What are the characteristics of spider animals? Information on the structure, general characteristics, life cycle and living habitat of spiders.
SPIDER, any eight-legged arthropod of the order Araneae, class Arachnida. Most abundant of all the arachnids—with about 40,000 species—the spiders are dominant predators best known for the spinning of silk from their bodies. They exist in vast numbers in all land habitats and consume insects as their principal food. They occur as permanent residents from polar ice to the depths of the hottest jungles and have been observed at heights above 20,000 feet (6,000 meters). One species, the water spider of Eurasia, has successfully invaded the water realm. It lives beneath the surface for long periods by carrying bubbles of air down to fill its diving-bell domicile. A few marine spiders live in watertight chambers in certain corals covered over at high tide, and they feed on small aquatic animals between high tides.
The Silk of Spiders.
The success of spiders as predators is based largely but not entirely on their use of silk. This material is produced within the abdomen in voluminous glands and dispensed through tiny spools and spigots on pliable, fingerlike, spinning organs called spinnerets. The silk is a fibrous protein that hardens into the familiar thread as it is ejected from the spinnerets. It is noted for its strength and elasticity. Viscid silk is produced in some glands and remains sticky for long periods. Man finds use for spider silk, which is 0.0002 inch (0.005 mm) thick or finer, as hairlines in lenses of surveying and laboratory optical instruments. Its use for clothing has been tried but is not commercially feasible.
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Most spider threads are double, since the simplest line usually comes from two spinnerets. Other threads frequently are composite lines drawn from different glands to produce simple draglines, attachment disks, covering sheets for egg sacs and retreats, dry swathing bands, viscid swathing films, viscid lines and drops, and many other kinds of silk. Most spiders never venture anywhere without putting down a dragline thread, which acts as a lifeline to prevent falls and, for many species, as a balloon on which to fly on the air currents for long distances. Many kinds of spiders spin intricate webs as snares to capture flying and jumping insects—a remarkable achievement paralleled among the other invertebrates only by the larvae of a few caddis flies and of true flies of the family Mycetophilidae.
Place in the Animal Kingdom.
Although often identified with insects, the spiders are only remotely related to them. The near relatives of spiders are scorpions, harvestmen, mites, whip-scorpions and other land arachnids. More distant ones are the aquatic king crabs and the fossil eurypterids. Fossil spiders are found in rocks of the Devonian Period, about 400 million years old, were well developed in the Carboniferous Period, and occur abundantly in Cenozoic amber.
Ancient animals already well developed when insects were coming into being, spiders became increasingly specialized and diversified through the ages to utilize the abundant food supply these creatures provided. Some specialize on ground and subterranean insects, others hunt them on herbs and shrubs, and still others spin webs to entangle flying forms. On the other hand, the spiders’ principal enemies are also insects—tiny wasps that parasitize spider eggs and larger, solitary wasps that are exclusive predators and use paralyzed spiders as provisions for their developing young.
Spiders and Man.
The Greek word for spider, arachne, recalls the myth of the Lydian maiden Arachne who, daring to challenge the goddess Athene to a contest in weaving, was turned into a spider and doomed to perpetual spinning. Long considered creatures of mystery and power, spiders often played a benevolent or potent role in ancient legends. More practically, spiders play an important role in nature as consumers of insects—sufficient reason for them to be valued by man. Their bodies even contribute much-needed proteins and fats otherwise not obtainable to the diet of some peoples.
Nevertheless, most modern men regard spiders with aversion because of the cobwebs they spin and their reputed venomous properties. The fact is that spiders rarely bite man, and their venoms have small effect, for the most part, on warm-blooded animals. The bites of very large spiders such as tarantulas can be painful, but most spider bites do not even break human skin. Hematoxins and neurotoxins in the venoms of a very few species cause local and systemic symptoms that are of variable severity in adults and graver in children, occasionally even causing death. In temperate regions the only spiders to be feared are the black widow and the brown recluse, which have neurotoxic venoms. In the tropics several other kinds are dangerous, notably the wolf spiders of southern Brazil and the burrowing tarantulas of Australia. A serum has been prepared for the wolf spider bites that quickly clears up the severe local lesions.
THE STRUCTURE OF SPIDERS
As in other typical arthropods, the body of a spider is encased in a stiffened outer covering. The body consists of two principal parts, the céphalothorax and the abdomen, each provided with distinctive appendages. The two parts are joined by a narrow waist, or pedicel.
Cephalothorax.
The cephalothorax represents a fused head and thorax. It is bounded above by a hardened shield, or carapace, which is ordinarily convex but may be flattened, smooth, or ornamented with lobes, spines, and prominences. The deep pits on the carapace in some species are sites into which the chelicerae of the females are fitted during the mating.
The head portion of the céphalothorax is bounded by more or less distinct grooves. It bears the eyes near the front edge. Most spiders have eight simple eyes resembling the simple eyes of insects, but some kinds of spiders have only six, four, or two eyes. A few cave spiders, in fact, have lost all their eyes or retain only vestiges of them. In some male dwarf spiders the eyes are carried to considerable heights on grotesquely formed spires. The floor of the céphalothorax is a flattened and frequently heart-shaped plate called the sternum. In front of it is the small lower “lip,” or labium, which forms the floor of the mouth. Around the sternum and in the space between it and the carapace lie the basal segments, or coxae, of the legs and of the two six-segmented, leglike appendages called pedipalpi that are situated on either side of the mouth. In most spiders the coxa of each pedipalp is fitted with an enlarged, sharp plate, the enaite—sometimes incorrectly called the maxillary plate—which aids in breaking the body of prey.
Chelicerae.
