Wild Side: Jumping spiders

A female Phidippus princeps waiting in ambush. — Matt Pelikan

Many people feel a visceral revulsion to spiders. And while the risks are vastly overestimated among the general public, the fact that a few species of spiders are dangerously venomous has given the whole the whole order a bad name.

But one spider family — Salticidae, the jumping spiders — breaks the rule by being almost popular among humans. Active, diurnal, fuzzy, and equipped with big central eyes like a cartoon puppy, jumping spiders seem almost like mammals; even arachnophobes may find them appealing. Some people even keep them for pets. But beyond their curb appeal, jumping spiders are highly evolved, formidable predators that routinely subdue astonishingly large prey items.

A female Phidippus princeps subduing a grasshopper nymp. — Matt Pelikan

Salticidae is a large family, indeed the largest among the spiders, comprising about 15 percent of all known spider species. Some 5,300 species have been described worldwide (many surely still await discovery), and more than 300 occur in the continental U.S. These spiders range from barely a millimeter in length to about an inch. Across this diversity, most species display a common body type, rounded, chubby, and rather short-legged by spider standards, making this group easy to recognize. But there are notable exceptions, such as jumpers that have evolved to resemble ants or even scorpions.

In the case of most jumping animals, leaping ability derives from an elongated pair of legs — long lever arms driven by powerful muscles (think of grasshoppers or kangaroos). But jumping spiders have evolved a totally different mechanism to power their impressive leaps: Contracting part of their bodies, jumping spiders raise the pressure of their internal fluids. Lymph forced into the legs snaps the legs straight, launching the spider into the air. These spiders, literally, jump by means of hydraulic power.

Random jumping is not much use, so jumping spiders have a targeting system on par with their leaping ability. A large, forward-facing pair of eyes (one of four pairs each jumper has) are highly evolved for precise vision. These eyes produce a high-resolution image (even a magnified one, in some cases), and respond to an extraordinarily wide range of light wavelengths. Special adaptations in the retina allow precise judgment of distance, and these eyes guide jumping spiders with pinpoint accuracy.

Jumpers tend to be fairly distinct in appearance, sometimes boldly patterned or colored. So this group lends itself to amateur study: A decent photo or even naked-eye view is often enough to identify a jumper, if you know what to look for. How many species of jumpers occur on the Vineyard is unknown, though the number is surely in the scores at least. Still, as is often the case with wildlife, a majority of the individual jumpers I encounter belong to a relatively short list of species.

I’ve found Island jumping spiders in habitats ranging from barrier beach to wet morainal woodland. These spiders are common in the garden, and even in the house: For some reason, some species find window screens to be congenial places to hang out. In part thanks to their body hair, which in some cases sports sticky tufts on the ends, jumpers can cling to nearly any surface (including glass), so look for them on walls and ceilings as well as on the floor.

While hunting techniques vary somewhat among species, the basic plan involves either slowly patrolling a promising area, or waiting motionless in ambush for suitable prey (meaning nearly anything less than two or three times the size of the spider itself). Jumping spiders stalk their prey, aligning their bodies with their target; when the victim is within range, the spider pounces, in a bound that may cross several inches and seems to happen instantaneously. They rarely miss, and once their fangs inject a first squirt of venom into the prey, the game is over.

Few jumper species build anything like a web, though they do produce silk. Often a leaping jumping spider trails a thread of silk as it springs, allowing self-rescue if the spider jumps over an edge or needs to disengage from an unexpectedly tenacious target.

Jumping spiders stage elaborate mating dances, in which bright colors or fluorescence (usually of the male) often plays a prominent role. Fertilized females lay eggs, usually in silken sacks, in a variety of settings, from in the open to under flakes of bark.

Many observers comment on the apparent intelligence of jumping spiders. While I doubt they’re very good at algebra, they do show very flexible behavior. Pet ones, for example, readily grow accustomed to being handled by humans. And wild jumpers seem able to map out large areas of habitat with great precision, moving around to exploit especially promising hunting areas.

As fearsome as they are to their prey, jumping spiders are docile animals, running rather than fighting when disturbed, and often tolerating respectful handling. They pose no medical risk to humans, and in a domestic setting, provide useful control of problematic insects like pantry moths, ants, or carpet beetles. If you find one, take a moment to appreciate its unique adaptations and appealing behavior.