Araneae (spiders)

Order Araneae

Spiders. Over 42,00o species have been described so far, and around 660 in the U. K. Spiders range in size from 0.4 - 300 mm.

The Spider body

diagram of a female spider

The prosoma and opisthosoma (abdomen) are joined by a pedicel (see right which shows the underside of a mature female).

The chelicerae (singular is chelicera) end in fangs which usually have poison glands, (see the diagram below and the photograph of the Tegenaria mature male below).

The anal tubercle (see above) is present in some species, while in others there is just a single, small excretory pore forming the anus.

diagram of a spider head

diagram of spider body parts

Spiders can have six or eight eyes (see right and above) and the arrangement of the eyes is used in identification; however, vision is poor in most species, and used mainly for detecting movement. The wolf and jumping spiders which hunt down their prey usually have better vision. Spider eyes are not compound like those of insects, but simple, and usually arranged in two rows.

The fovea is a dark depression behind the head region where internal muscles are attached.

The cardiac mark shows where, just below the cuticle, the heart lies.

 

Diafram of a female spider

Most spiders have what are called book lungs (see the drawing on the right). These are blood-filled slices of tissue separated by an air space. Most species have one pair of lungs.

Spider Reproductive organs

Each mature female has a pair of sperm storage sacs or spermathecae. The spermathecae are shed with each moult, so the female spider must mate after each moult. The epigastric fold (see above) is where the female genital opening, the epigyne, is located. It is a hardened reddish-brown area, and is seen only in mature females. The palps or pedipalps of the males are modified to transfer sperm into the female's epigyne (genital opening) see above and left, which is located on the dorsal side of the opisthosoma. The epigyne of the female and swollen palps of the male (see above) are only seen in mature adults. In females the palps are always simple. Females spiders are usually larger than males.

Internal anatomy of a female spider
parts of a spider leg

Spider legs

Spider legs are divided into 8 parts - see the drawing above left. There may be 2 or more claws, and these may have teeth like a comb. Hunting spiders have a pad of hairs which, along with a sticky secretion, enables the spider to get a good grip on most surfaces.

The femur is the equivalent of our thigh, the patella - our knee cap, the tibia - our shin, and the metatarsus, tarsus and claws our foot and toes.

As a rough guide hunting spiders have 2 claws and web builders have 3.

Tegenaria duellica mature male showing palps, chelicera, fang and eyes

spider spinnarets
above spider spinnarets

Spider silk

The silk is produced in the silk glands (see the drawing below) and comes out from the spinnerets - see the drawings below and above (the number of spinnerets varies according to species, but 3 pairs is the most common) which are located on the underside posterior of the opisthosoma. There are 7 different types of silk gland, each producing a different type of silk, but no single species has all 7 types of gland. The silk is a liquid protein which hardens, under tension, in contact with the air, it is 5 times as strong as steel, weight for weight, and is elastic like rubber. It emerges as many very thin strands (the spinnerets have up to 600 ducts) which join up to form one very strong strand. It is drawn out of the spinnerets by the back legs, or by the weight of the spider's body.

Many spiders (except wolf and jumping) catch their prey in webs. And web construction can be used to identify to family or genus. Webs are the most obvious use of spider silk, but spiders use silk for much more:

  • webs to catch prey
  • wrapping to keep prey until it is required
  • retreats on or near the web
  • lining of burrows
  • sacs to protect eggs
  • safety drag-line when moving around
  • parachute to balloon and aid dispersal of young or small spiders
  • small sheet of silk made by males to deposit a droplet of sperm before it is taken up in the palp
  • safe cells to moult in
  • constructing trap doors
  • bridging lines between objects
  • during courtship some males tether the female before mating

For each of the above a different kind of silk is produced.

Spiders do not stick to their own webs because they either step on the non-sticky parts only, or secrete a special oil on their feet. Also their leg hairs decrease the surface area available to stick.

Silk is also used by the females to build egg sacs where the young remain for a few weeks.

The silk from Araneus diadematus is around 0.003 mm thick - that is just 10% of the thickness of silkworm silk.

Spider silk from webs is used by many birds (tits, warblers, gold crests) as part of their nesting material.

Spider Moulting

Spiders undergo and average of 5 and 10 moults in their lifetime, with the larger species tending to moult more, and can moult up to 20 times. Spiders moult their skins as many as 8 times before reaching adulthood.

The new cuticle is very soft and this means the legs are unable to support the body weight, so small spiders suspend themselves from a silk thread, and large species make a silk mat to lie on and moult on their backs to allow the legs to harden.

During the moulting process they can lose a leg, which will be regrown at the next moult. When the spider's cuticle is stretched to the limit the spider can grow only by moulting. Spiderlings may moult every few days, and increase in size with each moult. The time between moults increases with age. When a spider is about to moult it will stop eating, stay immobile, and some species seal themselves inside a silk-lined chamber while the old, outer layer separates from the new layer. The new cuticle is soft and pale.

The old cuticle or exuvium is often mistaken for a dead spider. The cuticle hardens and takes on normal colouration within a few hours.

Spider venom

The main purpose of spider venom is the speedy immobilization of prey. This enables the spider to prey on animals larger that itself, and animas 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 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.

Social spiders?

A few species of spider live in groups, and are considered social, e. g. Anelosimius eximius spins webs that can contain over 100,000 individuals.

Spider life cycle

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.

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.

A few days later, after it has absorbed all of the remaining yolk, it 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.

  • Some may be carried around and fed by their mother
  • Some may feed on their mother
  • Others may have to make their own way in the world as their mother will already be dead

The spiderlings are just small versions of the adult, and will continue to moult, growing larger with each moult. Only when they have reached sexual maturity will the epigyne - in females - appear.

Feigning death

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.