This order includes the true Spiders, which are characterised by the amalgamation of the cephalic and thoracic segments into a single mass, and by the generally soft, unsegmented abdomen, attached to the cephalothorax by a constricted portion, or peduncle. Respiration is effected by pulmonary sacs in combination with trachece. (Hence the name Pulmotrachearia, sometimes applied to the order.) The number of the pulmonary sacs is smaller in the true Spiders than in the Scorpions, being either two or four, opening by as many stigmata upon the under surface of the abdomen. Usually there are only two pulmonary stigmata, placed just behind the peduncle which unites the cephalothorax with the abdomen, on the lower surface of the latter. In the Mygalidae there are two posterior stigmata, leading into pulmonary sacs; and in other genera there are also two additional stigmata, which, however, open into tracheae, and not into pulmonary sacs.
The head bears two, four, six, or eight simple eyes; the mandibles are simply hooked, and are perforated by the duct of a gland which secretes a poisonous fluid; and the maxillary palpi are never chelate. The maxillary palpi of the females are almost always leg-like, and are often of the same form in the males. The latter, however, commonly have the ends of the palpi tumid, in which case they appear to be employed for the purpose of conveying the seminal fluid to the female, thus exercising a reproductive function.
Spiders (figs. 162, 168) are all predaceous animals, and many of them possess the power of constructing webs for the capture of their prey or for lining their abodes. For the production of the web, Spiders are furnished with special glands, situated at the extremity of the abdomen. The secretion of these glands is a viscid fluid, which hardens rapidly on exposure to air, and which is cast into its proper, thread-like shape, by being passed through what are called the "spinnerets." These are little conical or cylindrical organs, four or six in number, situated below the extremity of the abdomen, and possibly to be regarded as modified limbs. The excretory ducts of the glands open into the spinnerets, each of which has its apex perforated by a great number of minute tubes, through which the secretion of the glands has to pass before reaching the air. Many spiders, however, do not construct any web, unless it be for their own habitations, but hunt their prey for themselves.
Fig. 168. - Araneida. Theridion riparium (female).
The form of the web has been employed as a basis of classification of the Spiders, and amongst its numerous modifications, the following may be specially alluded to : Some forms (such as the common Garden-spiders) construct a web in the form of an incomplete or complete circle, with lines radiating from the centre. These have been termed " Orbitelariae." Others - the so-called "Retitelariae" - simply spin a thin suspended sheet for their web. Others (" Tubilelariae") construct a silken tube, inserted in any accidental cavity, its mouth being open and guarded by more or fewer threads. Lastly, others ("Territelariae") spin a silken tube in a hole formed by the animal itself, and close its mouth by means of a variously-constructed lid.
The Spiders are oviparous, and the young pass through no metamorphosis ; but they cast their skins or moult repeatedly, before they attain the size of the adult. Most Spiders deposit their eggs in silken nests or cocoons, often beautifully constructed, and sometimes carried about by the females. The males are generally smaller than the females, and of rarer occurrence.