Body plan Leg segments: }} Legs are labelled I, II, III, IV from anterior to posterior. }} Spiders are
chelicerates and therefore,
arthropods. As arthropods, they have: segmented bodies with jointed limbs, all covered in a
cuticle made of
chitin and proteins; heads that are composed of several segments that fuse during the development of the
embryo. Being chelicerates, their bodies consist of two
tagmata, sets of segments that serve similar functions: the foremost one, called the
cephalothorax or
prosoma, is a complete fusion of the segments that in an
insect would form two separate tagmata, the
head and
thorax; the rear tagma is called the
abdomen or
opisthosoma. The first appendages behind the mouth are called
pedipalps, and serve different functions within different groups of chelicerates. Spiders' chelicerae have two sections and terminate in
fangs that are generally
venomous, and fold away behind the upper sections while not in use. The upper sections generally have thick "beards" that filter solid lumps out of their food, as spiders can take only liquid food. Scorpions' pedipalps generally form large claws for capturing prey, However, in spiders, it occupies only the upper part of the abdomen, and blood is discharged into the hemocoel by one artery that opens at the rear end of the abdomen and by branching
arteries that pass through the pedicle and open into several parts of the cephalothorax. Hence spiders have open
circulatory systems. Like most
arachnids, including
scorpions, for example the tubules of
insects and arachnids develop from completely different parts of the
embryo. Except for the primitive
Mesothelae, of which the
Liphistiidae are the sole surviving family, spiders have the much more centralized nervous system that is typical of arachnids:
all the ganglia of all segments behind the esophagus are fused, so that the cephalothorax is largely filled with nervous tissue and there are no ganglia in the abdomen;
Sense organs Eyes 's main
ocelli (center pair) are very acute. The outer pair are "secondary eyes" and there are other pairs of secondary eyes on the sides and top of its head. The other pairs, called secondary eyes, are thought to be derived from the
compound eyes of the ancestral
chelicerates, but no longer have the separate facets typical of compound eyes. Unlike the principal eyes, in many spiders these secondary eyes detect light reflected from a reflective
tapetum lucidum, and
wolf spiders can be spotted by torchlight reflected from the tapeta. On the other hand, the secondary eyes of jumping spiders have no tapeta. The
visual acuity of some jumping spiders exceeds by a factor of ten that of
dragonflies, which have by far the best vision among
insects. 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. The downside is that the scanning and integrating processes are relatively slow. There are spiders with a reduced number of eyes, the most common having
six eyes (example,
Periegops suterii) with a pair of eyes absent on the
anterior median line. Other species have four eyes and members of the
Caponiidae family can have as few as two.
Cave dwelling species have no eyes (such as the
Kauaʻi cave wolf spider), or possess vestigial eyes incapable of sight (such as
Holothele maddeni).
Other senses As with other arthropods, spiders'
cuticles would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly bristles called
setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of
taste and
smell, often by means of setae. An adult
Araneus may have up to 1,000 such chemosensitive setae, most on the tarsi of the first pair of legs. Males have more chemosensitive bristles on their pedipalps than females. They have been shown to be responsive to sex
pheromones produced by females, both contact and air-borne. 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. Because they are able to tell the sexes apart, it is assumed the blood scent is mixed with pheromones. Spiders also have in the joints of their limbs
slit sensillae that detect force and vibrations. In web-building spiders, all these mechanical and chemical sensors are more important than the eyes, while the eyes are most important to spiders that hunt actively.
Locomotion Each of the eight legs of a spider consists of seven distinct parts. 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. The only
extensor muscles in spider legs are located in the three hip joints (bordering the coxa and the trochanter). As a result, a spider with a punctured
cephalothorax cannot extend its legs, and the legs of dead spiders curl up. and jumping spiders can jump up to 50 times their own length by suddenly increasing the blood pressure in the third or fourth pair of legs.
