Notes on Theridion mystaceum

From The Newsletter No. 58 July 1990

Nearly eighty years ago the little spider then known in Britain as Theridion denticulatum (Walckenaer, 1802) was known to be common on drystone walls in Cumbria (Britten, 1912). In 1974, Locket, Millidge & Merrett indicated that the name had been pre-occupied and that the next available name was Theridion melanurum Hahn, 1831. They also noted that we have another species in Britain, Theridion mystaceum L. Koch, 1870, which closely resembles T. melanurum. The two species have similar, rather attractive abdominal patterns, both are less than 3 mm long and, although T. melanurum is usually larger, there is overlap in size ranges and so this is not a reliable character. On the Continent they seem to occupy different biotopes, with melanurum generally able to tolerate drier situations, but this does not seem to be the case in Britain.

In the autumn, sub-adult specimens are sometimes to be found in houses where they overwinter along the tops of walls in the angles where these meet the ceiling. It was in such a situation that I used to find them, without knowing which of the two species they were, twenty-five years ago, in our private sitting room when my late wife and I managed the White Swan Hotel at Stratford-upon-Avon. The atmosphere in that centrally heated room was very dry and, apart from a few threads of silk which collected fine dust, the spiders made no webs and did not catch any prey. In the spring the little spiders became active and made small webs nearer to the light from the window and caught minute flies. This intake of food apparently triggered off the final moult and the spiders became adult. At the beginning of 1983, three of these tiny spiders of uncertain identity were collected in the utility room of my house at Keswick. One specimen was a sub-adult male and the other two were sub-adult females. The spiders were kept separately in small glass vials with polythene closures. A small piece of dead leaf was put into each vial to provide something for the spiders to cling to and provide a foundation for whatever sort of web they could make in this limited space. Early in April they were seen to be making webs and small chironomid flies and minute Owl Midges (Psychodidae) were introduced as prey which the spiders caught and ate. By mid- April they had become adult.

The two females were put into the same vial to see if they would tolerate one another. However, one immediately attacked the other and killed it and commenced feeding upon it. The male spider was then put into the vial. The males are smaller than the females and this one approached the female cautiously signalling its presence by vibrating palpi. The female retreated from the prey which the male took over. It then severed the silk lines by which the dead female was suspended and moved it away from the living female. A few minutes later the male climbed across the web towards the living female, jerking the silk lines with its tarsal claws. The pair then mated: this took over an hour. The process was observed through a hand lens. I did not see a sperm web made or any sperm induction by the palps, but this may have been done before the spider was moved from its own vial. When mating had terminated the male went across to the dead female, drumming its palpi, and tried to mate with it. The following day the male was removed and killed by immersion in alcohol and, when it was examined very carefully under a binocular microscope, it was identified as Theridion mystaceum.

The supply of food to the female was continued. Eventually in a gravid state, the female refused food and became torpid for a few days. On the 20th May it laid about fifty eggs deposited in a small cocoon covered with dark green silk. On 16th June the eggs commenced to hatch and the spiderlings remained together in a cluster. Later they became active and attacked each other and some were observed to be feeding on others and eventually only nine remained. When these showed signs of attempting to disperse after they had moulted, they were released. The female had by now commenced feeding again and on 1st August another cocoon was produced which was smaller than the first and contained about thirty eggs.

The glass vial was kept on a window sill and when it happened to be exposed to the sun, the female was seen to move the globular egg cocoon from the high temperature round to the shady side of the dead leaf. From this second cocoon only six spiderlings survived to pass through the first moult. Not all the remainder had been eaten; some died before they had been completely hatched from the eggs. The female died on 18th August. The young spiders which dispersed, would, if not eaten by predators, grow to the sub-adult stage and start to over-winter in the late autumn to begin the cycle all over again. As far as I am aware this is the first time that the life cycle of this species has been recorded, and of course it shows that this can be done by patient observance, with the minimum of equipment without leaving the house! At the same time I was also curious about the other species T. melanurum. When I went to look for specimens I found them to be quite common, and gravid females were numerous at the beginning of June on drystone walls in Borrowdale and at Buttermere. The webs are spun in the open spaces about 20 mm between the flat slate stones. Some of these are rather irregular in shape but in favourable situations they can be somewhat conical in shape, open at the top and bottom, rather like flimsy lampshades. Looking at my record cards, I saw that I had drawn one of these when I first collected the species at Rosthwaite in Borrowdale and at Wastdale Head in 1950 which the late Dr Alex La Touche identified for me as Theridion denticulatum before the Ray Society volumes on British spiders were published.

References
Britten, H. (1912) The arachnids of Cumberland. Trans. Carlisle not. Hist. Soc. II: 42.
Locket, G. H., Millidge, A. F. & Merrett, P. (1974) British Spiders HI: Ray Society, London.
 
Added by John Partridge at 15:31 on Wed 11th Jan 2012. Return to Summary for Theridion mystaceum