Spiders, scorpions, mites, ticks, and their relatives are members of the class Arachnida. The arachnids comprise about seventy thousand species of terrestrial arthropods. Animals in this class have eight walking legs; most are carnivorous, and many subsist on a liquid diet of blood or predi-gested prey. Spiders are the most numerous arachnids, accounting for forty-two thousand species belonging to one hundred different families. The number of spiders can be larger than anyone expected. A study in Great Britain counted spiders in a meadow, coming up with 131 spiders per square meter. Within the area of 36,150 square kilometers that composes the Netherlands, there are approximately five trillion spider inhabitants. Put together, these spiders could consume all fifteen million Dutchmen in merely three days.
Many people confuse spiders with insects. Although many similarities exist between spiders and insects, such as the presence of an exoskel-eton, the gas exchange system, and the circulation system, three conspicuous traits can serve to distinguish spiders and insects. First, spiders usually have four pairs of legs compared to insects' three. Second, insects have compound eyes whereas spiders have singular eyes with lenses. Third, insects have antennae while spiders do not.
The body of a spider has two distinct parts: the cephalothorax, consisting of the head and breast, and the opisthosoma, or abdomen. The back of a spider is referred to as its dorsal side and the bottom is its ventral side. The eight legs, two jaws, and two feelers (palps) are connected to the cephalothorax. The males have a bulb at the end of their palps, which is used to store and inject semen into the sexual organs of the female. There are usually eight eyes on the cephalothorax, although the number may vary from none to twelve. An extensive nerve system is made up of a brain located in the cephalothorax, and ganglia (the equivalent of nerves in mammals) that run through various sections of the body. The heart is situated at the front upper side of the abdomen. The silk-making spinners are found at the rear of abdomen. These spinners are linked to glands that produce a vari-
Kingdom: Animalia Subkingdom: Bilateria Phylum: Arthropoda Class: Arachnida Order: Araneae
Suborders: Mygalomorphae (the primitive spiders); Aranaeomorphae (the modern spiders); Mesothelae, with one family of spiders, the Liphistiidae
Geographical location: Every continent except Antarctica
Habitat: Diverse; mostly on land, inside and outside buildings, on or close to the ground, under stones, logs, litter, low or medium foliage, tall shrubs and trees, under bark; some live in freshwater and very few in salt water Gestational period: Varies; some female spiders can carry sperm for some time after mating until ready to produce an egg sac Life span: Many live for up to two years Special anatomy: Eight walking legs, four on each side of the thorax; eight simple eyes, each with a single lens, that are particularly sensitive to movement; an extensive nerve system; silk-making spinners; muscular jaws equipped to inject poison into prey ety of proteins, which when mixed polymerize to form silk. As the fluid silk is pressed through the spinners, a thread is made. The reproductive organs are located between the book lungs and the spinners. Running though the whole body is the alimentary canal, at the end of which is the excretory system.
Most spiders are equipped with poison glands to kill prey. The jaws are used to grab and crunch the prey. A pair of syringelike structures, which are hollow and extremely sharp, are found at the end of the jaws. They are used to puncture the body and inject poison into a prey. The venom is produced in special glands and stored in a special bladder, around which is a spiral muscle. This muscle contracts to eject the poison through the syringe into the victim. The poison, made up of proteins, amines and polypeptides, causes paralysis by disrupting the communication between the nervous system and the muscles. The poison and digestive enzymes cause the death of cells and dissolve the contents of the prey. The spider then sucks the prey empty, leaving a shell behind. Many spiders can give a nasty bite comparable to the stinging of a wasp; a spider sting can even be fatal to children and persons with weak constitutions.
Spiders use a trachea, a slit above the spinners that can be opened and closed, for admission of oxygen. Long small tubes run from this slit into the body. Gases are exchanged with the blood by diffusion. Many spiders also have book lungs, which are hollow, leaflike structures through which the blood flows. Many modern spiders have both tracheas and book lungs. With these two systems together providing extra oxygen, the modern spider has an advantage in having quicker and more sustained reaction times than the primitive counterparts with only book lungs. The circulating blood in a spider's body is colorless and called hemolymph. It transports nutrients, hormones, and cells in addition to oxygen. It is also used locally to raise blood pressure during molting and stretching the legs. Spiders have an
The Black Widow: Small but Deadly
The black widow spider, Latrodectus mactans, is found in the United States from northern Massachusetts, south to Florida, and west through Louisiana, Texas, Kansas to California, and also throughout Central America. The body length of the female reaches up to 5 inches, but males are smaller. Leg span for the female is 0.75 inches, with that of males slightly longer. The main diet includes beetles, cockroaches, crickets, flies, scorpions, and spiders.
