The main purpose of spider venom is the speedy immobilization of prey. This enables the spider to prey on animals larger than itself, and animals that have dangerous defences, e. g. stings. Once the prey is immobile the spider can kill and eat it or keep it alive, but wrapped safely in silk.
So spiders that prey on large insects or larger prey tend to have faster acting venom than those that prey on small or safe insects. Also a spider can control the amount of venom it injects as it bites. As far as humans are concerned, there are only about 200 species of spider with venom harmful to man; perhaps the most notorious is the Black widow and its relatives, also the funnel web spiders.
Spider venom has been used by man in the past. The American Indians and the Bushmen in Africa used to smear venom on their spears and arrowheads.
About twenty species of spider live in groups, and are considered social, e. g. Anelosimius eximius spins webs that can contain over 100,000 individuals. These spiders share foraging, web-building and maintenance, and care of the young. They are mainly found in the tropics. The social lifestyle might have evolved to cope with heavy rainfall and large rain drop size, also protection against ants, and the ability to capture and kill large prey.
Eggs. Spider eggs are round and approximately 1 mm in diameter. They are laid in a batch and covered with silk. The eggs vary in colour, and the number laid depends mainly on the size of the female and her health. For U. K. species this can range from 2 - 2000. Some species produce just one sac of eggs, whilst others may produce more, but the sacs will contain fewer eggs. Oonops domesticus lays just 2 eggs in each sac, and she will make fewer than 6 sacs in her lifetime. Egg sacs can be used to identify the type or even species of spider. Some egg sacs are abandoned soon after construction, e. g. the pirate spiders, some are guarded, e. g. Eplongnatha ovata, the Candy-stripe spider, and some are carried around by the female wherever she goes as with the wolf spiders.
Spiderlings. The eggs hatch about a week after they are laid, but the spiderlings stay in the sac. In most species, on hatching from the egg the spiderling has no hair, colouring, claws and it cannot feed itself, basically it is just an egg with legs. During this stage they continue to get nourishment from the yolk of the egg. Below is a photograph of an egg sac containing spiderlings. This sac was on the underside of a firewood log.
A few days later, after it has absorbed all of the remaining yolk, the spiderling moults. Only after this moult does it have hair, claws, the ability to see, eat and spin silk. Normally all of this takes place inside the egg sac, and it will remain in the sac for a few more days, or even months depending on the season. There may be some cannibalism during this time. The method of growth from this stage varies with species.
The spiderlings are just small versions of the adult, and will continue to moult, growing larger with each moult. Spiders usually moult between 5 and 10 times. Lost or damaged legs can be regenerated during a moult. Only when they have reached sexual maturity will the epigyne - in females - appear. Most spiders live for one or two years, although some female tarantulas can live for 20 years or more, and some tropical spiders, which breed throughout the year, live for just a few months. Usually females live longer than males.
A startled spider will fall to the ground with its legs curled around its body. It can be picked up and rolled around in the palm of your hand, and still pretend to be dead, It will not do this when attacked by another spider however.
The main methods of capturing prey are, hunting, trapping, ambushing and stealing. Except for stealing, the prey is usually alive when captured, and can be just about anything that moves, depending on the method of capture and the spider species. However some invertebrates are rarely taken, e. g. ants, some true bugs, certain beetles, woodlice and millipedes. Nearly all of these have glands that emit a noxious fluid when they are attacked.
Hunting spiders include the wolf spiders, and jumping spiders. These generally do not build a web, and have good vision. The spiders who trap are mainly the web builders, there are links to them here, the ambushing spiders tend to sit and wait and specialise on camouflaging themselves as flowers, grass etc. the most common are the crab spiders.
Webs are usually constructed at night. First the spider raises its body up and squeezes out some silk. The air catches the thread and it becomes attached to an object. Then the spider pulls the thread tight and attaches it to the object it is standing on. Now it has a thread bridge between 2 objects. It walks across the bridge strengthening it with silk as it goes. From this bridge a frame, or outline of the web is made.
Next the spokes running into the centre of the web are laid down. Then the spiral starting from the centre where a few threads placed close together are struck down to give the structure strength.
Then a fairly wide spiral on the margin, and finally the central spiral, starting from the margin in almost as far as the hub. The outer margin spiral is no longer needed, and so is destroyed. The final spiral thread is the only thread that is sticky. The silk from Araneus diadematus is around 0.003 mm thick - that is just 10% of the thickness of silkworm silk.
The type of web a spider builds can help identify which family it belongs to, but do remember that not all spiders build webs, e.g. wolf and jumping spiders.
There are the main types of web you will come across:
Araneus diadematus were sent into space and lived aboard Skylab II for some time. It was found that they could build webs in zero gravity after a few week's of trial and error, and that the webs were very similar to those built on Earth.