What's in a spider-skin?

I often write about the identification of small invertebrates, but every now and again I take a slightly more basic approach - maybe a particular structure, or a close look at a more familiar species. So, when I found a moulted spider-skin, it seemed like a good opportunity to look at some general spider anatomy.

Moulted spider-skin, about 6.5mm long from the jaws to the tip of the abdomen (i.e. not including legs or palps). Note how the jaws are dark as they are the most heavily chitinised part. Also, only one pair of legs points backwards.

Zooming in on the jaws, the fangs are clearly seen as overlapping hooked structures - the hole at the end is even visible where venom is injected. Other mouthparts are also visible - the thicker part the fang is attached to is the 'chelicera'. Underneath the base of this (this is a view from beneath) and towards the side, is the 'endite' to which the palp is attached, and between the bases of these is the 'labium'. Behind each fang, protruding forwards of the labium, the 'maxilla' has a 'scopula' of bristles pointing inwards, and right in the middle is the mouth. Essentially, the fangs stab and are moved by the chelicerae, the maxillae manipulate, and the labium provides a surface for them to work against when feeding.

Close-up of a section of leg. As well as the covering of small hairs, there are a few long, dark bristles. The green bars indicate these and their number and arrangement can be important features for the identification of some spiders.

The skin showing that the spider emerged with a split between the eyes (circled) and the base of the chelicerae. The sternum surrounded by leg-bases (coxae) is clearly visible.

The arrangement of eyes in three pairs. The lines indicate the six eyes, showing that the eyes in the outer pairs are closely connected with a shared base, and the central pair are also very close together. This arrangement suggests it is a member of the family Segestriidae (a juvenile male I think), though this is not certain. The two crosses indicate structures that look like eyes but are not (they are artefacts of the photo and were not visible down the microscope - probably just light reflecting off part of the carapace).

As you can see, I've focused mainly on the head as this is where a number of possibly unfamiliar structures are located that are hardened, and therefore remain in shed skins. There are many other structures of interest, but those in the abdomen are lost as it is soft and shrivelled. Also, this is a static specimen - in a live spider, the eyes are actually more-or-less tubular and moved by small muscles - what you actually see here are just the lenses - and of course the mouthparts would be mobile.

If you want to know more about spider identification in Britain, I've given details of a few books below, and there is a 'UK Spider Identification' facebook group, plus of course the British Arachnological Society.

Further reading

Jones, D. (1989). A Guide to Spiders of Britain and Northern Europe. Hamlyn, London.
Jones-Walters, L.M. (1989). Keys to Families of British Spiders (AIDGAP Guide). Field Studies Council, Preston Montford.
Roberts. M.J. (1987). The Spiders of Great Britain and Ireland (3 vols.). Harley, Colchester. [compact 2-volume paperback edition published in 1993, but still £100+ so maybe only for the truly keen!]
Roberts. M.J. (1995). Spiders of Britain and Northern Europe. HarperCollins, London. [plus later editions]