Strepsiptera systematics: past, present, and future

The twisted-wing parasites (Strepsiptera Kirby) are among the most obscure and enigmatic orders in the Insecta. Strepsipterans are endoparasites of 7 insect orders, and have developed complex specialized life histories accordingly. The number of described strepsipteran species is relatively small for an insect order (fewer than 650, with 43 currently known only from fossils), they have very unusual ecological, morphological, and genetic characteristics, and little is known about their host use. Equally convoluted is the history of their systematics, which has featured contention about their phylogenetic position in the insect radiation - a dispute termed "the Strepsiptera Problem."Within the order, relationships between lineages have been largely resolved with the advent of molecular data. However, limited availability of genetic data and specimen inaccessibility have restricted progress in the field. The fact that many species are described only from singular male specimens poses the biggest challenge to the resolution of strepsipteran phylogenetic relationships. In this review, we give a comprehensive background on the biology and taxonomy of this order, discuss the history of strepsipteran systematics, detail the current challenges of its study, and propose future action for its resolution.