This cobweb appeared in a toilet cubicle (see first photo). The segmented object to the left is, I assume, a captured insect. There is an opening at the top and a dark shape within that is presumably the spider. Stuck to the web are peculiar whitish lumps that may be eggs. Does anyone know what spider makes this kind of nest?
鈥� The web your correspondent has found is not that of a spider at all 鈥� it is in fact the cocoon of a female vapourer moth (Orgyia antiqua). When the spectacularly colourful tufty caterpillar of this moth is ready to pupate, it finds a suitable place in which to form a silken case around itself.
The female of this moth is flightless, but has a very strong pheromone that it uses to attract a mate. The moth discards the pupal skin after hatching out of it, and this is the apparent 鈥渃aptured insect鈥� on the left-hand side of the original photo (right).
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鈥淭he female moth is flightless but has a very strong pheromone that it uses to attract a mate鈥�
A female moth will sit on this cocoon waiting for a male. Flying males can detect the pheromone from miles away and head straight towards it.
The two moths will mate for a couple of hours (see second photo). Shortly afterwards, the female lays several hundred eggs on the outside of the cocoon and then dies. The eggs will overwinter and hatch the following year.
If a male doesn鈥檛 find the female in time, the female will still lay a crop of unfertilised eggs and die.
I have raised a couple of vapourer moths from larvae and one of them managed to attract a mate, which we found flapping at the window. They are a relatively common garden species but are rarely seen as adults, even though they are day-flying moths.
Tim Stripp, Exeter, UK
鈥� The cobweb is in fact a cocoon spun by the caterpillar of the vapourer moth. The caterpillar is hairy, and when the time comes for it to pupate, it incorporates its spines into the cocoon, which you can see in the original photo.
The flightless female emerges quite quickly, and looking closely at the photo (second one), you can see that her wingless body is packed full of eggs. Once the moth has mated, it lays its eggs on the surface of the cocoon and then dies.
The centre of the top of each egg darkens as the larvae develop, and when they hatch, their first meal is the egg itself. Looking almost like a mist, they then disperse on threads to find suitable food plants, such as deciduous shrubs and trees including plums, hence the name 鈥渧apourer鈥�.
Kathy Wallwork, Tunbridge Wells, Kent, UK
This article appeared in print under the headline 鈥淲eb of intrigue鈥