<P> Spiders have developed several different respiratory anatomies, based on book lungs, a tracheal system, or both . Mygalomorph and Mesothelae spiders have two pairs of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allow air to enter and diffuse oxygen . This is also the case for some basal araneomorph spiders, like the family Hypochilidae, but the remaining members of this group have just the anterior pair of book lungs intact while the posterior pair of breathing organs are partly or fully modified into tracheae, through which oxygen is diffused into the haemolymph or directly to the tissue and organs . The trachea system has most likely evolved in small ancestors to help resist desiccation . The trachea were originally connected to the surroundings through a pair of openings called spiracles, but in the majority of spiders this pair of spiracles has fused into a single one in the middle, and moved backwards close to the spinnerets . Spiders that have tracheae generally have higher metabolic rates and better water conservation . Spiders are ectotherms, so environmental temperatures affect their activity . </P> <P> Uniquely among chelicerates, the final sections of spiders' chelicerae are fangs, and the great majority of spiders can use them to inject venom into prey from venom glands in the roots of the chelicerae . The families Uloboridae and Holarchaeidae, and some Liphistiidae spiders, have lost their venom glands, and kill their prey with silk instead . Like most arachnids, including scorpions, spiders have a narrow gut that can only cope with liquid food and spiders have two sets of filters to keep solids out . They use one of two different systems of external digestion . Some pump digestive enzymes from the midgut into the prey and then suck the liquified tissues of the prey into the gut, eventually leaving behind the empty husk of the prey . Others grind the prey to pulp using the chelicerae and the bases of the pedipalps, while flooding it with enzymes; in these species, the chelicerae and the bases of the pedipalps form a preoral cavity that holds the food they are processing . </P> <P> The stomach in the cephalothorax acts as a pump that sends the food deeper into the digestive system . The mid gut bears many digestive ceca, compartments with no other exit, that extract nutrients from the food; most are in the abdomen, which is dominated by the digestive system, but a few are found in the cephalothorax . </P> <P> Most spiders convert nitrogenous waste products into uric acid, which can be excreted as a dry material . Malphigian tubules ("little tubes") extract these wastes from the blood in the hemocoel and dump them into the cloacal chamber, from which they are expelled through the anus . Production of uric acid and its removal via Malphigian tubules are a water - conserving feature that has evolved independently in several arthropod lineages that can live far away from water, for example the tubules of insects and arachnids develop from completely different parts of the embryo . However, a few primitive spiders, the sub-order Mesothelae and infra - order Mygalomorphae, retain the ancestral arthropod nephridia ("little kidneys"), which use large amounts of water to excrete nitrogenous waste products as ammonia . </P>

Where did the black widow spider get its name