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Spiders, are members of the Phylum Arthropoda which also includes the insects, centipedes, millipedes and crustaceans. Arthropods are characterized by having jointed legs and a hard exoskeleton, which must be shed as the animal grows. Spiders are further classified in the Class Arachnida, which includes animals with four pair of legs, and two body parts, the cephalothorax and the abdomen. The four pair of legs are attached to the cephalothorax. In addition to four pairs of legs, spiders have a set of fangs, or chelicerae, used to inject venom when capturing prey, a pair of pedipalps, and spinnerets. All but one family of spiders use their fangs to inject venom into their prey. The pedipalps, located between the chelicerae and the first pair of true legs, are used as sensory organs and, in males, are part of the reproductive system. Spinnerets, usually three pair, are used to produce the silk spiders use for webs. Most spiders have eight simple eyes, although some have six, a few two and some cave dwelling spiders have lost all eyes.
Neoscona crucifera build their webs in a spiral wheel-shape.
All spiders have spinnerets attached to the rear end of the abdomen, and all are capable of spinning silk. The silk is used for many different purposes. Many spiders spin webs to capture food; some also wrap their prey before eating it. Most use silk to wrap egg cases. Many use silk to create retreats under stones or within leaves or to line burrows or tunnels. Silken drag lines are played out to serve as safety nets. Newly hatched spiderlings release silk to act as parachutes allowing them to travel a safe distance from their predatory siblings.
The most obvious use of silk is the construction of webs. Orb weavers spin large vertical orb webs to capture insects. The spider may hide in a silken retreat attached to the web, or may hang in the web. Cobweb weavers weave tangled webs in which they hang upside down When an insect is trapped, the spider rushes to inject venom and subdue it. Some spiders wrap the prey with silk to secure it.
Neoscona crucifera photo by: Tara Turner (NHM)
Some spiders, such as include orb weavers, cellar spiders, and cobweb weavers, spin webs to capture their prey. Other spiders actively hunt for their food. Wolf spiders, jumping spiders, sac spiders and ground spiders are examples of this group. Still others lie in wait until an insect comes within reach. Crab spiders often hide in flowers until a fly or bee lands, then grab it. Trapdoor spiders hide at the mouth of their burrows until an insect walks past, then rush out to grab it.
Spiders with weak jaws, such as cobweb weavers and cellar spiders, inject digestive fluid into the insect's body with its fangs, then suck up the liquified tissues, leaving the exoskeleton intact. Those with strong jaws, such as tarantulas, wolf spiders and orb weavers, use their jaws to crush the insect while regurgitating digestive fluid, then suck up the liquified tissues as well.
Reproductive structures are not clearly visible until the spider undergoes its final moult. In the male, the last segment of the pedipalp is enlarged into a complicated structure. When ready to mate, the male spins a sperm web on which he places sperm from an opening near the spinnerets. He then sucks up the sperm with his pedipalp to store until mating.
The reproductive organ of the female, the epigynum, is located on the underside of the abdomen toward the front. The male approaches the female and transfers the sperm by placing his palp into the opening of the epigynum.
Although the Black Widow has a reputation for eating the male during mating, most males survive the procedure by approaching her carefully and leaving quickly. Different families follow specific courtship patterns. Orb web weavers signal their approach by strumming the threads of the web and waiting until the female indicates she is receptive. Jumping spider males perform a dance with their brightly colored front legs.
Following mating, the female will spin an egg case to protect her eggs. Cob web weavers hang the egg cases in their webs and remain close by. Green lynx spiders do the same. Wolf spiders attach the egg case to their spinnerets and carry it until it hatches. The young then spend their first weeks on their mother's back. Others, like ground spiders, leave their eggs in a silken case under a rock.
Many people are concerned about spiders in their homes and gardens and ask about controlling the population. However, spiders are a major factor in controlling insect populations, especially insect pests such as flies, mosquitoes and moths. And the only dangerous spiders in the Los Angeles area are the widows, the Black Widow and the Brown Widow. All others are considered harmless to humans.
Ground running spiders are extremely sensitive to chemicals and web building spiders seldom touch the substrate. Therefore, if an insecticide is used, it must be sprayed directly on the spider. Any insecticide is toxic and presents a potential hazard to beneficial insects, such as bees and butterflies, as well as to humans including children. Complete control is unlikely since spiders from adjacent areas will move in. Young of black widows have been observed to completely reinfest an area within one week. To discourage spiders indoors, webs can be removed with a broom. Webs on shrubs and buildings are best controlled with regular hosing with a water stream, the spiders will usually rebuild their webs elsewhere.