Tarantula anatomy II: the cephalothorax

My previous post looked at the abdominal structures seen in the shed skin of a tarantula - this time I'm moving forward a segment to look at the cephalothorax i.e. the fused head and thorax.

The thorax with the top removed

With the upper surface removed, the empty thorax shows a clear pattern - the central section (the inner surface of the sternum) surrounded by the eight cavities associated with the coxa (1st segment) of each leg. The thorax is of course empty as the spider has emerged, but a slightly different view shows how complex this must be.

The empty thorax looking forward into the chelicerae

Here the structure is clearer still - the sternum at the base, the eight cavities through which the legs will have been extracted, plus corresponding cavities at the front (top of the photo) where the chelicerae (fang-bearring jaws) and head were pulled free. The white tuft of hairs between the chelicerae remains as do some white thread-like connective tissues within the empty legs. The sternum also clearly shows small holes such as those where book-lungs were joined to the outosde air via spiracles (breathing pores). Looking inside the upper surface of the cephalothorax, further structures can be seen.

Inner surface of the cephalothorax showing sculpturing

Close-up of sculpturing within the cephalothorax

 The inner surface shows both vaulting associated with the muscle attachments of legs and chelicerae, forming a central point between the attachments of these appendages. The lower photo is a closer look and shows finer mesh-like sculpturing presumably associated with the muscle attachments and possibly the passage of bodily fluids around them and/or the locations of nerves such as those serving sensory bristles. Near the front edge, the remnants of the eyes are also visible as their surface is also part of the exoskeleton and hence moulted along with other structures. Compared to the overall size of the spider, the eyes are small and closely clustered, possibly indicating the importance of, for example, sensory bristles to an animal that spends much time in a dark burrow during the day.

Front view of the cephalothorax showing eyes

Closer view of the inside surface of the cephalothorax showing how the eyes protrude outwards.

The upper surface of the cephalothorax - this back-lit view shows that there are eight eyes arranged on a small dome.

So, although the spider has left, certain structures - as with the abdomen - remain; the third and final part will look at appendages, including legs and fangs!