Spiders belong to the class Arachnida which is part of the sub-kingdom Metazoa, that is until some scientist decides to redo the the classification system. They are also part of the phylum Arthropoda, this word is derived from the Greek and it means "jointed foot". The earliest of all Arthropods is thought to be the trilobite, which is an extinct marine animal, click the link if you want to know more. The first Arachnid is thought to have been a Scorpion like creature from about 500,000,000 years ago. Today there are more than a million classified species of insects, with more being discovered every day. The Arachnids now number about 80,000, with no doubt many more yet to find.The spiders above are the most common in the garden and the home, with a couple there just for interest value.
Britain has more than 600 species of spiders, all of them harmless to man. They prey on other creatures, have eight legs and a body divided into two parts: a combined head and thorax and a large abdomen. Most spiders have eight eyes, arranged in two rows of four.
Many insects are vegetarian but most arachnids are predatory, feeding mainly on insects. Unlike insects, arachnids have four pairs of walking legs, do not possess antennae or wings, and have simple , rather than compound eyes. The body of an arachnid is divided into two clear regions, except in the mites and the harvestmen, where it appears to be one piece. The front or anterior section is the prosoma to which the four pairs of legs, the chelicerae (the area of the mouth which holds the fangs), and the pedipalps (for holding, and in the male copulating) are attached. I am not going to get too involved with anatomy, if you have any questions send me an e-mail. The back part is called the opisthosoma or abdomen, this supports the anal tubercle, the tracheal tubercle through which the spider breathes, and of course, the spinners or spinnerets. Six different types of silk gland have been identified, all of which are found on the orb weavers, four of the glands produce different types of silk and the other two produce a viscous liquid which coats the silk of the web, both the spirals and the spiral thread itself.
Spiders produce silk threads which thickness for thickness, have the breaking strain of iron. Spider silk is only about 1/200th of a millimetre in diameter and is so light that if a spider could spin a strand around the world it would weigh less than 6 oz. The silk is so fine that some of it has been used in telescopic gunsights.
Spider eggs are roughly spherical, and about 1 mm in diameter; they are laid in a compact mass and covered to a greater or lesser extent with silk, forming a sac. The eggs are variously coloured, pale brown, pale yellow, pink, even bright green. The number of eggs laid depends upon the size of the female; larger spiders lay more and sometimes larger eggs. Underfed females lay less eggs but the size depends upon the species. Some spiders produce more than one sac, but there is a tendency for fewer eggs in the later sacs. One species of spider has been recorded to lay 2000 eggs in four sacs, whereas another only lays two eggs per sac.
The eggs are smooth at first but within a week or so before hatching the form of the spiderling can be seen on the surface of the egg. Usually the legs can be seen as parallel grooves with the smoother area indicating where the abdomen is being formed. The egg has an outer "shell" (the chorion) which is ruptured by the egg tooth which is on the base of each palp of the pre-larvae. The tiny hairless and blind creature must wait a few days to moult into a more advanced stage, the larva, which has rudimentary eyes and a few hairs but lacks poison and the ability to spin silk. Both of these stages are unable to feed and they subsist off the yolk within them. After a short period, the larva moults into a nymph or spiderling which resembles the adult in general form. At this stage, some cannibalism may take place within the sac, and those spiderlings which are disadvantaged in any way become a meal for those that aren't. Note that all of the above still takes place within the outer sac. A picture of spiderlings below.
As soon as the nymphs of most species are capable of feeding, they leave the sac, this minimises any cannibalistic tendencies and thus maintains the numbers of the new generation. Some spiderlings, such as some web makers, move only a short distance from the sac, and construct their own tiny webs in which they catch their own food, while others disperse by air.
Spiders are not actually able to fly but they are very light in weight and the pull of the slightest breeze on a short line of silk is enough to lift them aloft and carry them considerable distances. Aerial dispersal or ballooning is most noticeable in the autumn and the word "gossamer" is possibly a corruption of "goose summer", which used to describe those days in October when "goose down" is seen drifting through the air. When large numbers of spiders balloon, their silk threads tend to drift together and form patches which are readily noticed.
A spider intent on ballooning climbs to a high point and turns to face the wind. Several strands of silk are expelled and these are carried up into the air. The spider then stands on tiptoe and, when the force of the air currents is sufficient, the leg claws release their grip and aaaaawwwwaaaayyy we go!
If the spider lands in a suitable place, it must set to hunting food, it the place isn't suitable (a lake or the sea or an active volcano) then it doesn't matter does it!! All spiders are carnivorous and most are stimulated only by living and moving prey. The hunters are divided up into day and night shifts, though some feed at all times of the day. All spiders are well equipped with sensory hairs on their bodies and legs, and they can detect the slightest change in air currents about them thus indicating the movement of prey. The also have chemosensitive (hairs which smell) hairs which inform them of the nature of their intended prey, and with their lightening fast reactions, hunting by this method is just as good as by sight. Spiders will and often do feed on other spiders. Most hunters will attack prey which is smaller than themselves, and will run from prey which is larger (seems like a good idea to me). Digestion in spiders is an external affair, they cannot ingest solid food, so the digestive juices have to introduced into the immobilised prey. Those that have well developed teeth (chelicerae) tear their prey apart and pour digestive juices onto it, and eat the resulting soup. Those whose chelicerae are not well developed will inject the poison or digestive juice, and then suck up the soup through the same aperture or tube or fang, whatever you want to call it. Feeding is slow and a large fly can take up to 12 hours to consume.
As the soft cuticle of a spider's abdomen becomes stretched by the intake of food a point is reached where no further enlargement is possible. None of the harder sclerotized parts is capable of increasing in size because, as in all insects, the skeleton is on the outside (exoskeleton). Thus the poor old spider has to moult. This involves the spider splitting the old cuticle and shedding it to allow the new softer larger cuticle to be exposed, once exposed the new cuticle will harden. The nymphs moult frequently, every few days during which the size of the head end will increase as well as the abdomen, this does not happen with mature spiders . The interval between moults increases as the spider ages. Smaller species have fewer moults, about five less, while larger and more long lived species require up to double this number. Sometimes the moult does not go according to plan, legs get stuck etc., and the spider dies, or they can break a leg or palp to regain their freedom, they are very susceptible at this stage. Moulting is a very big subject and is only covered briefly here.
Reproduction, is a process, which the male carries out with great care, as it usually ends in his death. The courtship ritual is elaborate, especially in the sighted spiders, and for good reason as it calms the predatory tendencies of the female and makes her more receptive. In the case of humans this is done with the aid of money, presents and dining out... The male has intromittent organs on his palps, and he must construct a sperm web, upon which he deposits a drop of sperm, the palps are then dipped into this with the embolus taking up the sperm like a fountain pen filling and storing it within the palp. (This is a very simplified explanation).
Once the female is receptive, the male approaches and, by crawling under or over the female (in the hunting species) the palps are applied one at a time or, in the more primitive species, both at once, and the sperm is transferred to the female. The males do not live long once they have matured irrespective of the longevity of the female. In Tegenaria (House spider species) the male dies, only then is he eaten by the female. The female lives on and catches more prey and lays her eggs. This can take place long after the death of the male. As explained earlier the eggs are laid in sacs.
© Stuart M Bennett 2000
