But there's more to a spider than eight legs and pedipalps. Here's a look at some of their other characteristics.
Basic Body Plan
Spiders have two main body parts: the prosoma (also called the cephalothorax) and the abdomen (also called the opisthoma). These are joined by a short, narrow stalk called the pedicel. A spider's eyes and chelicerae (its jaws, which are equipped with venom glands and fangs), are on the prosoma – there's no separate head. A spider's silk-releasing organs, called spinnerets, are on the far end of the abdomen.
The number of body parts helps to distinguish spiders from other arachnids and arthropods. For example, daddy longlegs, those spindly-legged arachnids often confused with spiders, have only one body part – the abdomen. Insects have three – head, thorax, and abdomen.
The Eyes Have It
Most spiders have eight eyes, arranged in patterns that vary for particular groups of spiders. An expert can often identify a spider just by looking at its eyes.
Interestingly, just because a spider has lots of eyes, that doesn't mean it has good vision. In fact, by human standards, most spiders have lousy eyesight. But great vision isn't particularly important for the spiders that build webs – at least, not when catching a meal is concerned. Their prey, after all, comes to them. However, spiders that actively stalk and hunt down their prey have excellent vision.
Spiders aren't the only arthropods with the ability to produce silk. Certain insects, such as silk moth larvae, do so as well.
Spider silk – made up of protein – is produced in glands inside the abdomen. Each silk gland leads to a particular spigot that opens to the outside through one of several paired spinnerets. A spider "reels out" silk by gently pulling it from a spigot with its two hind legs.
Silks of Different Ilks
Different silk glands produce different kinds of silk for different purposes. For example, female spiders produce a certain kind of silk to create their egg sacs. The webs of many spiders are made from a couple of different kinds of silk – a strong, stretchy silk for the web's basic framework and another, sticky variety that makes getting away that much harder for trapped insects.
Although all spiders make silk, not all of them spin webs to catch their dinner.
Making Sense of Spider Senses
What would life be like if you could taste through your legs and hear with your hair? If you can imagine such a concept, then you might have some inkling of what it must be like to be a spider.
Spiders, in fact, do taste, and also smell, through special sensory organs on their legs, as well as on their pedipalps. And they hear – or, more specifically, they sense vibrations – through hairs and tiny slits distributed over much of their body.
Picking Up Vibes
A spider's sensitivity to vibrations is finely tuned. For example, spiders can distinguish between different types of prey hitting their webs, such as a moth from a fly from a honeybee. This sensitivity to motion "tells" a spider what to expect so it will know how to handle a potentially dangerous meal.
The ability to tell one vibe from another also comes in handy during courtship: the males of web-building species often woo females by plucking a species-specific pattern on the females' webs. If a male simply blundered into a female's web without first introducing himself, he would risk becoming her meal instead of her mate.