Strepsipterans (Order: Strepsiptera)
An illustration of a male strepsipteran showing the halteres.
Illustration by Halvard.
These little insects live as parasites on other insects. As you can imagine, that means they must be small - no more than 4mm long in most cases. They are obligate parasites - which means that that is the only way they can live. And this means that they must have evolved to do this alongside their hosts, over many millions of years.
The name Strepsiptera comes from the Greek strepsis (a twist) and pteron (wing) - hence the are sometimes called twisted wing parasites. It is the hind wings that are twisted, at least when in flight, but the front wings are interesting too. These are much smaller than the hind wings, and they act more like balancers than wings. Balancers are also called halteres. There is another order of insects that has halteres - the Diptera, or two-winged flies. But in the flies it is the hind wings that are the balancers, not the front ones.
There are around 600 known species of these creatures in the world, and they are classified into nine families. The males look like the one in the diagram, with strange looking antennae and the unusual shaped wings, and although they do not feed they spend their time flying around flowers, all the while looking around for females.
In most species of Strepsiptera the females stay inside the insect they are parasites of (their host). This means that the females don't need wings. Neither do they need antennae, nor even legs. However, in a few types of Strepsiptera, the females get their own back on the free flying males by being able to reproduce by parthenogenesis. This means that they don't need males at all, and just lay eggs and produce offspring asexually, without males.
Most Strepsiptera are parasites on planthoppers and various bees and wasps. They can produce sterility in their insect hosts, so they must have some effect on the naturally occurring numbers of their host species. They also have another peculiar effect - they seem to make female host insects more like males, and males more like females.
In most cases the life cycle is essentially as follows:
- Males fly about, sniffing around for females which are inside bees or other host insects.
- Then they mate with the female, who doesn't move from the host insect during this process.
- The eggs develop and hatch inside the female's body.
- The larvae hatch from the eggs and walk out onto the outside of the host using their six legs.
- Then they hail a taxi - actually, another host adult, which they grab onto and which eventually goes home to its nest.
- It's the nest these larvae are interested in, because there will be larvae of the host insect there. They disembark from the taxi, pay the fare (I made that bit up) and enter a larva of the host insect.
- Then, they moult into a legless maggot, and live inside it, feeding away, and mature into an adult shortly after the host eventually emerges from its pupa.
In some cases strepsipterous larvae are born with legs and then develop into forms, including the adult female form, that don't have legs (this is called hypermetamorphosis). After that they just develop by incomplete metamorphosis.
Classification of Strepsiptera
In terms of where to place this group among the insects, some entomologists regard them as somewhat related to beetle groups, on the basis of similarities among the larvae.
The largest families are the Stylopids (Stylopidae) which live on bees and wasps, and Halictophagidae and Elenchidae, which feed on planthoppers. The other families are the Mengenillidae, Mengeidae, Bohartillidae, Corioxenidae, Callipharixenidae and Myrmecolacidae.
Where to find them
As mentioned above, the males may be seen flying around flowers, but the easiest way to find these parasites is by collecting some of their host insects and trying to keep them alive while waiting for the parasites to emerge. Host insects likely to be parasitised look bloated and tired. You might spot the female parasite, sticking her head out from the host's abdomen.
The logo of the Royal Entomological Society features a Stylopid strepsipteran.
Related links: Strepsipterans (Order: Strepsiptera)
- Checklist of Strepsiptera
- Scientific American: Bug-Eyed
- Strepsiptera - Discover Life
- Strepsiptera - Tree of Life
- Systematic and Developmental Biology of Strepsiptera (Insecta)
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