Thigmotaxis

1 Jan

Centipedes, like many other small creatures exhibit a curious behaviour called thigmotaxis – they like to squash themselves into corners in order to maximise body contact with surfaces.

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This is a common brown centipede resting against the side of a plastic sandwich box.

The reason small soil dwelling creatures do this is because they are particularly vulnerable to changes in humidity, they dry out quickly, or become water logged. The behaviour is so overwhelming that it can mask other reactions to external stimuli such as vibration, chemicals, etc..

Years ago I spent long hours studying the behaviour of Lithobius forficatus L. (the common brown centipede) as a Ph.D. student in order to discover what the beautiful structures on its hind legs were for:

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This is a diagram of the underside of a male centipede. You can see that the fattest segments of the last four pairs of legs, closest to the body, are equipped with a row of interesting pores. These are the coxal pores. Under the pores lies a curious tissue, known as the coxal organ.

You can see the pores more clearly here:

Coxal pores dimensions

At very high magnification the organ looks like this:

LM coxal pores

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Under the electron microscope the cells look for all the world like ‘kidney’ or ‘malpighian tubule’ (insect kidney) cells. On this basis it was classified as a ‘typical transporting epithelium’, a not very helpful description as we had not a clue what might be transported.

The Behavioural experiments proved compelling and, using a circular choice chamber to get around the centipede’s natural tendency to flatten itself to a wall I was able to demonstrate that the coxal pores were responsible for releasing a sex hormone, or pheromone attractive to members of the opposite sex.

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I came to the conclusion that the main problem facing centipedes (and anyone living under ground) was not drying out, but becoming water logged. So the coxal organ is most probably very good at getting rid of excess water (centipedes living in xeric or dry habitats have very small coxal pores, or none at all), and in so doing chucking out a useful ‘come on’ signal to other centipedes of the same species. The pheromone chemistry is phenolic and related to the chemistry of centipede cuticle (hardly surprising given that the coxal organ is modified cuticular epithelium which normally secretes the centipedes exoskeleton).

Sticking closely to a surface is not my thing. I prefer to venture out and discover things anew. Multifunctionality is common amongst amongst biological systems – it is the stuff of evolution and natural selection: a hand becomes a wing (pterosaur, bird, bat); a wing becomes a diving tank (Great Diving Beetle’s plastron); a zoologist become a woodworker…… A close study of centipede backsides was instructive in beginning a small voyage of discovery for me, a gift by a true mentor J.Gordon Blower, the ecologist and millipede man who pointed them out to me whilst smoking a number 6 filter tipped in his nicotine stained lab way back in 1979.

JG Blower 1

I have always thought the phrase ‘there is no need to re-invent the wheel’ the dullest of aphorisms. Re-invention is human, discovery and rediscovery a divine gift. Get out there and look at something very small, or something very big, but please do go and look, because your discoveries will be unique.

PMH Littlewood. Fine structure and function of the coxal glands of lithobiomorph centipedes: Lithobius forficatus and L. crassipes (Chilopoda, Lithobiidae) 1983. J. Morphology  Vol 177

PMH Littlewood. The chemosensory behaviour of Lithobius forficatus. 1. Evidence for a pheromone released by the coxal organs (Myriapoda: Chilopoda).Journal of Zoology Vol 211 January 1987

PMH Littlewood and JG Blower. The chemosensory behaviour of Lithobius forficatus. 1. Evidence for a pheromone released by the coxal organs (Myriapoda: Chilopoda).Journal of Zoology, Vol 211, 1987

PMH Littlewood.The water relations of Lithobius forficatus and the role of the coxal organs (Myriapoda: Chilopoda).J.Zoology, Vol  223, 1991

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