The physique of the spider looks apparently
bipartite: actually the body consists out of three parts: HEAD
(Cephalon), CHEST (Thorax) and
By the insects this division is more clearly visible; with the spiders on the other hand, head and chest are merged together in what we call a head-chest-piece or CEPHALOTHORAX (Prosoma); the ABDOMEN(or Opisthosoma) is joined to this "front-piece" by means of a narrow stalk. Through this stalk, (the PEDICEL) all connections between, stomach, the central nerve-system (in the cephalothorax) and the intestinal system, the bundles of nerve fibres (in the abdomen), run, while the blood-supply of the head takes place by way of the pedicel. The abdomen is softer than the cephalothorax, which is covered with chitin. From time to time this chitin-layer gets , together with the skin, casted off: the spiderMOULTS.
The ABDOMEN can have a strong coat of hair; the hairs lie close by the body (adjacent growth of hairs) and it is NOT JOINTED.
Spiders have EIGHT EYES, save some exceptions that have six, four, two or no eyes at all. The perception area of a spiders eye is larger than that of one single facet of the compound eye of an insect. We distinguish clearly the day-eyes (dull black) and the night-eyes(shiny). SIZE and PLACEMENT of the eyes are important identification characteristics. Position and size of the eyes give clues about the way of life of the animal.
To the front of the head we see the very sensitive FEELERS having the outlook of small legs, and which are called PEDIPALPS or PALPS. They consist of six movable connected members and are - by a living spider - always in motion.
Between the PALPS the big, two-part, strong CHEEKS, the CHELICERAare situated. Almost all Belgian and Dutch spiders have their chelicera in a plane perpendicular on the longitudinal axis of the body, with the pointed hooks moving to each other.
At the bottom-side of the cephalothorax we notice in the front, close behind the chelicera, the MOUTHPARTS (Mandibulae), grouped round a small mouth-opening. They consist out of an UPPER LIP(Rostrum), mostly concealed behind a strong hair growth, so very difficult to find, a LOWER LIP(Labium), mostly bigger and clearly visible, and both OUTGROWS of the first member of the palps; these outgrows are commonly, but faulty, called MAXILLA(Maxillae) ; real maxilla are lower jaws, which these protrusions are apparently not!
Clearly distinguishable is finally the CHEST SHIELD or Sternum, which covers the complete under side of the cephalothorax and to which the legs are fixed.
At the ABDOMEN, which can adopt several different forms, we usually find at the top various coloured markings and at the bottom, in the HIND END OF THE ADOMEN, esome small protrusions: the SPINNERS, with which the spinning threads are produced. Most species have three pairs of spinners: a posterior, a median an anterior. Some spiders with strong pronounced forwards pointing cheeks have four pairs of spinners. Some groups of spiders have a kind of small plate, located before the spinners, the SIEFE-PLATE (Cribellum), which can be considered as a deformed fourth pair of spinners and is characteristical for the CRIBELLATE SPIDERS (Cribellatae). The same applies for a small elevation, an irregular conical organ, which is located by some species before the spinners and bears the name COLULUS . The cribellum kept the original function (making spinning threads), the colulus not.
Close behind the spinners the by means of an anal tubercle closed anus is found. Immediately behind the pedicel we see, at the ventral side of the abdomen, in the neighborhood of the genital region, a transverse groove with in the center the sexual organ. With young animals (juveniles) and the adult males, the genital region is rather inconspicuous, by adult females this region is more pronounced, while round the sexual organ several small chitinal knobs and ledges are to be seen. The sexual organ, the knobs and ledges compose together the external part of the genitals, the EPIGYNE, one of the most important identification characteristics, as long as we're dealing with adult females.
In the already named transversal groove lies at left and right side of the epigyne a respiration opening, the(STIGMA). These respiration openings are the outlets of both lungs. All spiders possess TWO lungs, some species even FOUR; they 're so called BOOK LUNGS or TRACHEAL FANS, that borrow their name to their internal build; they contain a huge number of "sheets" which are placed next to each other as the pages of a book, through which the blood circulates and the gas exchange between the blood and the open air takes place. Spiders with two book lungs have also two tubular respiratory organs(TRACHEA) which are located more backwards, each with its own opening (two stigmata) or with one shared opening (stigma).
The legS consist all of seven members, that are all unequal in length and are, beginning from the cephalothorax, named as follows: the HIP (Coxa), the THIGH RING (Trochanter), the THIGH (Femur), the KNEE (Patella), the SHIN (Tibia), the MIDDLE FOOT (Metatarsus) and the FOOT (Tarsus).
All these members are usually covered up with hairs and spines of unequal length. The last members of the legs (metatarsus and Tarsus) have by different species a kind of brush, which consists out of a row sturdy, bowed and sometimes serrated hairs, at the ventral side: the SCOPULA. Furthermore there can be some spreaded long, thin and feathered hairs present on the legs, which are in connection with the nerves and can be considered as sensory organs, probably HEARING HAIRS (Trichobothria)
Some spider species have at the upper side of
the metatarsus of exclusive the fourth leg a comb of small, sturdy close
grouped hairs: the CALAMISTRUM.
ALL spiders with a calamistrum have also a cribellum (seeve plate).
At the extremity of the tarsus (foot) there are always two,
sometimes three claws. If a third claw is present, this is usually smaller
than both others. All claws are comb-shaped toothed. By some species the
third claw is missing, but is replaced by a bundle of
STICKY HAIRSthat permit the animal to climb
on smooth surfaces and such. This spiders have also a SCOPULA.
The complete arrangement of the tarsa is tuned in to the aim they 're used for: walking on the threads of the own web without sticking on it, walking on vertical walls and, by the web-spiders, the construction of the web. By the legs one can recognize often the way of life of the spider: the legs of web-spiders are long and small; those from hunting spiders short and thick.
The INTERNAL BUILD typifies itself by a short gullet which goes from the mouth to the sucking stomach, located in the cephalothorax. The emptying of the prey does not happen with the mouth -arts, but with the stomach, which functions literally as a suction pump. From the stomach on, the intestine system runs through the pedicel and enlarges itself in the abdomen. The intestinal system possesses a huge number of dead end extensions (DEAD END SACKS), as a result of which the absorptive capacity of the intestinal wall enlarges.
The cephalothorax contains also a CENTRAL NERVEMASS consisting out of two parts. The blood circulation is open. At the dorsal side of the abdomen lies an enlarged blood vessel: the BACK VESSEL, which functions as a heart that through irregular contractions the blood flow. There are no fother blood vessels: the blood, which contains no red corpuscles, flows free throughout the body and between the different organs.