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7 May 1999 |
Ref 99/99 |
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A THRIPS' HOME IS ITS CASTLE… |
To the thrips insects of arid central Australia, the task of defending their homes against predators and competitors is as serious a job as that facing any inner-urban human.
But some thrips have evolved tactics even more socially repellent than their human counterparts: Dactylothrips priscus, for instance, has no compunction about farting in the face of an invading ant, when it pokes its head into the thrip's home.
Laurence Mound, former head of entomology at the British Museum, and now Research Fellow with CSIRO Entomology and the Australian National Insect Collection (ANIC), confesses to having taken a surreptitious sniff at this anti-ant spray.
"It's extremely pungent, and took two tinnies to wash away the taste! The ant recoils like a shot. I'd love to know what's in it. It must contain some extraordinary chemicals which might be really useful as insect repellents, or in other ways."
The thrips' lack of neighbourly social graces are the product of evolution under the extraordinary pressures imposed by the Outback's harsh, arid and wildly fluctuating climate over tens of millions of years.
Although some thrips are crop pests - they are small plant sap-sucking insects - many others live on the various Acacia (wattle) plants in central Australia, either in individual or group homes.
Their domiciles range from the abandoned holes in acacia leaves left by leaf-miners or plant galls, to structures specially created from elaborately glued together leaves or spun fibres.
"These domiciles represent extremely valuable resources in an arid region. They provide the thrips with food, as well as protection from enemies, competitors and harsh conditions. The need to defend them is therefore high, and you'd expect that thrips would over time develop some pretty potent defensive strategies."
In one species, Carcinothrips leai, the female boasts a pair of large claws resembling those of a mudcrab, to do the job of building her fortress and keeping would-be invaders at bay.
As a counter-strategy another kind, Xaniothrips, has evolved large spines along its abdomen which it uses to evict any resident thrips before taking over her residence.
Members of another group, as yet undescribed by science, employ a large spear-like spine on their head to carve their way into the glued-up residence and expel its inhabitant.
Others still have evolved specialised armour plates on their body, which they use to block the entrance to a burrow they have take over from a leaf-miner or gall insect.
Laurence points out, "Clearly, any form of domicile is a valuable resource to small insects in the arid conditions under which most Acacia species grow. The highly specialised adaptations we see suggest that thrips put a high premium on defending their turf - or on ways to oust another species of thrips from its burrow so they can occupy it."
Despite an arsenal of aggressive weaponry and behaviour, many kinds of thrips also operate in a strong social environment with their own kind. It is possible that the harsh, dry environment occupied by so many of these thrips makes domicile protection and dispossession vital. The same environment has driven them to develop social behaviours that enhance home ownership.
Social behaviour is particularly clear in the thrips that induce galls on these Acacia trees. In some of them, the first generation of adults develop as soldiers. These will kill intruders into their gall, but often die themselves in the process. They thus lose their own opportunity to breed.
"Altruism (mateship) is thus not just an expression of human morality or political aspiration, it has been an important survival strategy in Australia's hostile environments for millions of years." says Laurence. "It's another insight into the extraordinary way in which this unique continent of Australia has shaped the life that inhabits it," he says.
The thrips insects themselves may seem insignificant. But
the evolutionary processes that have given rise to their sociality
have more general significance. And it is questions about the
evolution of social behaviour that drive this international research
programme involving scientists from Canada and from Flinders
University, as well as Laurence Mound at CSIRO.
More information:
Dr Laurence Mound, CSIRO Entomology 08 8950 7100
E-mail: laurence@ento.csiro.au
Mr Malcolm Robertson, CSIRO Entomology 02 6246 4040
Commonwealth Scientific and Industrial Research Organisation
(Australia's largest scientific research organisation)
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