Sunday, April 22, 2007

Spider Amaurobius similis

After a somewhat lengthy break (apologies dear readers) it is time to get back to the task of identifying all the life in my garden.

Spring has sprung and with it I have begun to realise the size of my self-appointed task: my garden is humming with insects, and plants are shooting up faster than I can catalogue them. I am currently stockpiling photographs in the hope of returning to them during the quieter winter months. Enough preamble, on to today's posting:

Under some old slates in the corner of my garden (at (1.8,1.5) - see here) I found the spider in the photo left (click on it to enlarge). Based on the beautiful colour plates in my Field Guide to Spiders (Michael J. Roberts, publ. Collins) my indentification for her is Amaurobius similis (the lace webbed spider). Amauorbius ferox is apparantly closely similar. I can't be certain, but I'm going with similis on the basis of my spider's relatively large (for the UK anyway) size: I measured her as 16mm from from front foot to back. A. similis is normally smaller it seems.

Why her? Because, as I learn from book above, male and female spiders can be distinguished by the shape of the palps - the short pair of stubby 'legs' that stick out forwards from the spider's 'head'. Only one palp can be seen in photo 1. Enlarging photo 2 both are clearly visible. The palps of male spiders end in swollen 'bulbs', whereas in females they simply taper to a point (as in photo 2) or sometimes end in a small claw.

The male palps are designed for 'scooping up' sperm off the web during mating, before depositing it in the female's epigyne, an opening on the her underside. Under magnification the male palps show up as fantastically complicated: an intricate arrangement of hooks, nodules and even in some species inflatable 'balloons', all structures evolved to maximise the fit to the female of the species' epigyne. It seems you can make some progress identifying spiders by their general size and markings, but it seems microscopic examinations of the palp/epigyne is the 'acid test' for spider i.d. Dr Roberts' book is full of meticulously detailed pen and ink line drawings of palps and epigynes. I made some effort with a handlens to examine my female's epigyne, but I wasn't confident to be able to separate similis/ferox, so we'll all have to live with the uncertainty (unless some expert out there can remove it based on the photos ?).

Two other points of interest are visible in photo 2. Firstly the spiders massive mouth parts: the chelicera. These are not themselves the fangs (which are only barely visible in photo 2 at the tips of the chelicera) rather they are crushing mouth parts designed for grinding and chewing as the spider squirts digestive juices onto the unfortunate prey. Secondly, the tiny circle of eyes - I can only convince myself of four in the photo though there should be eight - staring beadily up at the camera.

The common name for A. similis "the lace webbed spider" derives from the extrodinarily fine silk webs it can produce. According to my copy of the Biology of Spiders (Rainer F. Foelix, publ. Oxford Uni. Press - a fascinating read, more in future postings) this may be only one-hundred-millionth of a metre in diameter, so fine that it scatters light and appears blue. This silk is produced from a special array of tiny 'funnels' known as the cribellum situated at the spider's rear. The cribellum accompanies the more familiar silk-producing spinnerets and is not present in all spiders. Members of the genus Amaurobiidae (5 species in Britain and N. Europe) are distinguished by having a cribellum that is sparated into two parts. Spiders with a cribellum also have a tiny array of hairs known as the calamistrum, situated on the hind legs and arranged as a comb. The spider uses this for combing-out the fine silk as it emerges.

Finally, a word about the fate that may await my female. If she mates successfully she will spin a silk 'nest' into which she will lay her eggs and carefully protect them until they hatch. A likely first meal of the young spiderlings will be...their mother. Gruesome stuff!

1 comment:

Jim said...

Hello Henry,
I am absolutly fascinated with spiders and found your articles very interesting, I have been studying British spiders for year i love the small jumping spider and the `Salticus scenicus`or zebra spider is my personal favourite thank you very much,