Spider of the Month - NovemberAmaurobius similis

This month we have something totally different - a cribellate species, Amaurobius similis, which in terms of size is a relative monster - well at least compared to  last month's species.  Males are big enough at 6-8mm, but the females are even more impressive ranging from 9 up to 12mm in length.

We've come indoors for this species.
Previous "spiders of the month" have been chosen because in my pitfall traps I've caught lots of them at the time of the year.
In contrast, very few specimens of this species have appeared in my pitfall samples. There have been a few in pit-traps in leaf litter on the wooded island of Inchcailloch on Loch Lomond. More frequently, we have caught these spiders in corrugated paper traps wrapped around tree trunks - again in the woodland of Inchcailloch, but also on the trunks of conifers on afforested peat bogs.
In actual fact, this is not quite accurate: just one or two specimens belonged to A. similis and usually they were of the congener A. fenestralis. This latter species is, in effect, the "outdoor" equivalent of A. similis; both of these species inhabit cracks and crevices, but the latter exploits those of human dwellings. For A. fenestralis, this is reflected in the fact that we caught more inside the spaces of corrugated papers traps (wrapped around the trunks of trees) than on the exposed surfaces of sticky traps at the same locations.
There is also a third species, larger in size and more fearsome in appearance as its name suggests - Amaurobius ferox - which tends to live under stones.
Female A. similis on web.
Normally, A similis lives in holes and crevices such as one might find in sheds and outside window frames of houses, where they spin a rather diffuse web of lacy silk around the entrance to their retreat - as shown here.

In autumn, though,  they frequently come inside.
Indeed, just now a spider which might well be A. similis is starring in a television advert for a battery powered mini-suction device (essentially a pooter, which is a traditional arachnologists' collecting device in the form of a tube held in the mouth so that specimens can be sucked up - either into a specimen tube or trapped against a filter in the tube). The purpose of the "electric pooter" is to enable the nervous house-holder to remove the spider without having to touch it, but then safely release the poor animal, unharmed, outside the house!

So, what does cribellate mean?

If you look at the female A. similis shown above on her web, you can see that she is combing out silk with her fourth leg. There is a good reason for this.
cribellum As you know, spiders use their spinnerets to spin silk which they use for all sorts of functions.
Amaurobius produces a special kind of silk from its cribellum, which is a pair of sieve-like plates just in front of the spinerets, as shown in the diagram on the left. The silk is very characteristic, with a blueish colour when fresh.

The silk is produced from the cribellum in a large numbers of fine strands, which are teased out by a double (in the case of Amaurobius) row of curved bristles on the metatarsus of the fourth (hind) leg.
This structure is called a calamistrum and is shown in the diagram on the right here.



The picture of A. similis is again from the book by Dr. Mike Roberts : The Spiders of Great Britain and Ireland, published by Harley Books in 1985, from which the other illustrations have been highly adapted. The image of the female combing her silk is after Bristowe, W.S. (1958): The World of Spiders, published by Collins.
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