[6] Anatomically, spiders (as with all arachnids) differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax or prosoma, and the opisthosoma, or abdomen, and joined by a small, cylindrical pedicel.

However, as there is currently neither paleontological nor embryological evidence that spiders ever had a separate thorax-like division, there exists an argument against the validity of the term cephalothorax, which means fused cephalon (head) and the thorax.

Spider-like arachnids with silk-producing spigots (Uraraneida) appeared in the Devonian period, about 386 million years ago, but these animals apparently lacked spinnerets.

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.

[14] However, a few primitive spiders, the suborder Mesothelae and infraorder Mygalomorphae, retain the ancestral arthropod nephridia ("little kidneys"),[13] which use large amounts of water to excrete nitrogenous waste products as ammonia.

[13] The principal pair at the front are of the type called pigment-cup ocelli ("little eyes"), which in most arthropods are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup.

[28] This acuity is achieved by a telephotographic series of lenses, a four-layer retina, and the ability to swivel the eyes and integrate images from different stages in the scan.

Various touch sensors, mostly bristles called setae, respond to different levels of force, from strong contact to very weak air currents.

[34] The jumping spider Evarcha culicivora uses the scent of blood from mammals and other vertebrates, which is obtained by capturing blood-filled mosquitoes, to attract the opposite sex.

The part closest to and attaching the leg to the cephalothorax is the coxa; the next segment is the short trochanter that works as a hinge for the following long segment, the femur; next is the spider's knee, the patella, which acts as the hinge for the tibia; the metatarsus is next, and it connects the tibia to the tarsus (which may be thought of as a foot of sorts); the tarsus ends in a claw made up of either two or three points, depending on the family to which the spider belongs.

Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, spiders and a few other groups still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors.

Spiders reproduce sexually and fertilization is internal but indirect, in other words the sperm is not inserted into the female's body by the male's genitals but by an intermediate stage.

The ancestral arrangement ("haplogyne" or "non-entelegyne") consists of a single genital opening, leading to two seminal receptacles (spermathecae) in which females store sperm.

In this species the male will penetrate its pedipalps through the female's body wall and inject his sperm directly into her ovaries, where the embryos inside the fertilized eggs will start to develop before being laid.

[13] Baby spiders pass all their larval stages inside the egg sac and emerge as spiderlings, very small and sexually immature but similar in shape to adults.

[51] In one exceptional case, females of the jumping spider Toxeus magnus produce a nutritious milk-like substance for their offspring, and fed until they are sexually mature.

[13] The water spider Argyroneta aquatica build underwater "diving bell" webs that they fill with air and use for digesting prey and molting.

Walking insects form most of the prey of most deinopids, but one population of Deinopis subrufa appears to live mainly on tipulid flies that they catch with the backwards strike.

Laboratory studies show that Portia's instinctive tactics are only starting points for a trial-and-error approach from which these spiders learn very quickly how to overcome new prey species.

[73] The golden wheeling spider, Carparachne aureoflava, of the Namibian desert escapes parasitic wasps by flipping onto its side and cartwheeling down sand dunes.

The basic radial-then-spiral sequence visible in orb webs and the sense of direction required to build them may have been inherited from the common ancestors of most spider groups.

[93] The oldest known exclusively terrestrial arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period, and had a triangular cephalothorax and segmented abdomen, as well as eight legs and a pair of pedipalps.

[95] Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider at the time of its discovery.

Hence Attercopus and the similar Permian arachnid Permarachne may not have been true spiders, and probably used silk for lining nests or producing egg cases rather than for building webs.

[101] Spiders (Araneae) are distinguished from other arachnid groups by several characteristics, including spinnerets and, in males, pedipalps that are specially adapted for sperm transfer.

[113] The Mygalomorphae, which first appeared in the Triassic period,[96] are generally heavily built and ″hairy″, with large, robust chelicerae and fangs (technically, spiders do not have true hairs, but rather setae).

[134] It may be an exaggerated form of an instinctive response that helped early humans to survive,[135] or a cultural phenomenon that is most common in predominantly European societies.

[137] Cooked tarantulas are considered a delicacy in Cambodia,[138] and by the Piaroa Indians of southern Venezuela – provided the highly irritant bristles, the spiders' main defense system, are removed first.

[141][142] According to her main myth, she resisted her father Enki's sexual advances by ensconcing herself in her web,[142] but let him in after he promised her fresh produce as a marriage gift,[142] thereby allowing him to intoxicate her with beer and rape her.

[142] In a story told by the Roman poet Ovid in his Metamorphoses, Arachne (Ancient Greek for "spider") was a Lydian girl who challenged the goddess Athena to a weaving contest.