THE WORLD OF LIFE with Douglas Drenkow

The "cephalothorax" (front body region) of an arachnid, never with antennae or wings, typically bears several pairs of simple eyes, one pair of often fang-like (sometimes poisoned) "chelicerae", one pair of often leg- or claw-like "pedipalps", and four pairs of true legs. The "abdomen" (rear body region) is often broadly joined to the cephalothorax.

In such arachnids as scorpions or spiders, gases are exchanged through "book lungs", which open through slits on the outside of the body and have many "leaves" inside.

The body of a daddy-long-legs, or harvestman, is small, with long, thin legs. There is no distinct "waist", where the cephalothorax joins the abdomen, which is visibly segmented underneath (Compare these features with those of spiders, below, some of whom -- "daddy-long-legs" spiders -- otherwise look very similar).

The body of a scorpion is large, usually slender. There are one pair of leg-like "pedipalps", with claws, and four pairs of true legs. The long, segmented abdomen forms a tail with poison sting.

The body of a spider bears four pairs of legs and sometimes a pair of leg-like pedipalps. The "waist", where the cephalothorax joins the unsegmented abdomen, is fairly narrow. Silk is often spun by "spinnerets" at tail end of abdomen (Silk is a liquid protein that hardens into a highly flexible, yet extremely strong fiber when drawn out into the air). Colors range from pale or drab to dark or colorful, often with contrasting markings.

The body of a mite is typically microscopic and oval. There are almost always four pairs of legs. The "waist", where the cephalothorax joins the unsegmented abdomen, is broad. Colors range from pale or drab to dark or colorful.

The body of a tick is similar to that of a mite but typically larger, up to about 1/4" long.

METAMORPHOSIS

Young arachnids typically look like small adults.

During their mating dance, a male scorpion deposits on the ground a "spermatophore", which is taken-in by the female. Female scorpions brood their fertilized eggs within their body and carry their young on their back.

After a species-specific identification procedure (such as a highly visible dance for a hunting spider or a telltale plucking of the web for an orb-weaver), a male spider typically transfers sperm from a special receptacle in his leg-like "pedipalp", which he has filled with semen, into the "seminal receptacle" of the female. The females of only a few species of spiders, such as black widows, eat their mates; although because male spiders are typically smaller than the females of their species, the males undoubtedly survive in large part because of the elaborate, species-specific courtship procedures.

Although they cannot truly fly (as winged insects can), many young and other small spiders "balloon", or "parachute": The spider climbs to a high point, lets out a line of silk, and floats away on the breeze to faraway places (Clouds of such silken lines torn loose by strong winds form the "gossamer" that fills the sky on clear fall days). Silk is also used by various spiders for communication lines between potential mates, cocoons for eggs (as guarded by the females), or linings of burrows for the rearing of the young.

Although most lay eggs, some mites give birth to already hatched young and can, thus, build-up huge populations in short periods of time. Except for some wormlike "gall mites", with fewer legs than normal, the "larva", or hatchling, of a mite or tick has three pairs of legs; it gets its fourth, final pair after its first molt: At this point it becomes a "nymph", which -- after subsequent growth and molting -- becomes an adult.

HABITATS

Arachnids are extremely abundant on the land.

Scorpions hide during the day but search for arthropod prey at night, in their usually warm and dry habitats.

Spiders are some of the most common of all animals, found especially in shady, humid spots, such as under the leaves of plants. Silk is sometimes used by wandering spiders for path markers or for "draglines", which act like the safety lines of human mountaineers.

Mites are extremely common in soil, litter, leaf mold, and (occasionally) plant galls; under stones and bark; and even in fresh- and salt-water. All of these habitats have moisture, at least as humidity: Mites are very small and, thus, very vulnerable to drying out. Some mites, like all ticks, live on or in other animals as parasites.

FOODS

Most arachnids prey on other small invertebrates, especially insects.

The largest of spiders (such as tarantula bird spiders or fishing spiders) may capture and eat small vertebrates; but most spiders prey on insects, usually in considerable variety (although beetles with hard shells, ants, and wasps are eaten the least). Typically, a spider uses its fangs to grab its prey and inject it with a paralyzing or deadly nerve-poison. Spiders typically drink in the contents of their prey's body, liquefied by digestive enzymes either regurgitated onto the prey or injected by the fangs.

Silk is used by various spiders for constructing shelters from which to ambush prey or for fashioning stationary or thrown snares. The familiar orb webs are particularly effective at catching flying insect prey. Web spiders are usually less selective than hunting spiders in their choice of prey. Because web spiders respond to vibrations and hunting spiders respond to sighted movements, only live prey is taken by either type of spider.

Insect flight probably evolved as an escape from hunting spiders, on the ground; and web spiders probably evolved in response to flying insects.

Each species of mite has its own feeding habits -- scavenging wastes, preying on or parasitizing a select group of invertebrates or vertebrates, or sucking sap from a few to many species of plants.

DAMAGES/BENEFITS

So very abundant, arachnids are vital to maintaining the "balance of nature" (Outbreaks of insect pests often result from sometimes unintentional harm to populations of arachnids).

Although scorpions typically use their sting to prey on various insects, a few species are dangerous to us human beings.

Although a few spiders are dangerously poisonous, although "cobwebs" may be a nuisance, and although spiders are generally thought of as "creepy", spiders probably eat more insects -- often pests -- than do all the world's birds! If such valuable insects as honey bees are protected from spiders (especially orb weavers), these predators usually do much more good than harm in farms, groves, and gardens.

Some mites decompose and, thus, recycle natural materials; plant "parasites" sometimes make leaves turn yellow and fall off; animal and human parasites can cause serious injury to their "hosts"; and still other mites are predators or parasites of other invertebrates, often pests, sometimes beneficial (such as honey bees).

The feeding of ticks on vertebrates can cause irritation or even paralysis (from a venom injected by the ticks); and ticks are notorious for spreading blood-borne diseases (caused by various micro-organisms), such as spotted fever, relapsing fever, tularemia, Texas cattle fever, and Lyme disease.

Arthropods (Arthropoda)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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