Missouri is home to more than 300 kinds of spiders. Some are the size of a pinhead and are easily overlooked. Others are surprisingly large, with a leg span of 4 or more inches.
Spiders, along with ticks, mites, harvestmen and scorpions, belong to the class Arachnida. Unlike insects, which have six legs, spiders have eight. They have no antennae and two-piece bodies. A spider has silk-spinning structures called spinnerets at the back end of its abdomen, and it usually has eight eyes of various sizes and shapes grace its face.
A spider's mouth parts, too, are different from an insect's. Instead of mandibles capable of chewing, spiders have fang-tipped jaws called chelicerae. With these, they pierce their prey and inject a venomous fluid that immobilizes it. Digestive juices dissolve its internal tissues. The spider's small, tube-like mouth, aided by strong abdominal muscles, pumps and sucks the victim until it is a shriveled husk. A strong-jawed spider, such as the yellow garden spider or the tarantula, often mashes its prey between its chelicerae while ejecting digestive juices over it.
Size is helpful when determining the two suborders of spiders, although other characteristics are more diagnostic. The Mygalomorphae, which includes the tarantulas and trapdoor spiders, are generally large, with stout bodies, stout legs and jaws that move vertically. They also tend to be long-lived, some up to 25 years. The Araneomorphae, which includes garden spiders and orb weavers, generally have thinner bodies, spindly legs, and have jaws that move horizontally. The majority of spiders in Missouri belong to this suborder.
Spiders live in virtually every type of habitat in Missouri—and in staggering numbers. US arachnologists have estimated populations ranging from 30,000 spiders per acre in Mississippi woodlands to more than 2 1/2 million individuals in a grassland acre.
All spiders are potential predators on many arthropods, especially the insects. Most prey upon grasshoppers, flies, moths, caterpillars, leafhoppers, some bees and ants, and other spiders. Spiders eat more insects than birds and bats combined. Because of this, spiders are a boon on agricultural lands, destroying huge numbers of crop-damaging insects. Since each spider in a field may consume a least one insect per day, their cumulative effect on insect populations is significant.
The worst enemies of spiders usually are other spiders, but some insects, like the assassin bug and mud dauber wasp, prey upon them, as do bats, shrews and birds. Some orb weaving spiders construct a zigzag pattern of silk, the stabilimentum, at the hub of their webs which, scientists hypothesize, may deter birds from flying into the silk structure. But it might also help birds locate an orb weaver in order to prey upon it.
While some Missouri spiders rarely live longer than a year, others can live up to two or three years. Some hibernate in winter under tree bark or rocks, or in cellars and attics. But many die within one warm season, leaving the future to over-wintering spiderlings or a brood of encased eggs. Spiderlings emerge in early spring from their winter hiding places or from egg sacs suspended from vegetation or from flattened silk sacs constructed on leaves or in flower heads. Some spiders leave egg sacs in burrows under rocks, while others, such as wolf spiders, carry the nursery with them.
To disperse most (although not all) young spiders travel by climbing to the tops of grass blades, fence posts or shrubs, elevating their abdomens and throwing out silken threads. Caught by the air currents, the tiny arachnids appear to fly, although spiders never develop wings. This is known as ballooning.
Spiders grow by molting, or ecdysis. In this process, the spider casts off its tight outer body cover—its exoskeleton—after secreting a new, larger one underneath. Spiderlings gradually develop into adults in this way. Some have peculiar color patterns that change as they approach adulthood. Few spiders molt after sexual maturity, but some, such as female tarantulas, do.
All spiders exhibit similar premolting behavior. They do not eat, become lethargic and retreat into silken molting quarters in a burrow, under a leaf or in a corner. The outer skeleton splits along the upper body portions and the spider gradually slips its body and legs from the old casing, much like taking off a skin-tight glove. The actual molting process varies among species and can take from less than 15 minutes to a full day. Molting spiders are particularly vulnerable; they are unable to move away or fight back because they must rest until their new exoskeletons harden.
Identification of spider species is generally difficult for the novice and expert alike. Spider classification is based on external structures that include eye arrangement, number of hairs and claws on the legs and the complicated structure of reproductive organs. Understanding the specialized technical vocabulary in many spider keys often requires an arachnologist's help. The field entries listed below will help you identify some of Missouri's more common spiders.
Your next woodland walk offers the opportunity to get acquainted with these interesting creatures, which are so undeserving of their dreadful reputations. After all, a spider acts as a spider would.
All spiders have silk glands, although not all use silk in spinning webs.
When drawn and stretched from the spinnerets, located at the rear of the abdomen, the liquid silk solidifies into tiny strands that are both strong and elastic.
Spider silk is stronger (greater tensile strength) than a thread of steel of the same weight.
Spiders use silk for web building, for capturing prey, for sperm transfer, for lining hibernating, molting or living chambers and constructing egg cases, for draglines and mating bowers.
Most spiderlings use silk for wind-borne travel.
Many people dread the thought of a spider bite, but few in the United States are bitten by spiders and even fewer die from them. Relatively few spiders are capable of piercing human skin. While it is true that all Missouri spiders have venom, the toxicity varies with the species. Most spider venom is harmless to humans but may cause temporary skin discoloration, irritation or swelling, much like a mosquito bite. Large spiders are capable of inflicting a painful bite but rarely do so unless consistently provoked. Spiders generally flee upon human approach, contrary to the tales of science-fiction writers.
Of the two potentially harmful spiders found in Missouri, you are more likely to encounter the brown recluse than the black widow. Both can inflict bites that can cause severe pain and infection. Deaths due to black widow bites are extremely rare, and no deaths have been proven to be caused by brown recluse bites. Deaths attributed to spiders usually occur in individuals who have an extreme allergic reaction or immune deficiency to the spider's venom.
Although the chance of being bitten by a venomous spider is extremely slim, you can reduce the likelihood by heeding the following:
Collecting spiders is a good way to get to know them better and appreciate them more. If you hunt with a camera, you may capture some splendid photographs of these colorful and unusual creatures.
Many spiders are active at night, and you can find them by shining a flashlight into vegetation. Wolf spider eyes reflect light, causing a night meadow to appear to twinkle.
You can easily capture spiders using a small glass jar. You can also use a pillowcase or canvas net to sweep vegetation, or to place under bushes as you shake them.
Collection for scientific, school or personal research sometimes requires the preservation of specimens in 70-percent ethyl or rubbing alcohol. If you pursue such studies, remember to make careful notes of the spider's location (on plants, rocks, soil, etc.), habits, appearance and coloration prior to capture, and any further information that may help you to identify your specimen. Remember, although specific spider identification can be difficult, the broad categories are more easily distinguished.
Spiders have been residents of the earth for 400 million years. Primitive spiders found preserved in amber look remarkably similar to our present-day species. Whether the prospect of observing these eight-legged creatures excites you or not, one thing is certain—spiders are here to stay.