Sub-order a. Mesothelae (see figs. 60 to 62).—Opisthosoma distinctly segmented, furnished with 11 tergal plates, as in the Amblypygi; the ventral surface of the 1st and 2nd somites with large sternal plates, covering the genital aperture and the two pairs of pulmonary sacs, the sternal plates from the 6th to the 11th somites represented by integumental ridges, weakly chitinized in the middle. The two pairs of spinning appendages retain their primitive position in the middle of the lower surface of the opisthosoma far in advance of the anus on the 3rd and 4th somites, each appendage consisting of a stout, many-jointed outer branch and a slender, unsegmented inner branch. Prosoma as in the Mygalomorphae, except that the mesosternal area is long and narrow.
Family—Liphistiidae (Liphistius, *Arthrolycosa).
Sub-order b. Opisthothelae (see fig. 63).—Opisthosoma without trace of separate terga and sterna, the segmentation merely represented posteriorly by slight integumental folds and the sterna of the 1st and 2nd somites by the opercular plates of the pulmonary sacs. The spinning appendages migrate to the posterior end of the opisthosoma and take up a position close to the anus; the inner branches of the anterior pair either atrophy or are represented homogenetically by a plate, the cribellum, or by an undivided membranous lobe, the colulus.
| Fig. 62.—Liphistius desultor. Lateral view. |
| I to VI, Appendages of the prosoma cut off at the base. o, Ocular tubercle. prae-gen, The prae-genital somite. 1 and 2, Sternites of the first and second opisthosomatic somites. 3 and 4, Appendages of the third and fourth opisthosomatic somites, which are the spinning organs, and in this genus occupy their primitive position instead of migrating to the anal region as in other spiders. 5, Tergite of the fifth opisthosomatic somite. 11, Eleventh opisthosomatic somite; an, Anus. |
| (Original.) |
Tribe 1. Mygalomorphae.—The plane of the articulation of the appendages of the 1st pair to the prosoma (the retrovert) vertical, the basal segment projecting straight forwards at its proximal end, the distal segment or fang closing backwards in a direction subparallel to the long axis of the body. Two pairs of pulmonary sacs.
Families—Theraphosidae (Avicularia, Poecilotheria). Barychelidae (Barychelus, Plagiobothrus). Dipluridae (Diplura, Macrothele). Ctenizidae (Cteniza, Nemesia). Atypidae (Atypus, Calommata).
Tribe 2. Arachnomorpnae.—The plane of the articulation of the appendages of the 1st pair to the prosoma horizontal, the basal segment projecting vertically downwards, at least at its proximal end, the distal segment or fang closing inwards nearly or quite at right angles to the long axis of the body. The posterior pulmonary sacs (except in Hypochilus) replaced by tracheal tubes; the anterior and posterior pairs replaced by tracheal tubes in the Caponiidae.
Principal families—Hypochilidae (Hypochilus). Dysderidae (Dysdera, Segestria). Caponiidae (Caponia, Nops). Filistatidae (Filistata). Uloboridae (Uloborus, Dinopis). Argiapidae (Nephila, Gasteracantha). Pholcidae (Pholcus, Artema). Agelenidae (Tegenuria). Lycosidae (Lycosa). Clubionidae (Clubiona, Olios, Sparassus) Gnaphosidae (Gnaphosa, Hemiclaea). Thomisidae (Thomisus). Attidae (Salticus). Urocteidae (Uroctea). Eresidae (Eresus).
Remarks on the Araneae.—The Spiders are the most numerous and diversified group of the Arachnida; about 2000 species are known. No noteworthy fossil spiders are known; the best-preserved are in amber of Oligocene age. Protolycosa and Arthrolycosa occur in the Carboniferous. Morphologically, the spiders are remarkable for the concentration and specialization of their structure, which is accompanied with high physiological efficiency. The larger species of Bird’s Nest Spiders (Avicularia), the opisthosoma of which is as large as a bantam’s egg, undoubtedly attack young birds, and M’Cook gives an account of the capture in its web by an ordinary house spider of a small mouse. The “retrovert” or bent-back first pair of appendages is provided with a poison gland opening on the fang or terminal segment. Spiders form at least two kinds of constructions—snares for the capture of prey and nests for the preservation of the young. The latter are only formed by the female, which is a larger and more powerful animal than the male. Like the scorpions the spiders have a special tendency to cannibalism, and accordingly the male, in approaching the female for the purpose of fertilizing her, is liable to be fallen upon and sucked dry by the object of his attentions. The sperm is removed by the male from the genital aperture into a special receptacle on the terminal segment of the 2nd prosomatic appendage. Thus held out at some distance from the body, it is cautiously advanced by the male spider to the genital aperture of the female.
 | Fig. 63.—Ventral view ofa male mygalomorphous spider. |
I to VI, The six pairs of prosomatic appendages. a, Copulatory apparatus of the second appendage. b, Process of the fifth joint of the third appendage. M, Mouth. pro, Prosternite of the prosoma. mes, Mesosternite of the prosoma: observe the contact of the coxae of the sixth pair of limbs behind it; compare Liphistius (fig. 61) where this does not occur. stg, Lung aperture. gn, Genital aperture. a, Anus with a pair of backwardly migrated spinning appendages on each side of it; compare the position of these appendages in Liphistius (fig. 61). | |
| (From Lankester, “Limulus an Arachnid.”) |