Arizona-Sonora Desert Museum Digital Library

Description

This 3 to 4 inch (70 to 100 mm) large bodied, burrowing spider is commonly seen during the summer rainy season in southwestern deserts. The female is usually a uniform tan color. The male has black legs, a copper-colored cephalothorax and a reddish abdomen. Their burrows can be as large as 1 to 2 inches (25 to 50 mm) in diameter, with some strands of silk across the opening.

Distribution and Habitat

The Arizona blond tarantula is typically found in saguaro-dominated plant communities. There are many similar species throughout the desert southwest, but they are difficult to differentiate.

Ecology

Tarantulas are nocturnal predators that never venture far from their burrows unless it is mating season. In winter they plug their burrows with soil, rocks, and silk and survive in a relatively inactive state. During this time the animals live off stored fat reserves.

Tarantulas have an interesting defensive capability in addition to venom. Some of the hairs on the top of the abdomen are specialized for defense. These urticating hairs, as they are called, are tipped with backward pointing barbs. If a tarantula is threatened in any way, it brushes these hairs into the face, paw or other body part of its attacker. Once these hairs are embedded, they are irritating and very difficult to remove because of the barbs.

Life History

Male tarantulas mature when they are 10 to 12 years of age, at which time they leave their burrows in search of females. Upon finding the burrow of a mature female — she's usually at least 10 years old — the male will announce himself by stroking the silk at the top of the burrow and tapping particular sequences that the female responds to. During mating, the male must reach under the female to insert his pedipalp into her gonopore to deposit sperm. He is particularly vulnerable to predation by the female when mating. The male's first pair of legs has a “spur” located behind the knee which he uses to hold the female above him during copulation. After copulation the male makes a hasty retreat. The female lays her eggs in a burrow, sometimes staying with them. The young remain in the burrow until they disperse.

Each time a female tarantula molts, typically once a year, she also molts the lining of her epigynum (the female reproductive structure) where the sperm are stored, so she must mate again before she can produce fertile eggs. The many tarantulas seen on the roads in Arizona during the summer rains (July, August, September) are usually males searching for mates. The male tarantula does not survive long after his summer mating. Sometimes the female makes a meal of the male, or another predator kills him. Sometimes he dies of exposure to heat and cold. Even in captivity, out of harm's way, males only survive a few months after mating.

Spiders and spiderlike animals belong to the class Arachnida.

Arachnids differ from most other arthropods in having no antennae. All adult arachnids have four pairs of legs and have no wings. They usually lack mandibles, having instead fang-like mouthparts to pierce and break up prey.

Spiders are soft-bodied arachnids with two body parts: the fused head and thorax, called the cephalothorax, and the abdomen, also known as the opisthosoma. Spiders have four pairs of walking legs and a fifth pair of appendages, located just behind their mouthparts, known as pedipalps. Pedipalps are not used for locomotion but often assist in touching and maneuvering prey. Each male pedipalp is equipped with a special structure that is used to transfer sperm to the female. This apparatus makes the male palp enlarged; it is often described as resembling a boxing glove. The mouthparts, called chelicerae, each end with a fang. The fang is connected to the venom gland, which enables the spider to inject venom into its prey.

Venom

All but a few species of spiders are venomous. (A notable non-venomous example in the Southwest is the feather-legged spider.) While most spiders are venomous, few are dangerous to humans. In Arizona, only the widows (black widow) and the brown spiders (brown recluse types) have venom dangerous to people. Other species may bite and cause some local swelling, but unless one is allergic to the venom, no medical attention is necessary.

Venom in spiders has two functions: prey capture and defense. In its most common use, spiders bite their prey and inject venom, which immobilizes the prey and starts the process of digestion. Spiders have no teeth and rely on the venom to liquefy their prey in order that their stomachs, known as sucking stomachs, can draw in the meal. Some of the larger spiders, such as tarantulas and wolf spiders, have projections on the inside of their chelicerae that help break the prey into smaller pieces to aid digestion. These hunting spiders typically leave a small pellet of crushed exoskeleton after eating, whereas the smaller web-building spiders leave an empty shell of the former prey, neatly cocooned in silk.