The black widow possesses one of the most powerful venoms. She earns her name from her gruesome habit of devouring the tiny, harmless male spider after mating with him. The black widow's poison is called a neurotoxin—it attacks the nervous system and blocks the transmission of nerve signals to the muscles, causing convulsions, paralysis, and intense pain. Every summer, the male black widow goes searching for a mate. The female simply waits in her web, hanging upside down as usual. When he finds a female, the male approaches very carefully and signals to the female that he is not prey by tapping out a coded message on the web. As soon as the mating is over, however, he must escape or become an easy meal for the female.
open blood circulation system with the heart located in the back of the abdomen. Blood vessels transport the blood to the heart but thereafter the blood flows freely in the open spaces between organs. The heart is an open tube with valves which is hung in a cavity. Elastic muscles around this cavity contract, enlarging the tubes and forcing blood to flow in only one direction. The size of the heart is closely correlated with the size of the trachea system.
A number of nerves extend from the brain to the legs, eyes, and the rest of the body. The brain occupies about 20 to 30 percent of the cephalothorax volume. Spiders have several sensory organs with which to sense and react to their surroundings. They have simple eyes, each with a single lens, which are particularly sensitive to movement. Spiders have neither ears nor sense of taste. However, they are able to detect smell with scent-sensitive hairs located on their legs. With the brain and all sense organs, spiders are sharp hunters.
With its enormous strength, spider silk is an extraordinary material. A thread of silk the thickness of a pencil has enough strength to stop a Boeing 747 flying at full speed. Humans simply do not yet know how to duplicate such a material. Silk threads are produced by several glands located at the spider's abdomen. Every gland produces a thread for a special purpose: glandula am-pulleceae for the silk of the walking thread, glandula pyriformes for the attaching threads, glandula acinoformes for the encapsulation of prey, glandula tubiliformes for the thread of cocoons, and glandula coronatae for the adhesive threads. A thread is made up of polymerized protein molecules. The smallest measured thread was only 0.02 micrometer yet a web made up of it is capable of stopping a bee flying at full speed. The thread is also very elastic and can be stretched 30 percent without breaking. Spider webs take a variety of shapes and function to trap prey, produce cocoons, and provide hiding places for the spiders.
Male spiders are often smaller and more colorful than the females. Males can also be recognized by what appears to be a fifth pair of legs. These are actually palps with bulbs for injecting their sperm into a female during mating. During breeding season, males search for females. Once the female is found, the male has to avoid being mistaken for prey by the female. Male spiders of different species use different ways to announce to the female that they are interested in mating. If the signals are right and the female is ready, mating occurs. After the mating, the males of some species must be extremely careful or they will become an easy meal for the female. The females lay their eggs and tend the young. The kingdom of spiders goes on.
See also: Arachnids; Arthropods; Circulatory systems of invertebrates; Food chains and food webs; Molting and shedding.
Emerton, J. H. The Common Spiders of the United States. Reprint. New York: Dover, 1961. An extensive review of identified spiders, their distribution, and habitat within the United States.
Foelix, Rainer F. Biology of Spiders. 2d ed. New York: Oxford University Press, 1996. Thorough and in-depth examinations of anatomy, physiology, and many other aspects of spiders.
Mascord, Ramon. Australian Spiders. Rutland, Vt.: C. E. Tuttle, 1970. Provides useful information on spiders found in Australia.
Roberts, Michael J. Spiders of Britain and Northern Europe. New York: HarperCollins, 1995. Descriptions of common spiders inhabiting Great Britain and northern Europe.
Simon-Brunet, Bert. The Silken Web. Chatswood, New South Wales, Australia: Reed, 1994. Details the construction and functions of diverse forms of spider webs.
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