblaei, is an adaptation to overwintering as eggs in temporary ponds. C. cooki is univoltine C. trivittatus immatures occur mostly in permanent lentic habitats where this species overwinters as a 4th instar larva. This species may be uni- or multivoltine or have a 2 yr life cycle. Behavioral differences are evident between C. trivittatus and C. cooki. Only C. cooki larvae are capable of ingesting ostracods. Male adults of C. trivittatus form large swarms. Adult females are the main dispersing agent of C. trivittatus while limited evidence for C. cooki suggests that both sexes of this species disperse. Under laboratory conditions, C. trivittatus male adults live up to 7-8 days and female adults up to 12 days. A reconstructed phylogeny of some Chaoborus spp. is provided. The subgenera Schadonophasma and Chaoborus s.s. are both monophyletic and are sister groups. An upper Oligocene Chaoborus fossil indicates that the minimum age of the speciation events which gave rise to the ancestor of these 2 groups is 25 million years. A reconstructed phylogeny of Schadonophasma species indicates that C. cooki and C. nyblaei are more closely related to each other than either is to C. trivittatus. Zoogeographic considerations suggest 2 possible hypotheses for the origin of these species.