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Spiders


Spiders are a sub-group of Arachnids, a class within the group of Arthropods, and are typified by the following features:




ANATOMICS

The dorsal side of the cephalothorax is made of chitin what is called carapax. Directly behind the head is a groove, the fovea. The zone just behind the eyes is called the clypeus. 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 bottom of the abdomen. The jaws are located below the eyes and are called chelicerae and its extremities are constituted by fangs, the piercing parts that deliver the poison 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 rear legs, they can comb the silk to turn out fluffy wool like silk to make the so-called hackle bands on their webs.


VISION

Spider have different number of eyes (from none up to 12), they are singular and called ocelli, are located in the front and the main pair is always standing in the middle and have a different construction. 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: the light sensitive cells point towards the incoming light and is consequently referred to as indirect retina. In the other eyes the light sensitive points away from the light and have an indirect retina. These lesser eyes give the spider a much broader field of vision than the main eyes and are used to provide them the ability to evaluate distance. During hunting, these secondary eyes grab the movement of the prey and the main eyes are used to focus as it moves closer to the victim.


FEEDENG AND DIGESTION

The spider captures its prey with the legs and uses its sharp fangs to inject the poison abounding in liquefying enzymes: the digestion is partially external in the quarry itself. The liquefied contents are delivered 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. 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 go on with the digestion. In this process, the food is reduced to a molecular size. The midgut finally ends in the cloacal chambers where the faeces are collected.


BREATHING

Spider breathing system is very different from ours and two independent structures are involved: book lungs and tracheae. Book lungs represent the older respiratory system organized in stacks of plates where the blood flows through allowing the gas exchanging. This double gas diffusion system (in/out) is not very efficient so, throughout the evolution, mainly in more active spiders, tracheae partly or almost entirely replaced the book lungs. The tracheae are tubes located at the underside of the abdomen where there is the entry hole for oxygen and straight run to the organs and tissues with no splitting. Generally, in every family of spiders, each system have a altered weight in the balance of oxygen.


BLOOD CIRCULATION

The heart is arranged in a cavity called pericardium, sited below the heart spot which is detectable in the dorsal side of the abdomen. The blood system is classified as an open scheme because of no blood vessel like in the human one: the tissues are irrigated by running free blood that pours out straight from the arteries extremities. The transport of oxygen is similar to the human one where it circulates bound to haemoglobin, a molecule containing iron; in spiders oxygen link to haemocyanin, a molecule including copper, that once oxidized assume a blue/green colour typical of the spiders blood. The blood is known to contain some cells concerned with blood clothing, with defences against infections and wound healing. There's big differences in the cardiac rhythm between big spiders (30-70 beats/min) and smaller ones (up to 200 beats/min), instead the blood pressure is nearly equal to men (0,18 atm) and double during the changing of the skin: it seems as blood pressure works as an hydraulic pump that help out walking.


REPRODUCTION

The reproduction is often a lethal mission for males who pay with their life at the end of a complicated game including lot of tricks and tactics. First the male has to produce sperm from its dedicated organs located in the abdomen, then the seminal liquid flow through tubes and reach the genital opening in the middle of the epigastric channel sited in the ventral surface of the abdomen behind legs. For reproducing it needs to transfer sperm in the appropriate reservoirs in its palpal organs so it begins with assembling a small web and a drop keeps out from its genital opening, then it uptakes the liquid with its palps exploiting both capillary and gravitational forces. Finally it has to approach the female following the correct ceremony with no mistakes, life is the price; when the male is accepted it quickly connects its palps to the female epigyne inserting sperm and then it tries to flee.



SENSE ORGANS

Tactile, tasting, slit organs and proprioceptors are located just beside the eyes. Tactile perception is due to hairs that completely cover the body of the spider: any movements translates in a nervous impulse that gives information about the environment (sounds included). Tasting sensors con be found inside the mouth and on the extreme part of the palps where there are sunken hairs conveying volatile substances. Slit organs are arranged around the body and pick up vibrations, stress and gravitational forces. Finally proprioceptors are dislocated in the appendages and work as movement/position detecting system.