Thursday, July 26, 2007

Earwigs make good mothers

There's something about insects--natural history in its purest and most alluring form--at the TLS, Gaden S. Robinson reviews James T. Costa's appealing-sounding book The Other Insect Societies:

Aphids are the new hot property in studies of social insects. Widespread caste differentiation among gall-making species from two families is a comparatively recent discovery, and there are numerous variations on the theme. A generalized scenario involves a foundress mother aphid inducing, by a combination of mechanical damage and possibly the injection of growth-inducing chemicals, a hollow gall in a leaf in spring. The foundress mother may have been the product of sexual reproduction on an alternative hostplant in autumn – hostplant alternation is common in many aphids. She then populates the gall by giving birth to first instar nymphs (rather than laying eggs); these are all daughters produced parthenogenetically – ie, each is a clone of the mother. While some daughters are conventional aphids like their mother and mature to resemble her precisely, others more resemble small scorpions with muscled raptorial forelegs and forward-directed dagger-like stylets – the stabbing mouthparts that in the familiar greenfly are plugged into a leaf or stem to suck sap. These agile, aggressive soldiers never moult and mature. Their sole function is to defend the colony. In one Taiwanese species, the soldiers mount suicide attacks against mammals, including man, and this appears to be primarily a defence against squirrels eating the Styrax gall that is home to the colony. If we consider the cloning aphid as a superorganism, the foundress might be considered to have split herself into an array of feeders and breeders (for the daughters repeat the cloning process) and mailed fists. The numerous parthenogenetic generations that may ensue through the summer, coupled with the remarkable fecundity of aphids, place some of these aphids potentially among the biggest superorganisms in existence. But it is often not as simple as this. There is evidence in many species of numerous foundresses being found in one gall; there is cannibalism, parasitism, competition and a host of other dirty tricks as well as co-operation between different clones. Teasing apart the genetic and ecological advantages, and determining the relatedness (or not, in view of the dirty tricks department) of aphid “families” has become a compelling and very active research area.

Another book I heartily recommend is Thomas Eisner's extraordinary For Love of Insects, which should be in everyone's collection....

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