Twisted-winged Insects (DLiA summer interns list)

Twisted-winged Insects

Strepsiptera

Summary ⁵

The Strepsiptera are an endopterygote order of insects with nine extant families that include about 600 described species. They are endoparasites in other insects, such as bees, wasps, leafhoppers, silverfish, and cockroaches. Females of most species never emerge from the host after entering its body, finally dying inside it. The early-stage larvae do emerge because they must find an unoccupied living host, and the short-lived males must emerge to seek a receptive female in her host.

Appearance and biology ⁵

Males of the Strepsiptera have wings, legs, eyes, and antennae, though their mouthparts cannot be used for feeding. Many have mouthparts modified into sensory structures. To the uninitiated the males superficially look like flies. Adult males are very short-lived, usually surviving less than five hours, and do not feed. Females, in all families except the Mengenillidae, are not known to leave their hosts and are neotenic in form, lacking wings, legs, and eyes. Virgin females release a pheromone which the males use to locate them.

In the Stylopidia, the female's anterior region protrudes out of the host body and the male mates by rupturing the female's brood canal opening, which lies between the head and prothorax. Sperm passes through the opening in a process termed hypodermic insemination. The offspring consume their mother from the inside in a process known as hemocelous viviparity. Each female then produces many thousands of triungulin larvae that emerge from the brood opening on the head, which protrudes outside the host body. These larvae have legs and actively search out new hosts. Their legs are partly vestigial in that they lack a trochanter, the leg segment that forms the articulation between the basal coxa and the femur),

Strepsiptera of various species have been documented to attack hosts in many orders, including members of the orders Zygentoma, Orthoptera, Blattodea, Mantodea, Heteroptera, Hymenoptera, and Diptera. In the strepsipteran family Myrmecolacidae, the males parasitize ants, while the females parasitize Orthoptera.

Strepsiptera eggs hatch inside the female, and the planidium larvae can move around freely within the female's haemocoel; this behavior is unique to these insects. The larvae escape through the female's brood canal, which communicates with the outside world. The larvae are very active, because they only have a limited amount of time to find a host before they exhaust their food reserves. These first-instar larvae have stemmata (simple, single-lens eyes). When the larvae latch onto a host, they enter it by secreting enzymes that soften the cuticle, usually in the abdominal region of the host. Some species have been reported to enter the eggs of hosts. Larvae of Stichotrema dallatorreanurn Hofeneder from Papua New Guinea were found to enter their orthopteran host's tarsus (foot). Once inside the host, they undergo hypermetamorphosis and become a less-mobile, legless larval form. They induce the host to produce a bag-like structure inside which they feed and grow. This structure, made from host tissue, protects them from the immune defences of the host. Larvae go through four more
instars, and in each moult the older cuticle separates but is not discarded ("apolysis without ecdysis"), so multiple layers form around the larvae.
Male larvae pupate after the last moult, but females directly become neotenous adults. The colour and shape of the host's abdomen may be changed and the host usually becomes sterile. The parasites then undergo pupation to become adults. Adult males emerge from the host bodies, while females stay inside. Females may occupy up to 90% of the abdominal volume of their hosts.

Adult male Strepsiptera have eyes unlike those of any other insect, resembling the schizochroal eyes found in the trilobite group known as the Phacopina. Instead of a compound eye consisting of hundreds to thousands of ommatidia, that each produce a pixel of the entire image - the strepsipteran eyes consist of only a few dozen "eyelets" that each produce a complete image. These eyelets are separated by cuticle and/or setae, giving the cluster eye as a whole a blackberry-like appearance.

Very rarely, multiple females may live within a single stylopized host; multiple males within a single host are somewhat more common. Adult males are rarely observed, however, although specimens may be lured using cages containing virgin females. Nocturnal specimens can also be collected at light traps.

Strepsiptera of the family Myrmecolacidae can cause their ant hosts to linger on the tips of grass leaves, increasing the chance of being found by the parasite's males (in case of females) and putting them in a good position for male emergence (in case of males).

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