Mr and Mrs Smith's Life Science

Spiders

The first ancestral chelicerates probably evolved about 600 million years ago. They are now distinguished from the other arthropod groups by the possession of (at least) six pairs of appendages. These normally include four pairs of walking legs, a pair of chelicerae and a pair of pedipalps. They have no mandibles, no antennae and the body is divided into two, not three, sections, as in the Uniramia. They are however normally bilaterally symetrical, have a through gut, have uniramous appendages, a non-calcareous exoskeleton and are gonochoristic.

Spiders, numbering some 36,000 known species, belong to a huge group of invertebrates called arthropods. So do insects, crustaceans, centipedes, millipedes, and other animals characterized by paired and jointed legs (which is what the word "arthropod" means), segmented body, and an exoskeleton.

A lot of people think spiders are insects, but the two are only distantly related. Spiders share a closer kinship with scorpions, ticks, mites, daddy longlegs, and other arthropods that have, as their most obvious characteristic, eight legs arranged in four pairs. These eight-legged arthropods are called arachnids. Insects, in contrast, have six legs arranged in three pairs.

Besides having eight legs, spiders and other arachnids have an extra pair of appendages called pedipalps. Pedipalps are a little like hands: they help arachnids feel their surroundings and hold on to prey and other objects.

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.

Silk Spinners
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.

Being a spider means, for the most part, being alone all of your life. That's because spiders, with only a few exceptions, are naturally solitary creatures. They do manage to socialize long enough to court and breed, although even this amount of interaction has its drawbacks for some spiders: after performing their vital services, the males of a few species become the female's next meal. However, by helping nourish the new mom, the male spider contributes to the survival of his offspring.

In Pursuit of Prey
All spiders are carnivorous, and insects make up the bulk of most spiders' food. But just about any small invertebrate – including other spiders – is fair game. Even a few vertebrates, such as frogs, fish, birds, and rodents, occasionally find themselves in the fangs of these formidable predators. (You bet there are some big spiders out there!)

Spiders are amazing food-catching machines. Even the most common methods and "tools" they use to make a living – the basic web, for example – are marvels of evolutionary ingenuity. Here's an overview of some of the ways spiders do what they do best.

Silken Snares
When most people think of spider webs they probably think of the spoked, roundish, and more-or-less regular constructions called orb webs. Although these beautiful webs may look like they'd take their tiny architects all day to design and build, many orb weavers can whip one out in less than thirty minutes. Most orb web spiders spiders build a new web every day, recycling their silk supply by eating the old web.

Orb webs may be the most elegant of the silken snares, but they certainly aren't the only ones. There are lots of variations on the theme, from elaborate tunnels and tubes to the tangled cobwebs that house spiders build in ceiling corners. There's also the minimalist approach of bolas spiders, which manage to catch their dinner on a single silken line that they hurl at passing prey.

Lurking for Lunch
Web weavers are rather sedate creatures much of the time. But when the vibrations of a struggling victim signals a catch, they spring to life and head for the action. Experience and an oily coating on their feet help spiders avoid getting stuck as they skirt across the threads of their web.

Once a spider reaches its prey, it usually subdues the animal by biting it, injecting a paralyzing venom, and wrapping it in silk – or, conversely, by wrapping it in silk and then giving it a venomous bite. If times are plentiful and the spider isn't particularly hungry, it may save the meal for later. But if it is hungry, it starts digesting immediately – before it even begins consuming it.

Spit and Suck
Pre-digestion is a must for spiders, who don't have a mouthful of teeth to help them break down their food. To start the digestion process, a spider spits up from its intestinal tract a drop of liquid and deposits it onto the prey animal, momentarily marinating it in digestive juices. Then, with help from powerful contractions in its throat and stomach, the spider sucks down a portion of its liquified meal. It repeats this "spit and suck" process until nothing but the hard, indigestible parts of the victim remain.

Webless Wanderers
About half of all spiders don't build webs to catch their meals. Instead, they either lie in ambush for their prey or, in a few cases, they actively stalk it. These webless spiders are often called "wandering" spiders, a reference to the fact that they are less sedentary (though not by much, in some cases) than their web building relatives.

Many wanderers do build a kind of silken nest – either wedged among vegetation or in a shallow burrow – but this nest doesn't serve as a bug snare. Instead, it's a hiding place, or retreat, within which the spider waits for passing prey. When it sees or feels movement nearby, the spider rushes out of its retreat, pounces on the animal, and delivers a paralyzing bite. Then it uses the same basic feeding techniques as web weavers, digesting the animal in advance and sucking in its liquid meal.



Spider Moms
Within a few weeks after mating, female spiders are ready to lay their eggs. Many enclose the eggs in a silk sac, called an egg case, that protects them and maintains the correct temperature and humidity for their development. Other females forego building an elaborate case, instead laying the eggs inside their retreats and covering them with silk threads.

Female spiders lay anywhere from a few dozen to several hundred eggs, depending on the species. Once the eggs are laid and the egg case complete, some spiders move on, leaving the future of their progeny to the whims of chance. Others stay with the egg case and guard it until the eggs hatch. And a few, such as wolf spiders, take mothering much further: they carry their egg case, attached to their spinnerets, wherever they go. Then, for a week or so after her spiderlings hatch, a female wolf spider carries her young around too — as many as a hundred or so, all crowded onto her back.


Up, Up, and Away
For many spiders, life starts out with a far-flung adventure. After they hatch, and when they're little more than speck-sized, the spiderlings travel with the wind to strange new lands on a tiny silk filament that they spin for this special purpose.

This spider "flight," called ballooning, can take young spiders high into the atmosphere (ballooning spiders have been caught on airplanes!) and hundreds of miles from their place of origin. Many of the spiderlings don't make it — they end up in water or in a hungry bird's belly, for example — but enough survive to set up shop wherever they may land.