Spiders are members of the arachnid order Araneae. They are characterized by producing silk, having venom glands with fangs, having eight legs, and lacking wings. Like scorpions, spiders breathe using a gas-exchange system similar to that of insects. Inside the abdomen is a set of flat respiratory grids called book lungs. These are connected to the outside through openings called spiracles. In some species, the spiracles are also connected to a system of tiny tubes called trachea. The trachea or book lungs exchange oxygen directly into the arachnid's tissues without the use of a blood-circulatory system.

The anterior pedipalps and chelicera of spiders are much reduced when compared with those of scorpions. One benefit of having smaller anterior feeding appendages is that they do not interfere

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