Strepsiptera are entomophagous parasitoids which exhibit extreme sexual dimorphism. The adult male is free living, whereas the adult female is neotenic and permanently endoparasitic in the host, except in the family Mengenillidae where the female late larval instar (like the male) emerges from the host to pupate externally. The only other free-living stage of the order is the 1st instar larva which emerges viviparously from the neotenic female and seeks a new host to parasitise.
Strepsiptera are parasitic in Thysanura, Blattodea, Mantodea, Orthoptera, Hemiptera, Diptera and Hymenoptera, but their hosts are principally among Hymenoptera and Auchenorrhyncha (Hemiptera). Parasitism by Strepsiptera is termed stylopisation.
The reduced mesothoracic wings, together with the expanded hind wings in Strepsiptera, give them a superficial resemblance to Rhipiphoridae (Coleoptera); they have therefore been included by several workers as a family in the Coleoptera (Arnett 1968; Crowson 1960, 1981; Ross et al. 1982). Strepsiptera differ from the Coleoptera in thoracic structure, in the structure and position of their fore and hind wings, in the absence of the gula and trochanter, and in the formation of the puparium (Kathirithamby 1989). The use of the hind wings for flight, and of the modified metathorax to accommodate the flight muscles, suggest that Strepsiptera have a sister group relationship with the Coleoptera.
Kinzelbach (1971a, b) and Kathirithamby (1989) give detailed accounts of the morphology, internal anatomy, biology and systematics of the order.
Endopterygote Neoptera with reduced mandibulate mouthparts, extreme development of metathorax, reduced prothorax, and without differentiated trochanters in fore and mid legs. Adult males free living, with functional hind wings and small haltere-like fore wings. Females larviform, viviparous; usually parasitic, in puparium, and then with secondary progoneate genital apertures. Heteromorphosis during larval growth, 1st instar larvae free living and active, later instars parasitic.
Of the 484 species described from various parts of the world, only 39 are Australian (Perkins 1905; Lea 1910; Ogloblin 1923, 1926; Kogan and Oliveira 1964; Kinzelbach 1971a; Kifune and Hirashima 1983, 1987, 1989; Drew and Allwood 1985; Kathirithamby 1989, in press a, b). Many more species await description (as many as 160 of them from Australia).