Behavior And Ecology

Mating behavior in spiders varies greatly from one group to another and in some cases involves complex courtship displays. Prior to mating, the male typically deposits a droplet of sperm from the genital pore onto a small, silken platform called a sperm web. The droplet is then drawn into the copuiatory organ at the tip of each male palp in a process called sperm induction. Sperm is stored in the palp until mating takes place. Following acceptance of a male's advance by a female, copulation is accomplished by indirect insemination in which the male transfers the sperm from his palps to the spermathecal ducts on the underside of the female's abdomen. Although the males of many species die soon after mating, others refill their palps with sperm and may mate with other females one or more times thereafter. Contrary to popular belief, most males either walk away or hurriedly retreat without being attacked and eaten by the female.

Oviposition typically occurs within a few weeks after mating. The female spins a silken sheet onto which she extrudes fertilized eggs from her genital opening, forming a mass in which the individual eggs are cemented together. She then protects the eggs by spinning multiple layers of silk to form the egg sac. More than one egg sac may be produced, as commonly observed in black widow spiders. Although some species of spiders remain with the eggs to protect them until they hatch, most spiders provide no maternal care and leave the eggs to hatch unguarded.

Spiders have been very successful in exploiting a wide range of ecological habitats, including islands. This is accomplished, in part, by their specialized dispersal behavior called ballooning. Spiderlings, and even adults of many small spiders, are carried aloft on silken threads by which they may be conveyed by wind currents over considerable distances. To become airborne the spider orients into the wind, lifts the tip of its abdomen, and extrudes a strand of silk from the spinnerets. When the strand is long enough to provide enough buoyancy to support its body weight, the spider releases its grip on the substrate and floats away.

Spiders can be categorized into three major groups based on their general behavior and feeding strategies: burrowers, vagrants or wanderers, and web builders. Burrowing spiders usually excavate their burrows in the soil of suitable sites, where they remain more or less permanently throughout their lives. They typically capture prey which comes within reach of the burrow entrance or make short excursions to capture food and return to the safety of the burrow. Among this group are many of the ground-dwelling tarantulas, trap-door spiders, and burrowing wolf spiders. Vagrant spiders tend to wander extensively and may or may not regularly return to a given location where they have constructed a retreat. They do not produce a silken web for capturing food, but instead hunt or ambush their prey. Examples include many wolf spiders, jumping spiders, sac spiders, gnaphosids, and ctenids. These spiders commonly enter homes during their hunting forays and occasionally bite humans. The brown recluse spider also falls into this group. It differs from the others, however, by actually living indoors rather than wandering inside incidentally, and it makes a flimsy web. Web-building spiders construct various silken structures to detect and capture potential prey. The webs may be sheetlike, as in diplurids and agelenids, or more complex trapping webs, like those of comb-footed spiders and orb weavers.

Burrowing and vagrant spiders usually rely on their physical size and strength to capture food, together with sufficiently potent venoms to subdue their prey quickly. The size of acceptable prey items is often positively correlated with the size of the spider itself. Web-building spiders, on the other hand, tend to rely on the use of silk to ensnare or immobilize their prey. They are able to utilize a wider range of prey items, often capturing and feeding on insects and other arthropods much larger than themselves. Those that construct aerial webs also have access to a wider variety of flying insects than do ground-dwelling spiders. The venom of web-building spiders is typically less potent than that of non—web-building species, accounting in part for the fact that bites of even the larger web builders are usually quite harmless.

Recognizing that virtually all spiders possess venom glands, it is not surprising that when they are threatened, species with chelicerae large enough to pierce the skin will bite humans and other animals. In most cases the reaction is minor, usually limited to mild, localized pain and slight to moderate swelling at the bite site. The severity of the reaction is dependent on the species of spider, its size, and the amount of venom injected.

The venom varies greatly among different spider taxa in terms of chemical composition and its effects on different animals upon injection. Components include a wide range of proteases, esterases, polyamines, free amino acids, histamine, and specific toxic compounds unique to individual groups or species. Whereas some venoms are primarily cytolytic, causing the destruction of cells and tissues with which they come into contact, others act as neurotoxins or disrupt normal blood functions. For details on the biochemistry and pharmacology of spider venoms, see Bucherl (1971), Bettini (1978), Duchen and Gomez (1984), and Geren and Odell (1984).

Most problems warranting medical attention are not due directly to bites but to secondary infections. In other cases involving the more venomous species, reactions can be much more severe, occasionally causing deaths. See the following sources for information on venomous spiders in different regions of the world: North America (Wong et al., 1987), South America (Lucas, 1988), Europe (Maretic and Lebez, 1979), South Africa (Newlands and Atkinson, 1988), and Australia (Southcott, 1976; Sutherland, 1990).

The following accounts address spider problems of particular medical importance.

The term tarantism has special significance from a medical viewpoint. It refers to a condition in which individuals allegedly bitten by a "tarantula" spider experience a wide range of symptoms, including tremors, hyperactivity, difficulty breathing, muscular rigidity and priapism (painful penile erection in males), sweating, and uncontrolled crying. In its most extreme form it can lead to fainting spells, delirium, and convulsions. Although descriptions of this syndrome can be traced back as early as Aristotle's writings in the fourth century BC, it was most prevalent in Europe during the Middle Ages. It is believed to have been named after Taranto, Italy, where an epidemic of tarantism occurred in 1370. From there the phenomenon spread throughout Italy to present-day Croatia, Spain, and other parts of the Mediterranean. The only cure was thought to be prolonged and vigorous dancing to special, lively music to induce perfuse sweating and eventual

Allergy Relief

Allergy Relief

Have you ever wondered how to fight allergies? Here are some useful information on allergies and how to relief its effects. This is the most comprehensive report on allergy relief you will ever read.

Get My Free Ebook

Post a comment