Anatomy and Biology

SizeSpiders occur in a large range of sizes. The smallest, Patu digua from Colombia, are less than 0.37 mm (0.015 in) in body length. The largest and heaviest spiders occur among tarantulas, which can have body lengths up to 90 mm (3.5 in) and leg spans up to 250 mm (9.8 in)

The average huntsman spider species is about 1 inch (2.5 centimeters) long with a leg span of up to 5 inches (12.7 cm). The giant huntsman spider, however, has a leg span of up to 12 inches (30 cm), making it the largest spider by diameter; it is often described as being “the size of a dinner plate.

The giant huntsman spider (Heteropoda maxima, “the largest”) is a species of huntsman spider (Sparassidae), a family of large, fast spiders that actively hunt down prey. It is considered the world’s largest spider by leg span, which can reach up to 12 inches (30 centimeters).

Physical traitsSpiders, unlike insects, have only two tagmata instead of three: a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (also called the opisthosoma). The exception to this rule are the assassin spiders, whose cephalothorax is divided into two parts by an elongated “neck”. Except for a few species of very primitive spiders (family Liphistiidae, also called segmented spiders), the abdomen is not externally segmented. The abdomen and cephalothorax are connected with a thin waist called the pedicle or the pregenital somite, a trait that allows the spider to move the abdomen in all directions. This waist is actually the last segment (somite) of the cephalothorax and is lost in most other members of the Arachnida (in scorpions it is only detectable in the embryos). Unlike insects, spiders have an endoskeleton in addition to their exoskeleton.

The cephalothorax 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 legs, eyes, chelicerae and other mouthparts, and pedipalps.

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 liquified 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 frontmost pair of appendages are the pedipalps (or just palps), at the base of which are coxae or maxillae next to their mouth that aid in ingesting food. Since they do not have antennae, spiders use specialised and sensitive setae on their legs to pick up scent, sounds, vibrations and air currents.[citation needed]

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. 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).

Respiratory systemSpiders have developed several different respiratory anatomies, based either on book lungs. 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. This system has most likely evolved in small ancestors to help resist desiccation. The trachea were originally connected to the surroundings through a pair of spiracles, but in the majority of spiders this pair of spiracles has fused into a single one in the middle, and migrated posterior close to the spinnerets.

Among smaller araneomorph spiders we can find species who have evolved also the anterior pair of book lungs into trachea, or the remaining book lungs are simply reduced or missing, and in a very few the book lungs have developed deep channels, apparently signs of evolution into tracheae. Some very small spiders in moist and sheltered habitats don’t have any breathing organs at all, as they are breathing directly through their body surface. In the tracheal system oxygen interchange is much more efficient, enabling cursorial hunting (hunting involving extended pursuit) and other advanced characteristics as having a smaller heart and the ability to live in drier habitats.

Lifespan

In general, males have a shorter lifespan than females for two reasons: they move about more when searching for mates, so are more open to predation; they also eat practically nothing in the adult stage since they only want to mate. Varies in every species of course, but these are common generalities. In general, males live for ~1-1.5 months.

Females of most spiders (excluding tarantulas and other mygalomorphs, as well as the Filistatidae and Eresidae) tend to die either after laying their eggs, or after the eggs hatch and the juveniles are dispersed. Considering that they store sperm, time of copulation isn’t a good estimate of when the female will die. However, we do know that they have a short life cycle. Maximal span is 2-3 years, with most of them dying before 1 year of age. It actually depends on the climate a lot – in colder, northern areas, the lifespan is longer, and in warmer southern areas, the lifespan is shorter. This is because in most spiders, the adults don’t moult.

Mygalomorphs, filistatids and eresids do moult in the adult stage, however. They live for more than 3 years, and barring predation, they can go for 10 years easy. A colleague from the Amazon tells me he knows of one that’s been alive for ~20 years – a popular visitor attraction, the jungle guide takes people there and pokes it with a stick, since it’s always under the same tree.