Silk production The abdomen has no appendages except those that have been modified to form one to four (usually three) pairs of short, movable
spinnerets, which emit
silk. Each spinneret has many
spigots, each of which is connected to one silk
gland. There are at least six types of silk gland, each producing a different type of silk. Silk is mainly composed of a
protein very similar to that used in insect silk. It is initially a liquid, and hardens not by exposure to air but as a result of being drawn out, which changes the internal structure of the protein. male spiders do not produce ready-made
spermatophores (packages of sperm), but spin small sperm webs onto which they ejaculate and then transfer the sperm to special
syringe-styled structures,
palpal bulbs or palpal organs, borne on the tips of the
pedipalps of mature males. When a male detects signs of a female nearby he checks whether she is of the same species and whether she is ready to mate; for example in species that produce webs or "safety ropes", the male can identify the species and sex of these objects by "smell". A few exceptions exist, such as
Parasteatoda tepidariorum. In these species the female appears to be able to activate the dormant sperm before oviposition, allowing them to migrate to the ovarian cavity where fertilization occurs. The only known example of direct fertilization between male and female is an Israeli spider,
Harpactea sadistica, which has evolved
traumatic insemination. 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. Males of the
genus Tidarren amputate one of their palps before maturation and enter adult life with one palp only. The palps are 20% of the male's body mass in this species, and detaching one of the two improves mobility. In the
Yemeni species
Tidarren argo, the remaining palp is then torn off by the female. The separated palp remains attached to the female's epigynum for about four hours and apparently continues to function independently. In the meantime, the female feeds on the palpless male. In over 60% of cases, the female of the Australian
redback spider kills and eats the male after it inserts its second palp into the female's genital opening; in fact, the males co-operate by trying to impale themselves on the females' fangs. Observation shows that most male redbacks never get an opportunity to mate, and the "lucky" ones increase the likely number of offspring by ensuring that the females are well-fed. However, males of most species survive a few matings, limited mainly by their short life spans. Some even live for a while in their mates' webs. DirkvdM Nephila clavipes.jpg|The tiny male of the golden orb weaver (
Trichonephila clavipes) (near the top of the leaf) is protected from the female by producing the right vibrations in the web, and may be too small to be worth eating. Orange spider egg sac.jpg|Orange spider egg sac hanging from ceiling Gasteracantha mammosa spiderlings next to their eggs capsule.jpg|
Gasteracantha mammosa spiderlings next to their eggs' capsule Lycosidae female carrying young.jpg|
Wolf spider carrying its young on its abdomen Females lay up to 3,000
eggs in one or more silk egg sacs, Like other
arthropods, spiders have to molt to grow as their
cuticle ("skin") cannot stretch. In some species males mate with newly molted females, which are too weak to be dangerous to the males. and an
Australian female trapdoor spider was documented to have lived in the wild for 43 years, dying of a parasitic wasp attack.
Size (
Theraphosa blondi), the largest spider by mass Spiders occur in a large range of sizes. The smallest,
Patu digua from Colombia, are less than in body length. The largest and heaviest spiders occur among
tarantulas, which can have body lengths up to and leg spans up to .
Coloration Only three classes of
pigment (
ommochromes,
bilins and
guanine) have been identified in spiders, although other pigments have been detected but not yet characterized.
Melanins,
carotenoids and
pterins, very common in other animals, are apparently absent. In some species, the
exocuticle of the legs and prosoma is modified by a
tanning process, resulting in a brown coloration. Bilins are found, for example, in
Micrommata virescens, resulting in its green color. Guanine is responsible for the white markings of the European garden spider
Araneus diadematus. It is in many species accumulated in specialized cells called
guanocytes. In genera such as
Tetragnatha,
Leucauge,
Argyrodes or
Theridiosoma, guanine creates their silvery appearance. While guanine is originally an end-product of protein metabolism, its excretion can be blocked in spiders, leading to an increase in its storage. Environmentally induced color changes may be morphological (occurring over several days) or physiological (occurring near instantly). Morphological changes require pigment synthesis and degradation. In contrast to this, physiological changes occur by changing the position of pigment-containing cells. An example of physiological color change is observed in
Cyrtophora cicatrosa, which can change its body color from white to brown near instantly. ==Ecology and behavior==