Biologists classify spiders as predators (animals that eat other animals). Most spiders are very small and inconspicuous, so we often fail to realize how really common they are. Estimates of spider densities in natural environments range from over eleven thousand to nearly three million individuals per acre! Because of this, it is no surprise that they contribute to the natural control of insect populations. These tiny predators are a vital component of natural ecosystems, their prey include a variety of invertebrate animals, many of whom are pests. Perhaps owing to the fact that most species do not specialize on a particular prey type, spiders are rarely intentionally employed as agents in the biological control of agricultural or garden pests. Recent research has demonstrated the importance of spiders in controlling pest outbreaks.
Spider reproduction depends upon the amount of food that they have eaten, thus a spider population often reflects the number of prey in a given area. For a particular type of spider there is a wide range in the number of young which are produced. In one species I documented that the range of eggs laid by female spiders varied between 4 and 850. An average female spider produces about 100 eggs but some species produce over 3000 eggs several times during a lifetime. The young spiderlings can travel great distances by a remarkable form of flight called ballooning. The tiny juveniles climb to the top of a stem or blade of grass and release long threads of lightweight silk. As the breeze catches these fibers, the spider is launched into the air, sometimes to fly hundreds of miles. Spiders have been captured in special traps thousands of feet up in the air and are often the first animals to arrive in a disturbed habitat (recently plowed field, burned area). For example, spiders began to colonize the slopes of Mt. St. Helens only months after the volcanic ash cooled. Spiders have many enemies. Spiders are not only cannibals but they are also the principal prey of a variety of different wasps. In addition, they are commonly eaten by birds. When we apply pesticides, we inadvertently kill many of the beneficial organisms as well; including large numbers of spiders.
Scientists have described about 37,000 types (species) of spiders worldwide. In North America alone, we have at least 3,800 different species. In Ohio, over 520 species have been recorded, but there are undoubtedly many more types to be found here. Most people know that spiders have eight legs; did you know that most types have eight eyes too? In addition to their walking legs, spiders have a pair of short leg-like "palps" located at the front of the body. These palps are used by the males to transfer sperm to the females. Some other animals with eight legs are sometimes confused with spiders. Perhaps the most common animal that is confused with a spider is the "daddy-long-legs" or "harvestmen" which are related to spiders but have only one major body division (spiders have two; a combined head-thorax and an abdomen). Ticks and mites also have eight legs and one major body division.
Comparison of Arachnid body forms
One way to classify the spiders is by their predatory strategies. Not all spiders spin webs to capture prey. Some species are active hunters, others wait at the entrance to their burrow or at a flower for insects to pass nearby. Here are a few of Ohio's most commonly observed spider types: arranged by the type of snare that they construct to capture prey.
Very few spiders are known to bite humans. Nearly all spiders are venomous, but few are dangerously poisonous. Most spiders will attempt to escape, but they may bite if cornered. Bites occur most frequently because a spider is trapped in a glove, shoe or fold in clothing. Even though most spider bites are not serious, if you are bitten you should seek medical attention as soon as possible. It will help the doctor if you can take the offending spider with you so that it can be identified. Capture the spider in a tightly sealed container if possible. Even a squashed spider may be identifiable if all of the parts can be saved.
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