Amaurobius hole in a wall...
...and its owner with a trapped a dronefly
The unusual behaviour relates to reproduction. Females lay their egg clutch during June and July inside their holes and sit over it for three weeks, when she opens the egg sac and the spiderlings hatch. The interaction between the spiderlings and their mother stimulates her and inhibits the maturation of the eggs of a second clutch. The female is not agressive and instead solicits the young to migrate to her ventral side and lays immature eggs, the so called trophic eggs, for the spiderlings to eat.
(from Kim and Roland 2000)
Spiderlings deprived of this extra food do not survive as well, and gain weight much more slowly. A few days later, the female dies and the spiderlings promptly cannibalize her, gaining further weight and increasing their chances of survival when they disperse. The trophic eggs are fertilized and apparently viable, so the female is actually increasing the survival of her first clutch by sacrificing a possible second. Does she produce more or less offspring overall as a result? Kim and Roland, in experiments in which they compared spiderlings with access to trophic eggs with deprived spiderlings, estimated that the mother produced an average of 104 viable spiderlings of the first clutch at day 6 when fed with trophic eggs, compared with a total number of 102 when they removed females from the first clutch and allowed to lay again. This similar number of total spiderlings is, however, misleading as six out of the 10 females removed from their offspring either had abortive clutches or ate them themselves, suggesting that the viability of the second clutch as a replacement clutch is decreasing and the eggs of the second clutch could be specialising in their function as trophic eggs. Trophic egg feeding might have evolved as a way of reducing intra-clutch cannibalism at a time in the spiderling's life when they cannot produce the sticky silk required to catch their own prey. Next time you see Amaurobius holes in your wall, spare a thought for these hair-raising behaviours happening so near you at night.