The Characteristics of the Insect Order Strepsiptera

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Strepsiptera: a strange word for a strange group of animals. The title identifies them as a kind of insect so they must have six jointed legs. What does the name mean? First, it defines the taxonomic status of the animals it names. It is an order in the Class Insecta which is a class in the Phylum Arthropoda. All arthropods have an exoskeleton with jointed legs. Insects are the class of arthropods with six jointed legs, which distinguishes them from the multi-legged millipedes and centipedes and from the eight-legged spiders and mites. The Class Insecta is large, diverse, prolific and hugely successful. The common orders include the beetles (Coleoptera or 'hard-wings'), butterflies and moths (Lepidoptera or 'scaly-wings'), ants, bees, wasps (Hymenoptera or 'clear-wing') and of course, the ever-present flies (Diptera, meaning 'two wings'). These four orders represent the vast majority of insects on Earth, but there are another twenty-seven known and described orders of insects, and the Strepsiptera is one.  The reader probably noticed that the four big Insect orders all had names ending in -ptera, the Greek word for wings. As in those orders, the first part of this order's name,  "strep", means something about their wings, in this case 'twisted'. Strepsiptera are parasites that spend most of their lives living in insect hosts such as bees and ants. About 600 species have been described. 

Strepisterans have full life cycles that involve a metamorphosis in a pupal case from larva to mature adult. The female is worm-like, with no legs or wings.  When she makes her puparium, the head end sticks out of the host and exudes a pheromone to attract the male.  When he finds her, he uses his wasp-like style to penetrate the brood sac covering that lies between her head and thorax. This is called hypodermic insemination. The eggs hatch into larvae within the brood sac. One female can produce hundreds of larvae, all with tiny legs that they use to crawl out into the wider world where they seek new host insects to infect. The larvae can be carried by the female host to her eggs and larve or they can crawl until they find the nymph of the host species. In either case, they find their way inside between the plates of the host's protective exoskeleton. Then they feed on the hosts' fluids and tissues until they metamorphose, either into worm-like females or free-living, flying males.

Only males ever live free-flying lives; the females remain safe and wingless within their hosts. The wasp-like males have four wings, but the forepair are reduced in size and similar to the halteres of flies. In flies, the halteres, which act like gyroscopes, form from the back wings. If the Strepsiptera use them for a similar purpose, then this is an example of analogous structures that evolved separately (convergent evolution). In the Strepsiptera, the back pair of wings are fully developed, but slightly twisted so that they rest at a different angle to the forewings. It was this characteristic that led to their name. The males have no functional mouthparts, living for only a few days and having only one purpose during this time. They exist to track down females by their pheromones and fertilise them before dying. Another distinguishing characteristic seen in the males which can be used to identify them as Strepsiptera are their flabellate or fan-shaped antennae. They are covered with sensoria that probably are used to pick up the most elusive pheromone trails.

The host species seem relatively unaffected by these strange little parasites.  Although the larvae feed off the host, its life span does not seem to be shortened by this. This means that strepsipterans probably will not be generally useful as biological controls. Using Strepsiptera to control the bugs that are pests of coconuts has been tried, with limited success. Other than this limited potential for biological control of insect pests, Strepsiptera have no economic or other value to human beings. For this reason and their secretive habits, little is known about the majority of species.

Sources:  Britton etal 1979  Insects of Australia chapter 31 Strepsiptera by E Reik pp622-635  CSIRO Canberra Australia


http://www.ento.csiro.au/education/insects/strepsiptera.html     http://tolweb.org/Strepsiptera 

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