Spider

Spiders (order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom,[2] and spinnerets that extrude silk.[3] They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms.

Etymology

The word spider derives from Proto-Germanic *spin-þron-, literally 'spinner' (a reference to how spiders make their webs), from the Proto-Indo-European root *(s)pen- 'to draw, stretch, spin'.[10]

Feeding

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.[13] The families Uloboridae and Holarchaeidae, and some Liphistiidae spiders, have lost their venom glands, and kill their prey with silk instead.[19] Like most arachnids, including scorpions,[14] spiders have a narrow gut that can only cope with liquid food and two sets of filters to keep solids out.[13] 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.[13]

The stomach in the cephalothorax acts as a pump that sends the food deeper into the digestive system. The midgut 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.[13]

Capturing prey

The best-known method of prey capture is by means of sticky webs. Varying placement of webs allows different species of spider to trap different insects in the same area, for example flat horizontal webs trap insects that fly up from vegetation underneath while flat vertical webs trap insects in horizontal flight. Web-building spiders have poor vision, but are extremely sensitive to vibrations.[13]