Dead Spiders
We have a lot of spiders in our garden and some venture into the house. In the autumn we are treated to the sight of large arachnids scurrying across the living room floor in the frantic dash to find a mate. Sometimes they choose to use our premises as their last resting place and usually it falls to me to sweep up their remains. It is only recently that I have paused to look at their bodies in any detail and invariably they are on their backs with their legs curled in towards. I have never paused to wonder why, that is until now.
Relative to the size of their body, the legs of a spider seem rather thin and slender. This is because they do not contain extensor muscles, only flexor muscles. Instead of using muscles to move its legs, the spider relies on a sort of hydraulic system that utilises haemolymph pressure, the arachnid equivalent of blood pressure. Its body is somewhat akin to a hydraulic system, which relies on a hydraulic chamber to through its limbs so it can stretch them out.
As their flexor muscles have grown grow larger and more powerful to compensate for the absence of extensor muscles, spiders are able to control the flow of the fluid so that they can rapidly increase and then decrease the pressure of the haemolymph fluid when they reach their prey and grip them tightly. Thei muscles also allow them to climb virtually anywhere.
However, when they are dying or have died, spiders lose control of their body fluids and without the haemolymph pressure legs, they naturally curl inwards towards their body. Also, because a spider’s centre of mass, the point at which its mass is evenly distributed, the cephalothorax and abdomen are heavier than their thin legs, gravity pulls them on to their backs when their legs are weakened from a lack of haemolymph fluid.
While curled legs are invariably a sign that a spider is dead, some spiders like the male nursery web spider, use it as part of a mating ritual while others may use it as a technique to ward off the attentions of a predator. It is also possible that a spider is dehydrated or stressed when they pretend to be dead.
If you are not sure whether a spider is dead or not, just blow on it gently. They do not like to be disturbed and a gentle waft of air will send a live one scurrying away and merely rock a dead one back and forth. Another tell-tale sign is the state of its body which immediately starts to decompose upon death. If it is dull and wrinkly like a raisin, has its legs curled inwards and is on its back, it is almost certainly dead.
