Notes on Pirata hygrophilus

From The Newsletter No. 11 December 1974

Being a biology teacher I am constantly trying to introduce little bits of practical work into my lessons, especially practical work with spiders, which most biology syllabuses tend to ignore completely. One fascinating piece of behaviour is cleaning behaviour and I never fail to become absorbed in watching this. Pirata hygrophilus is a useful animal from this point of view because it can be induced to perform cleaning behaviour in the following way: Half fill a steep-sided plastic container in water (a margarine tub or similar is ideal) and place a spider on the surface. The spider will rush about but be unable to leave the steepness of the sides of the container. After a short time it will settle down in one place and usually carries out cleaning behaviour fairly soon afterwards. In fact I once kept a spider on water for 2 hours with a strong light shining on it and it spent most of its time cleaning itself. This does raise the question as to whether it is 'cleaning' or 'waterproofing'. One needs a fairly good light to see what is happening and this produced a second rather interesting and helpful observation. The spider is in contact with the water via its legs, but only the tip of the leg is touching. The weight of the spider at these points of contact cause depressions in the water surface. The depression in the water surface is a measure of the weight being supported.

If a strong light is shone down from directly above the spider a shadow is cast on the bottom of the container. This shadow is that of the spider but there are also shadows resulting from the depression of the water surface. The size of these shadows are directly related to the size of the depression, which ie related to the weight being supported. Using this rather simple observation, one can see the relative weight distribution of the spider and in Pirata hygrophilus the first pair of legs seem to bear little of the weight. Most is borne by legs 2 and 3. This changes during cleaning behaviour.

Normally the spiders hold themselves up so that the body is clear of the water, but occasionally they relax and the abdomen rests on the surface, but not for long. Pirata spp. have relatively long legs and I felt that it might be interesting to make similar observations on a short-legged spider. Consequently I obtained some specimens of Salticus scenicus, full-grown or almost so and tested them. As might be expected they were unable to raise their abdomen clear of the water surface and were continually trying to escape. However, they were unable to jump and spent most of their time desperately 'paddling' with their legs. Perhaps the most interesting observation was that almost all of the ventral surface of the abdomen was in contact with the water and, though I cannot definitely say, I suspect that the lung spiracus and tracheal spiracle were certainly blocked by water.

These observations raise lots of questions some of which I will try to answer in the future. In the meantime I pass on the observations to anyone who is interested.
 
Added by John Partridge at 15:42 on Tue 3rd Jan 2012. Return to Summary for Pirata hygrophilus