Tarantula anatomy I: the abdomen

A few days ago, I was chatting with a friend of a friend on facebook after they posted a picture of the moulted skin of their pet Mexican Redknee Tarantula Brachypelma smithi. The upshot was that they offered to send me the skin so I could have a look at it under the microscope and see what interesting features were visible. It turns out that there were quite a lot - more than can fit into a single post - so here is part 1, looking at abdominal features.

Dorsal view of the tarantula skin as it arrived in the post

The skin was well packaged and in really good condition and shows first of all how the spider moults. The top of the abdomen and cephalothorax split and peel back as a long flap, and the spider emerges up and backwards, pulling its legs and other appendages free. The new exoskeleton - including the fangs - is soft and needs to harden, and hence the spider will not be able to feed for a couple of days after moulting.

Looking at the abdomen, there are the familiar long bristles that you might expect to see, but a closer view (and indeed touch) shows that the texture is actually very different. There is a dense covering of shorter, softer hairs which look and feel much like moleskin - quite unexpected if you don't know what to expect!

The soft hairs of the abdomen along with longer, coarser bristles.

The long bristles are important as they have a sensory function, whicle some others form an important defence mechanism, being brushed off towards potential predators using the legs. These 'urticating' hairs (the paler patch top left in the photo above) are much smaller but are barbed and cause irritation to areas such as the eyes. In the wild, the spider would spend most of its time in a burrow in an earth bank and use these hairs to deter predators such as coatis - though large and fearsome-looking, these spiders rarely bite and have only weak venom. In this genus, the urticating hairs are classified as Type III (there are six recognised types) which are 0.3–1.2 mm long and particularly irritating to mammals, including humans who sometimes develop a rash as an allergic reaction. The biochemistry of the hairs is poorly known - they appear to be chitnous and are certainly not made of living tissue - their irritating effects have been assumed to be physical (i.e. the effect of having barbed hairs stuck in your eyes/skin), but there may also be direct chemical effects, at least in some species.

The bases of two sensory bristles showing the attachment points that fit into 'sockets' in the exoskeleton. Mag x40

The fine hairs on the surface of a bristle. mag x100

Some of the small, defensive urticating hairs - note the thin attachment points and covering of barbs. Mag x40

Close-up of the barbs covering urticating hairs. Mag x100

The last feature I want to look at here are the book lungs, a series of flat membranes (lamellae) that spiders use for breathing via a pair of pores (spiracles) and which increase the surface area for gas exchange in much the same way as alveoli do in our lungs. They are named after their overall form which is similar to a stack of pages in a book. In this specimen, initially they were deflated and looked like quite unremarkable white masses, but when teased apart, some of the fine structure could be seen with the lamellae attached to branches leading to the spiracle and thus the outside air.

A deflated book lung.

Close-up of the book lung showing individual lamellae.

So, just a few abdominal features here - after all, the spider took its other organs with it! However, they are still interesting and there's more to come as I will be writing about the cephalothorax and appendages soon...