Arachnid

An arachnid is an invertebrate (an animal without a backbone) that has four pairs of jointed walking legs. Most arachnids, like spiders, ticks, scorpions, and mites, live on land and have two main body parts. Many arachnids prey on other invertebrates, while some are parasites. Unlike insects, however, arachnids have no jaws, antennae, wings, or compound eyes

(many tiny, individual eyes that altogether make a single image that resembles a mosaic). Arachnids are probably the most unpopular type of arthropod (an invertebrate with jointed appendages and a segmented body).

An arachnid is a member of the phylum (or primary division of the animal kingdom) Arthropoda since it has a jointed body case, an exoskeleton (a hard, outer support structure), and no backbone. The class name Arachnida comes from a story in Greek mythology in which a young girl named Arachne spun a silken cloth more perfectly than did the goddess Athena, who became so angry that she turned the girl into a spider. Arachnids are sometimes thought to be insects, but what defines an arachnid and prevents it from being considered an insect is the number of legs it has. An arachnid has eight legs. An insect has six legs. A crustacean has ten legs. Thus a spider and a tick are arachnids, while a centipede and an ant are not. Among other characteristics of arachnids is their highly developed sense of sight. They also have a rigid yet versatile exoskeleton that, combined with muscles and jointed limbs, allows them to have great flexibility and mobility. Its skeleton armor also keeps them waterproof and prevents them from losing body fluids by evaporation.

The spider is a good representative of the arachnid class. Besides their eight walking legs, they have a modified pair they use for handling food and another modified pair of appendages they use as poison fangs or claws.

Most spiders breathe by means of "book lungs." These are special organs located on the abdomen; they include chambers filled with hollow plates connected to tubes that extend to the outside of the skeleton. Air enters via these tubes and passes over plates that are richly supplied with blood. The oxygen diffuses into the blood and carbon dioxide is released. Arachnids, like spiders, are usually hunters, so they have sense detectors in their front end as well as six or eight pairs of eyes. Web-weaving in order to trap prey is probably the most distinctive characteristic of a spider, although not all spiders spin sticky webs. Some literally jump on top of their prey. Weaving a web is instinctive behavior, and webs are made of liquid silk (really a protein) produced from glands in the abdomen and shot out through "spinnerets" at the rear of spiders' bodies. Sticky threads are added later to catch and hold any prey that gets tangled. Spiders wait in hiding and stay connected to their web by a single thread that acts like a fishing bobber and signals that something is caught. The spider's feet are coated with an oily film so that it will not be caught in its own web. The spider bites its prey, immobilizing or killing it, and wraps it in silk for a later meal. Other arachnids, like scorpions, simply paralyze their prey with a sting. Similar to a spider, scorpions consume only the prey's bodily fluids.

Unlike spiders and scorpions, ticks are parasites. They burrow their heads into the skin of a mammal and feed on its blood. A tick is able to take in 200 times its own weight in blood at a single feeding. Although they do not always kill their victims, ticks may sometimes carry infectious diseases, such as lyme disease, which may be fatal if left untreated.

Arachnids reproduce sexually (through the union of male sperm and female eggs) and therefore must mate to produce offspring. For spiders however, mating can be a dangerous act. Since spiders not only eat other spiders but also those of their own species, it is not unusual for the female to kill and eat the smaller male immediately after mating with him. To avoid being eaten, some male spiders lock fangs with the female while they mate. Others tie the female in silk before mating, and still others present her with a meal already wrapped and ready to be eaten. Females lay their eggs in silken cocoons and sometimes keep watch over them until they hatch.

[See alsoArthropods ]