A new and more detailed article can be found here: Spider information and a dutch version here: Spinneninformatie
The body of a spider consists of two easily recognizable parts. A prosoma or cephalothorax (composed of the fused head and thorax) and the abdomen (the rear body).
A narrow waist (pedicel) connects both parts. The upper side of the spider is called the dorsal side (i.e. the back) and the bottom the ventral side (i.e. the belly). The cephalothorax at the dorsal side is made of chitin what is called the carapax. The eyes are located in the front. Most spiders have eight eyes, some 6. But there are spiders with no eyes, 2, 4 and up to even 12 eyes. The eyes are singular and are called ocelli, unlike the insects that have compound eyes. The main pair of eyes is always the middle pair and has a different construction than the other eyes. The light sensitive cells point towards the incoming light and is therefore referred to as indirect retina. Except the crab spiders and jumping spiders, the main eyes are small or even absent in spiders with six eyes. The construction is similar to as in humans and these eyes have a high resolution. In the other eyes the light sensitive cells points away from the light and have an indirect retina. These secondary eyes give the spider a much broader field of vision than the main eyes and are used to give them the ability to judge distance. During hunting, these secondary eyes catch the movement of the prey and the main eyes are used to focus as it moves closer to the prey.
The back the prosoma is called the abdomen or opisthosoma. Directly behind the head is a groove, the fovea. The zone just behind the eyes is called the clypeus (forehead). The part on the dorsal site between the legs is called sternum. The palps in the front are used for sensory purposes. In the male the palps are modified for putting sperm into the epigyne of the female, that is situated on the underside of the abdomen. The jaws are located below the eyes and are called chelicera. The piercing parts of these jaws are the fangs that deliver the venom via a small hole at its tip.
The silk is produced by the silk glands and released by the spinners. Immediately in front of the spinnerets, in certain spiders, is the cribellum, a special plate, containing a row of openings through which many strands of very fine silk is produced. With their hind legs, they can comb the silk to produce fluffy wool like silk to make the so-called hackle bands on their webs. The spinnerets and the properties of silk
The trachea holes and the book lungs are the inlets for oxygen. These holes are situated at the underside of the abdomen. The number of trachea and book lungs and their position varies from family to family.
At the front dorsal side of the abdomen is the heart spot located. Below this spot is the spiders heart. Big spiders have a heart beat around 30 - 70 beats a minute whereas in smaller spiders the heart beat can rise up to 200 beats per minute.
The internal anatomy of a female (picture from reference 7)
Spiders have an alimentary canal (yellow), a blood vascular system (red), a breathing system (orange), a nervous system (blue), an excretion system (green), a reproduction system (white) and a set of glands for the production of silk (white at the rear).
The spider catches its prey with the appendages i.e. the legs. Then it uses its sharp fangs to inject the venom into the prey. This venom also contains enzymes that dissolve the prey. The digestion is partly external in the prey itself. The liquefied contents are sucked into the alimentary canal by the sucking stomach that is located in the cephalothorax. The particles in the food are filtered out by means of many hairs around and in the mouth. The filtering is so efficient that it can separate the particles in Indian ink from the liquid. From the sucking stomach the food is transferred via the intestine to the midgut which is located in the abdomen where secretory cells release enzymes which digest the food further. Resorptive cells take up tiny globules of food and continue the digestion. In this process, the food is broken down to a molecular size. The midgut finally ends in the cloacal chambers where the faeces are stored.
Meta segmentata eating a fly.
The breathing system of spider is different from ours. There are two separate systems involved, book lungs and tracheae. The book lungs are assumed to be the first respiratory system evolved which is later replaced by the tracheae. A book lung consists of a stack of plates. Blood flows through these plates and the gas is exchanged. The gases pass in and out by a diffusion process and it therefore not very efficient. In more active spiders tracheae replace part of the book lung or the whole book lung. The tracheae are tubes that do not branch but run from the opening on the outside directly into the tissues and organs.
The spiders blood system is know as an open system. There is no blood vessel system like in man but the blood pours out from the end of the arteries directly onto the tissue where it runs freely. The cavity around the heart is called pericardium. The blood is different from man. The oxygen is bound to hemocyanin, a molecule that contains copper, whereas in humans the oxygen is bound to hemoglobin, a molecule that contains iron. The color of oxidized copper is blue/green. Therefore, spiders have 'blue' blood. The blood contains some cells, though to be mainly concerned with blood clothing, defense against infection and wound healing. The blood pressure of spiders is high and is with 0.18 atm nearly equal to men. During the changing of the skin, this pressure is doubled. It is thought that the blood pressure is used as a hydraulic force for walking.
The reproduction is a dangerous task for the male spider that he often has to pay for with his life. It is a complex process involving a wide range of approaches and tactics.
The male has sperm production organs in it abdomen. Tubes lead to the genital opening that is located in the center of the epigastric furrow that can be found on the ventral side of the abdomen behind the legs. Before the male can mate, he has to transfer the sperms to the reservoirs in its palpal organs. To do this it makes a small web and a drop excluded from its genital opening. He then takes up the semen with its palp. The uptake of semen is probably a combination of capillary and gravitational forces. Then the dangerous part. He has to approach the female with the correct rituals. Failing these conditions is a death penalty. When the females accepts the male, the males connect its palp to the female epigyne and the semen is inserted.
The spider processes different organs to communicate with the environment.
The most observable organs are the eyes and which were described earlier. Beside the eyes the spider has tactile sensors, tasting sensors, slit organs and proprioreceptors.
The tactile sensors are the hairs that are spread all over the body. Movement of the hairs triggers a nerve impulse. That this system works can easily be tested by shouting to a spider. It will directly stop its activities because the sound waves move the tactile hairs and the slit organs. Touching a spider will have the same effect but that seems to be obvious.
The tasting sensors are located inside the mouth and on the end segments of the palps. On the palps are hollow hairs into which volatile chemicals enter. If a spiders manipulating its foods and touching it with its palps it is tasting it!
The slit organs are spread around the outside of the body that senses stress, vibration and gravitational forces. The proprioreceptors are found on the appendages and are used to detect and to sense the position of the legs and the direction of the movement.
Once the eggs are fertilized and the eggs are matured, the female lays her eggs. The mother has to take care of her unborn children because the elements, predators or parasites can harm them. The Scytodes and the Pholcus (daddy longleg) use the simplest form. They carry their children in loose bundles with them. They can do that because they live indoors and are not threatened by the weather. Other species like the wolf spiders put their eggs in a sac of silk and attach the sac behind their body. After their children are born, the female carries them for a while on her abdomen.
Many other species put their eggs in a sac of silk and guard them until the youngsters emerge. Often she will care for her youngsters until she dies.
Among some species, the children eat the mother. Some Araneidae guard their eggs until the mother dies in the autumn. The eggs stay unattended until they hatch the following spring.
Spiderlings of Araneus diadematus are clustered in a yellow ball that explodes on a slight disturbance as shown here.
Ed Nieuwenhuys, 30 August, 1998