Strepsipteran females spend their whole life inside a wasp. The males are rather more exciting, particularly in terms of convergence…

The Strepsiptera are a remarkable group of insects, whose systematic position has been a source of considerable debate. Although often allied to the flies (Diptera), this is now considered unlikely and a position close to the beetles (Coleoptera) seems more plausible, while still somewhat controversial. Colloquially known as twisted-winged flies, the strepsipterans have attracted attention for several reasons, such as their extraordinary life cycle with the female being effectively permanently endoparasitic (with implications for immune avoidance), living typically in wasps. The male performs a nuptial flight, during which it fertilises the only part of the female to “see” the outside world, the opening of her reproductive system. Aside from this, strepsipterans are also important in terms of evolutionary convergence, most notably with respect to halteres and specialised compound eyes.

Halteres in strepsipterans and other insects

In strepsipteran males, the forewings are modified into halteres, club-shaped gyroscopic balancing organs that serve to stabilise the insect in flight.These halteres are strikingly similar in form and function to those found in the dipterans, but in the latter group it is the hindwings that have been adapted. Thus, “the differentiation of wings into halteres occurred independently at least two times in evolution” (Pix et al. 1993, Naturwissenschaften, vol. 80, p. 374). However, this is not quite the end of the story. A remarkably analogous structure has evolved in the African lymexylid beetle Atractocerus brevicornis, which is an unusually fast flyer. Here, the elytra, modified forewings that are typically hardened and protect the membranous hindwings, have been adapted into haltere-like organs. They oscillate in flight and experiments showed that stable flight became impossible when they were removed. Furthermore, haltere-like structures may be present in the males of some coccoideans, an unusual superfamily of hemipterans known as scale insects. Most species parasitize plants and adult females usually become attached to the plant and secrete a waxy coating, which resembles the scales of fish or reptiles, for protection.

Compound eyes in strepsipterans and trilobites

Compound eyes, which consist of numerous ommatidia, small units with a lens focussing light onto a group of photoreceptors, are widespread amongst the insects and indeed other arthropods. However, the compound eyes of strepsipterans are strikingly similar to the schizochroal eyes of the extinct trilobites, most notably in terms of the remarkably large lenses and widely spaced ommatidia.During its nuptial flight, the male strepsipteran requires sharp eyes, as it must locate the reproductive organs of the female protruding from the host. Thus, its compound eye is transformed relative to other insects, to bear a series of enormous lenses embedded in a hard cuticle with typically rather prominent spines. Unlike the usual compound eye of insects, where typically each lens serves as only a point source, in the strepsipterans each lens can provide a separate image. How these are integrated by the brain is not known, but it is possible that this type of eye acts more as a general light collector rather than providing high acuity. Although schizochroal trilobite eyes were remarkably similar in structure, they differed in being composed of calcite. As an aside, another intriguing convergence with the calcitic compound eye of trilobites is found in the brittle star Ophiocoma wendtii. This echinoderm possesses remarkably equivalent calcitic structures in the tips of its arms that are derived from the mesodermal skeleton (stereom) and evidently form an array of microlenses with exceptional optical performance. And recall that, just as it is somewhat enigmatic what the hunting strepsipteran male sees, it is even more so for the brittle star, given that unlike the insect they lack a brain.