Thursday, May 19, 2016

Thanatosis


Thursday's Guardian Country Diary is about thanatosis - the strange behaviour of some animals that pretend to be dead when they are threatened. It's also known as 'playing possum', because its most famous exponent is the Virginia opossum, but many other animals do it, from grass snakes to tiny insects like this vine weevil that I found amongst primroses in a wood in Teesdale.
















Vine weevils are notorious horticultural pests because their larvae feed on the root systems of garden plants - especially those grown in pots. The first sign of their presence is wilting of the leaves, by which time they have destroyed most of the root system.

Vine weevils are parthenogenetic, laying fertile eggs without the presnece of a male to fertilise them, so it only takes one weevil to start a pest infestation.
















The adult weevil is a slow-moving cumbersome beetle that cannot fly, so when it's disturbed it simply tucks its legs under its body .....
















.... and either drops to the ground or rolls onto its back, keeping perfectly still. In this state it's hard to spot in the soil. Eventually, when it thinks the coast is clear, it stretches out a tentative leg, rocks itself until those sickle-shaped claws grip a leaf, hauls itself right-way-up and ambles away.




















Thanatosis is very common amongst weevils but it's most lively insect exponent is the click beetle, whose larvae are the notorious wireworms that burrow into potato crops. 

These little beetles play dead the moment that they are touched but they revive quickly, in a particularly alarming way.
















While they are on their backs a small peg locks the joint between their abdomen and their thorax, so when they flex their muscles great tension builds up. Eventually the attachment between the wing cases and thorax slips and they straighten instantaneously, somersaulting into the air and away to safety. It they land on their backs again they repeat the process, sometimes two or three times until they land right-way-up.


1 comment:

  1. I haven't noticed Weevils doing it but the Click beetles are regular performers. It is amazing how far they go. I have lost a couple in the van whilst trying to take pictures.

    ReplyDelete