Wednesday, 30 May 2007

Look before they leap

Rake over a handful of woodland leaf litter and you will almost certainly see any number of tiny creatures. Some will remain where you find them, some will move slowly away, seeking out shelter from the daylight to which they have been exposed, and others will spring away from you like miniature fleas. You probably know the last of these animals – the ones that leap – as ‘springtails’. They are familiar because they are found in many different habitats and are just as easily encountered when you tidy your flowerbed or turn over a patch of seaweed on the tide line. What you may not appreciate is just how many of them there are and just how successful they have been.

It has often been said that springtails are the oldest known group of insects, having existed on the Earth for at least 400 million years. Although the limited fossil evidence does indeed demonstrate this ancient heritage, springtails are only distantly related to true insects and are considered ‘primitive’ in comparison with their more diverse cousins. However, complexity does not always bring success and it is a testament to the functionality of springtail design that their basic body plan has changed relatively little over the last 40 million years. While evolutionary pressures are strong, a design that works can remain little changed over very long time periods. Charles Darwin paid little attention to the springtails, describing them as ‘wingless, dull-coloured minute insects’ but his close friend Sir John Lubbock was fascinated by them, writing, in 1893, a famous monograph on them. Lubbock described some 130 species (6,500 are recognised today), with many beautifully illustrated by A T Hollick, a deaf and dumb artist of precocious talent.

Lubbock gave these creatures the group name of Collembola, derived from the Greek words ‘colle’ (glue) and ‘embolon’ (piston) because of an adhesive organ on their undersides, used to attach them to slippery substrates. The name ‘springtail’ comes from a second appendage, the ‘furcula’, which enables the springtail to leap away from danger. Under normal conditions the furcula is held in place against the underside of the body by a catch. This catch is released if the springtail senses danger and the furcula slams into the substrate, launching the springtail into the air. Some species are able to reposition the furcula in flight so can jump again as soon as they land; others need a period of rest between jumps before they can fire again. With up to 40,000 springtails per square metre in good habitats you would think that we would take more interest in them. However, their tiny size seems to be a barrier to our interest, which seems a real shame.

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