Spider outside
Unlike an insect, the spider's body is in two sections. The head and thorax, bearing the eyes, mouthparts and legs, are fused together to form the cephalothorax. This is joined by a slim waist (pedicel) to the second body section, the abdomen, on which are found the silk spinning organs (spinnerets), the reproductive openings and the breathing organs (book lungs and/or tracheae).
The cephalothorax is covered above by a hard cuticular plate called the carapace - much like the hard 'shell' covering a crab. The simple eyes, usually eight, are commonly arranged in two rows along the front of the carapace (although eye arrangement and sizes vary). A depression in the middle of the carapace is the fovea, internally an attachment point for thoracic muscles. The mouthparts are placed below the front of the carapace. At the front are the two large jaws with their piercing fangs (the chelicerae), while behind the jaws on the underside there are two small cuticular plates (flattish blocks of cuticle) - an upper plate, the labrum (upper lip) which is hidden by a lower plate the labium (lower lip), clearly visible from below behind the jaws. These two plates form the roof and floor respectively of the tube-shaped mouth which opens just behind the jaws. A pair of plate-like maxillae flank the labium - each has a food cutting row or patch of teeth on the front end and they form the bases of a pair of short limbs, the pedipalps, that help with food handling, touch and taste sensing and, in male spiders, are modified as mating organs. Behind the labium is a larger and more obvious plate, the heart-shaped sternum which lies between the bases of the legs.
In mygalomorph spiders (trapdoor and funnel-web spiders) the large jaw bases project forward in parallel with their fangs folded back side-by-side underneath. To bite their prey these spiders must raise the front of the body, allowing the fangs to open like a pair of daggers for a downward strike. In the more common araneomorph spiders (redbacks, wolf spiders, etc.) the jaws are slung vertically under the front of the carapace. The fangs are hinged laterally and bite cross-ways against each other, like pincers. This is a more efficient arrangement for seizing and manipulating prey, especially on a web.
The jaws are flanked by the short, leg-like pedipalps that are both sensory and used to handle prey; and followed by four pairs of legs with two to three terminal claws. The two-clawed spiders are hunters (e.g., jumping spiders, huntsman spiders, ground spiders), most with thick hair brushes (scopulae or claw tufts) on the ends of the legs - these improve traction on smooth or sloping surfaces like leaves or tree trunks. Many of the three-clawed spiders are web builders, often with claws and hairs modified for silk handling (e.g., orb-weavers, gum-footed web spiders, lace-web spiders). Others, like the wolf spiders and spotted ground spiders, are active hunters/ambushers.
The abdomen is usually covered with thinner or more flexible cuticle - this allows for expansion with feeding or when eggs are developing. The thin waist or pedicel separating it from the cephalothorax allows movement of the abdomen, for example, during silk spinning and mating displays.
Beneath the abdomen are the breathing organs known as book lungs. Small openings called spiracles lead into air filled cavities into which the thin, leaf-like lamellae of the book lungs project - like rows of book pages. The outer surfaces over which air passes are covered by very thin cuticle from which peg-like struts project, keeping the lamellae from collapsing. Blood (haemolymph) circulates within the lamellae and gaseous exchange between blood and air occurs across their thin walls. There are two pairs of book lungs in mygalomorph and basal (less advanced) araneomorph spiders. Most other araneomorphs have the front pair of book lungs only, the rear pair being replaced by fine, cuticular tracheal tubes that divide within the body and allow more efficient gas exchange. A few tiny spiders living in moist, sheltered habitats have no breathing organs, gas exchange taking place directly across the thin body cuticle.
The gonopore or genital opening (from which eggs or sperm are released ) is placed in the genital groove between the front pair of book lungs. In most female araneomorph spiders there is another separate, plaque-like mating opening, the epigynum. At the end of the abdomen are the spinnerets (silk spinning organs), usually four or six in number, and the terminal anal tubercle on which the gut ends at the anus.
