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What are these soft white balls, about 5 centimetres long, found under a drain cover (pictured below)?

Barbie Short
Troon, Ayrshire, UK

The white blobs look exactly like the spider egg sacs that hang from the walls of the pit prison in Dundonald Castle in Ayrshire, UK. Apparently, these are made by European cave spiders.

I work as a tour guide at the castle. Sometimes, when visitors are halfway down the ladder that descends into the pit prison, I ask them not to disturb the spiders – which are large, black and usually stationary.

Sometimes, the visitors go very quickly back up the ladder, but others become very interested. In fact, some people come to the castle just to see the spiders.

The egg sacs are themselves full of tiny spiders. And yes, the prison is dark and dank, much like a drain.

Tim Laney
Bath, UK

The soft white balls hanging below the drain cover are the egg sacs of the European cave spider (Meta menardi).

These spiders aren’t rare, just rarely seen as they tend to live in dark locations such as caves, mines and – as in this example – sewers and drains. The silk of this spider is notable for its strength, which is needed to support the mass of the sizeable egg sac.

David Muir
Edinburgh, UK

These are the teardrop-shaped egg sacs of the cave spider M. menardi. Each sac, shown here hanging by a silken thread, contains a couple of hundred eggs.

The rather scary-looking adult spider has a 5-centimetre leg span and a 1.5-centimetre body length, but it rarely scares us humans as it is seldom seen. M. menardi is photophobic, living away from light, and so is a denizen of dark places such as caves, mines and sewers.

The egg sacs, however, are often constructed close to the entrances of caves and tunnels, as the developing spiderlings are strongly attracted to light, unlike their parents.

This is an evolutionary adaptation that encourages the young to disperse away from the adults to colonise new areas.

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