Directly beneath the front end of the cephalothorax are two appendages called chelicerae. These are the offensive weapons of spiders. It is believed that the chelicerae are derived from the same pair of primitive appendages that in crustaceans became the second antennae. Each chelicera is two-segmented. The basal segment is stout and often margined by a toothed groove at the farther end, while the outer segment is a movable spine, or fang, lying in a groove when at rest. The sharp fang is the part thrust into the prey, and near its end is a tiny opening through which venom flows into the wound. The poison glands, entirely contained within the basal segment in the tarantulas, extend farther back into the head as more or less voluminous pouches in true spiders.
Legs.
Each walking leg consists of seven segments named as follows, beginning with the one that joins to the body: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. At the tip of the tarsus are found two or three claws. The legs vary considerably in length among spiders. Some of them are long, fine stilts between which the body hangs, while others are stubby props. In walking, the spider moves the first and third leg of one side in conjunction with the second and fourth legs of the other side of the body. The remaining legs of both sides go into action while the other series is at rest, and the creature advances step by step.
Abdomen.
The juncture between the cephalothorax and abdomen is made by a narrow waist, or pedicel, that is the first true abdominal segment. Through this narrow channel pass the ventral nerve cord, a large artery, part of the midgut, and tiny trachael tubes. All of these structures are essential to maintenance of life in both body parts. Ordinarily the abdomen is a saclike structure without visible segmentation. Although covered by a hardened cuticle, it is usually much softer than the cephalothorax. In primitive spiders the top of the abdomen is armed with hard plates, called tergites, that indicate the original segmentation of ancestral forms. In many modern spiders the abdomen is covered in whole or part by a single plate and frequently armed with curious spines and processes, some of great length. This armor may discourage birds from eating them. In other spiders the abdomen is sometimes drawn out into a long tail, giving the animal a wormlike appearance. The under side of the abdomen rarely bears conspicuous prominences. Near the base of the abdomen—depending on the species —may be two openings to so-called book lungs, or air-filled sacs with numerous leaflike folds, and between them the genital opening. Farther back may be present a second pair of book lungs, a pair of tracheal spiracles, or a single median spiracle near the spinnerets. This latter condition is the most frequent one in higher spiders.
The copulatory organ of the mature female, the epigynum, is located in front of the genital opening beneath the base of the abdomen and takes many forms. At the tip of the abdomen is the anal tubercle with the anal opening. The chief excretory organs are coxal glands that collect waste into large saccules, pass it through convoluted tubes, and void it through the anal opening. The remarkably diversified silk glands in the abdomen, like the venom glands in the chelicerae, are thought to be modified coxal glands, homologous with the nephridia of roundworms. The only appendages of the abdomen are the fingerlike spinning organs called spinnerets, usually located below near the tip. These are believed to be derived from two-branched abdominal appendages that were used as swimming organs by ancestral forms.
GENERAL CHARACTERISTICS
All spiders are predaceous and none are parasitic like their relatives, the ticks. Spiders subsist on the body juices of living animals, chiefly insects, which are subdued with the spiders’ venom. The sharp edges of the endites and the chelicerae are used to crush and break the fresh body of the prey. At the same time, the body of the prey is bathed with digestive fluid. In this way the softer parts of the animal are broken down and predigested to a liquid state. The liquid is then sucked into the intestine by powerful muscles. It is doubtful that spiders ever imbibe solid material through the small mouth. Even small snakes, birds, and mammals are first reduced by the powerful digestive juices.
Sense of Touch.
The sense of touch is remarkably developed in spiders. It is centered in the more or less luxuriant covering of hairs and spines on their bodies. Many setae are extremely sensitive to touch and vibration, and some may be receptors for various chemical stimuli. Although the spider has poor eyesight-except for some of the diurnal hunters—and perhaps does not hear or smell, it has a keen knowledge of its surroundings by means of the sensory hairs.
Life Cycle.
Spiders lay eggs, cover them over with silken sheets, and mold the mass into an egg sac, each characteristic of the given species. The average number of eggs is probably less than 100, but some large spiders lay nearly 3,000 at one time, while minute species lay only 1, 2, or a few. Spiders develop gradually, as do grasshoppers, and resemble the adults through most of their early life except as larvae before the first molt inside the cocoon. They undergo from three to a dozen molts before they finally become adult. Tarantulas mature very slowly, requiring 9 or 10 years, and the females live as long as 25 or 30 years. Most northern spiders live a single year.
Courtship and Mating.
Instead of transferring male semen directly to the female by contact of genital openings, the male spider has developed an extraordinary sexual organ on the ends of the pedipalpi. This intromittent organ is comparable in general with a syringe or a hypodermic needle, and around it are protective sheaths and structures to facilitate the pairing. There is no internal connection with the gonads, so the semen must be transferred from the orifice to the copulatory device. To accomplish this, the male spins a little sperm web, deposits a globule of semen upon it, and then sucks the globule into the syringe in each pedipalp. The female has developed paired pouches for the storage of semen and retains the fluid for long periods, often fertilizing batches of eggs over long intervals. This has contributed to the incorrect belief that» spiders can also develop parthenogenetically.
The male sometimes runs considerable risk in approaching his mate, who is usually much larger and may be merely hungry and not ready for mating. Some males are killed because they fail to approach the female correctly or because, after being successful in mating, they fail to leave the premises before normal predatory instincts again dominate the female. However, this result is by no means the rule, as is sometimes supposed. Various routines have been devised by groups of spiders to gain recognition. The jumping spiders, with their good eyesight, posture and dance before their prospective mates, each in a characteristic manner, while the web spinners tweak the threads of the web in a certain manner until they are allowed to approach.
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