Araneae (spiders)

Order Araneae

Spiders. Over 40 000 species have been described so far, and around 660 in the U. K .

Spider parts of the body

The prosoma and opisthosoma 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 photograph of the Tegenaria mature male below).

Spiders can have six or eight eyes (see below) 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.

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

Reproduction. 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 palps 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 below) are only seen in mature adults. Females are usually larger than males.

Spider silk

The silk comes from the spinnerets - see the drawings below and above (the number varies according to species, but 3 pairs is the most common) which are located on the underside posterior of the opisthosoma. The silk is a liquid protein which hardens in contact with the air. It emerges as

many very thin strands which join up to form one very strong strand. Most 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

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. 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 legs

Spider legs are divided into 8 parts - see the drawing below 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.

Spider courtship, mating and behaviour

Courtship of the female by the male can be elaborate, but much of this is to make sure that the female can distinguish him from a juicy meal! Before mating the male spider deposits a drop of sperm on the surface of a small sheet of silk, and then transfers this to his palps which pick it up by capillary action. the male's palps fit into the female's epigyne like a key fits into a lock. This prohibits mating with the wrong species. In species which spin webs the male will pluck and vibrate the strands of the web to attract the female's attention. All spiders are very sensitive to vibrations. He may hang

around the web for days before making his move. Some species wait until the female is occupied with prey; others until she is about to moult. Some supply a prey item as a gift. In species with good eyesight such as the Lycosidae and Salticidae, the males signals his intentions with his legs and palps. The mature male above right was found wandering across my floor in search of a female. For this species this is common behaviour at the end of summer and during the autumn.

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.

Moulting. Spiders moult their skins as many as 8 times before reaching adulthood. During the moulting process they can lose a leg, which will be regrown at the next moult.

Spiders and gardens

• The Lycosidae will eat 5 - 15 small insects each day, and in California Pardosa ramulosa is an important predator of leafhoppers.
• The Salticidae also have a fondness for leafhoppers and will eat 4 - 8 per day.
• In China farmers build little straw huts beside their paddy fields for the spiders to overwinter in. In the UK money spiders protect the fields of winter wheat from aphids, and are active earlier in the year than ladybirds - another great aphid predator.

Amazing spider snippets

• It has been estimated that in one year the weight of insects caught and eaten by spiders in the U. K. is equal to the weight of the entire human population. This also exceeds the weight of insects eaten by birds.
• A hectare of meadow can contain over one million Linyphiidae (money spiders). This can be most easily seen when the webs are covered with the morning dew.
• 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.
• It is believed that spiders have been in existence for over 300 million years.
• The largest spider that ever lived was Megarachne which lived during the Carboniferous and is now extinct. It was 33 cm long with a leg span of around 50 cm!
• In France Tegenaria sp. are real eased into wine cellars as the presence of their webs is though to create feeling of age.
• In Belgium many brewers believe that spiders are instrumental in controlling the numbers of fruit flies (Drosophilidae).
• In 1936 a policeman on point duty on Lambeth Bridge in London held up traffic to allow a particularly large Tegenaria parietina to cross the road.