Why is the spider coldblooded?

Good question. There are many advantages to being cold blooded and a few disadvantages.

Spiders can tolerate extreme cold. They are cold blooded so they can almost freeze solid and recover with little damage. This helps them survive in seasonal climates. Of course they like it warm but they have a way of surviving should it freeze.

They are heat activated. When the sun shines and warms the spider up he can become much more active than when it is cold. this means that during the warm periods he can generate and use more energy. When it's cold he doesnt need to keep his body temperature regulated so he can minimise energy requirements living on very little food for long periods of time.

So survival is the key and it's probably this that the ancient spider devloped as a stratergy and that trait has been passed genetically to every decendant ever since.