Strepsiptera - Appearance and Biology

Appearance and Biology

Male Strepsiptera have wings, legs, eyes, and antennae, and superficially look like flies, though their mouthparts cannot be used for feeding. Many of their mouthparts are modified into sensory structures. 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 and legs. 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 thus 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 (which lack a trochanter, the leg segment that forms the articulation between the basal coxa and the femur), and actively search out new hosts. Their hosts include members belonging to 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, which is unique to these animals. The female has a brood canal that communicates with the outside world, and the larvae escape through this. The larvae are very active, as 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), and once they 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 there is separation of the older cuticle but no discarding ("apolysis without ecdysis"), leading to multiple layers being formed around the larvae. Male larvae produce pupae 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 Phacopida. Instead of a compound eye consisting of hundreds to thousands of ommatidia, each with a single lens and capable of producing a picture element (pixel), 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 may alter the behaviour of their hosts. Myrmecolacids may cause their ant hosts to climb up the tips of grass leaves, possibly to increase the spread of female pheromones to increase the chances of being located by males.