Spider anatomy

The anatomy of spiders includes many characteristics shared with other arachnids. These characteristics include bodies divided into two tagmata (sections or segments), eight jointed legs, no wings or antennae, the presence of chelicerae and pedipalps, simple eyes, and an exoskeleton, which is periodically shed.

Spiders also have several adaptations that distinguish them from other arachnids. All spiders are capable of producing silk of various types, which many species use to build webs to ensnare prey. Most spiders possess venom, which is injected into prey (or defensively, when the spider feels threatened) through the fangs of the chelicerae. Male spiders have specialized pedipalps that are used to transfer sperm to the female during mating. Many species of spiders exhibit a great deal of sexual dimorphism.

Spiders, unlike insects, have only two main body parts (tagmata) instead of three: a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (also called an opisthosoma). The exception to this rule are the assassin spiders in the family Archaeidae, whose cephalothorax is divided into two parts by an elongated "neck". In the majority of spiders, the abdomen is not externally segmented. The exception is the Liphistiidae, a basal family, which retains this more primitive character; hence they are sometimes called segmented spiders. The abdomen and cephalothorax are connected by a thin waist called the pedicel. Unlike insects, spiders have an endoskeleton (constructed of endosternites) in addition to their exoskeleton.

The cephalothorax, also called prosoma, is composed of two primary surfaces: a dorsal carapace and a ventral sternum. Most external appendages on the spider are attached to the cephalothorax, including the eyes, chelicerae and other mouthparts, pedipalps and legs.

Like other arachnids, spiders are unable to chew their food, so they have a mouth part shaped like a short drinking straw that they use to suck up the liquefied insides of their prey. However, they are able to eat their own silk to recycle proteins needed in the production of new spider webs. Some spiders, such as the dewdrop spiders (Argyrodes), even eat the silk of other spider species.

Spiders typically have eight walking legs (insects have six). They do not have antennae; the pair of appendages in front of the legs are the pedipalps (or just palps). Spiders' legs are made up of seven segments. Starting from the body end, these are the coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. The tip of the tarsus bears claws, which vary in number and size. Spiders that spin webs typically have three claws, the middle one being small; hunting spiders typically have only two claws. Since they do not have antennae, spiders use specialised and sensitive setae on their legs to pick up scent, sounds, vibrations and air currents. Some spiders, such as the Australian crab spider, do not have claws.

The pedipalps have only six segments: the metatarsus is missing. In adult males, the tarsus of each palp is modified to carry an elaborate and often species-specific structure used for mating (variously called a palpal bulb, palpal organ or copulatory bulb). The basal segments of the pedipalps, the coxae, next to the mouth, are modified to assist with feeding, and are termed maxillae, although they are not homologous with the maxillae of mandibulate arthropods. In mesothele and mygalomorph spiders, the maxillae are only slightly modified; in araneomorph spiders, the anterior edge is often saw-like and is used in cutting up prey.

Spiders usually have eight eyes, each with a single lens rather than multiple units as in the compound eyes of insects. The specific arrangement of the eyes is one of the features used in classifying different species. Most species of the Haplogynae have six eyes, although some have eight (Plectreuridae), four (e.g., Tetrablemma) or even two (most Caponiidae). Sometimes one pair of eyes is better developed than the rest, or even, in some cave species, there are no eyes at all. Several families of hunting spiders, such as jumping spiders and wolf spiders, have fair to excellent vision. The main pair of eyes in jumping spiders even sees in color.