Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Hexapoda
Class Insecta
Order Strepsiptera

Common names: stylops, strepsipterans

Overview

Strepsiptera are a small order of highly specialised parasites of other insects that are rarely encountered. They are characterised by a high degree of sexual dimorphism. Males are free-living their eyes large with well-defined facets, antennae branched, forewings reduced to knobs resembling the halteres of flies (Diptera), and hindwings large and membranous with reduced venation; their mouthparts are reduced and non-functional and their wingspan ranges from 1�8 mm. Apparently strepsipterans are short-lived, with some species perhaps surviving for only a few hours. Females are parasitic, and most do not leave their insect hosts. They lack wings, legs, eyes and distinctive genitalia, and can only be seen as a protruding outgrowth on their host. The exception is female Mengenillidae that are free-living with eyes, legs and antennae.

Distribution and diversity

Strepsiptera are widely distributed in a range of habitats but are extremely difficult to find. Many species are described from the free-living males that have come into traps; to collect the female requires locating and collecting a parasitised host. Around 600 species of Strepsiptera have been described worldwide and many more await description as the fauna of many parts of the world is very poorly known. Forty-two species in 15 genera from six families are described from Australia. However, based on specimens in collections, the fauna is estimated at around 160 species.

Life cycle

Female Strepsiptera attract males by releasing pheromones from the site they occupy on their host (i.e. only their head protrudes from their host). Sperm passes through an opening in the female�s head and from there directly into the body cavity (haemocoel). The female then produces a large number of active first-instar larvae with legs (triungulin) that are released just outside the host. They fall to the ground or on to plants and crawl around in search of a suitable host. Immature homopteran hosts must be discovered directly by larvae, but for hymenopteran hosts the larvae may climb onto flowers to be carried by bees or wasps back to their nest. The strepsipteran larva enter the larval or nymphal insect host through joints or sutures. It then moults (hypermetamorphoses) into a less mobile, grub-like second instar larva with no distinct head, legs or antennae. The larva grows and continues to moult inside the host's body cavity. After pupating in the host, the winged male emerges and flies in search of a mate, whereas the adult female remains larva-like inside the host.

Feeding

Larvae and female strepsipterans are endo- (internal) parasitoids of a range of insects. They feed within the host's body cavity on nutrients from the blood and non-vital tissues. Male stylopids have reduced mouthparts and do not feed.

Ecology

Strepsipterans are rarely seen or found but males sometimes are attracted to light. They parasitise several groups of insects, with varying levels of host specificity at the family or species level; most are not confined to a single host species. Particular species are known to attack silverfish (Thysanura), cockroaches (Blattodea), preying mantids (Mantodea), grasshoppers (Orthoptera), bugs (Hemiptera), flies (Diptera) or wasps and bees (Hymenoptera); Hemiptera and Hymenoptera are the most common hosts. In most cases they do not reduce the lifespan of the host significantly but are known to induce sterility and change host behaviour. Death of the host may occur with the emergence of the male or first instar larvae.