The second function of venom is for defense. A spider that is threatened by restraint or by touching may bite. The spider controls the amount of venom injected and may inject none at all. A black widow could bite when its hiding place under a rock or a log is exposed. People are sometimes bitten by brown spiders that have taken up residence in clothes that have been left piled on the floor or in a tent overnight.

Senses

Spiders typically have eight eyes, although some species have a reduced number. The brown spiders have only six, while some cave-dwelling spiders have no eyes at all. Although most spiders have eyes, only one group, the jumping spiders, rely on their eyes to hunt. Other spiders use their eyes to orient when wandering or to detect motion in the initial stages of prey capture. Spiders are equipped with special hairs, mostly on their legs, through which they feel, taste and hear. Even at a distance, spiders can “feel” where their prey is by the displacement of air around these hairs.

Silk

The diverse use of silk is the most characteristic feature of spider, which have at least four, but more typically six, spinnerets (appendages that spin silk). A spider may have up to six types of silk gland, each producing a different kind of silk. It is actually a complex strand of proteins that is produced as a liquid, and solidifies under tension. Silk is used to build webs, catch food, line burrows, protect eggs, detect prey (as trip lines), and even to aid in dispersal. This dispersal phenomenon is known as ballooning. Spiderlings (baby spiders) climb to the top of an object or plant and let out silk. The spiderlings are so light that, as the air currents take up the silk, the spider is transported, sometimes miles away. Some of the larger spiders, such as tarantulas, are too heavy to balloon, but most smaller species have this ability.

Ecology and Life History

Spiders are predators. They survive by eating other animals such as insects, crustaceans, or even other spiders. Thus when spiders get together it can be potentially life-threatening. But spiders have developed special ways of communicating with each other to avoid cannibalism. Male spiders warn female spiders of their presence by plucking their webs, exchanging chemical signals, tapping the surfaces females are resting on, or producing audible drumming or scrapings. Each species has a specific code, and its use prevents most cannibalism.

Among insects, the primary predators on spiders are spider wasps in the family Pompilidae. Different species of wasps have different strategies, but the result is the same: the wasp lays an egg on the immobilized spider, and when the wasp larva hatches, it eats the spider. Most wasp species do not hunt particular spider species, though they may prefer one over another. Interestingly, the spiders rarely defend themselves when attacked by wasps.

Male spiders, which reach maturity at an earlier age than females, must search for a mate. With the exception of tarantulas, spiders typically mate one time during their lives. The average spider lives for approximately one year, but some of the larger wolf spiders live about two years; and tarantulas may live up to twenty years. The male web-building spider sometimes guards the web of an immature female from other males, mating with her immediately after the molt in which she becomes an adult. Males of other species must go through an elaborate courtship of song and dance before the females will accept them. Once the female has mated, she can store sperm for several months up to a year. She may produce two to three egg cases during that time. Depending on the species, a few to a few hundred eggs may be laid per egg case, but usually each subsequent egg case is smaller with fewer viable spiderlings.

Adaptation to Desert Life

Spiders are fairly tolerant of environmental extremes. They can adjust their body temperatures to be higher or lower than the ambient temperature by, for example, sunning to warm up, or by escaping into the shade or a burrow to cool down. High body temperatures threaten the loss of water as vapor through transpiration; a loss amounting to over 20 percent of the spider's body weight is lethal. Spiders can drink, even from moist soil. Many spiders are nocturnal, which means that they are active only during the cooler, more humid parts of the day. Spiders can survive cold winter temperatures by moving to relatively warm micro-climates such as burrows or leaf litter; they curl up to reduce exposure, become rigid and reduce their metabolic rate.