Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/312

2i)2 AKACHNIDA [SPIDER*. concave, and always includes, either within its concavity or articulated to its under side, a (frequently complex) congeries of corneous lobes, spines, and spiny processes, in some instances articulated to each other so as to be capable of being opened out as by hinges. These are not developed until the spider arrives at maturity. Up to that period the digital joint has a tumid and somewhat semi-diaphanous appearance, and, although commonly smaller, bears the same general form that it has after maturity. These pro cesses, or, as they are called, palpal organs, are connected with the process of reproduction the fecundation of the female. This subject, however, we shall have occasion to refer to again. The palpi are generally furnished more or less with hairs, bristles, and spines, like the legs ; and like them, also, are reproduced after accidental loss. The labium is also another important portion of structure ; it is situated at the fore extremity of the sternum, between and in front of (or rather beneath) the maxillas; it is of varied form and size, and stopping the gap between the maxillae, it completes with them the inferior and lateral boundaries of the mouth, the falces forming its upper limits. The labium is always present, except in a new and very remark able Brazilian spider, Aphantochilus Rogersii (Cambr.), in which the labium is wanting, and yet not wanted, because the maxillae, in this instance, close up to each other, leav ing no gap between them. Between the labium and falces is another portion of structure, the tongue ; it is, no doubt, by the aid of this that the act of swallowing the juices of insects, when expressed by the falces and m axillae, is effected; that, in fact, the spider eats without being choked. No use hitherto has been made of the tongue in spiders for purposes of classification, but the form of the labium is very useful for the characterisation of genera. These parts falces, maxillae, labium, and tongue thus complete the mouth-parts of a spider. The sternum has already been mentioned ; its form varies from oval to heart-shaped, and sometimes somewhat pentagonal; its convexity also varies considerably; its use in classification is rather limited, but with its clothing of hairs, bristles, or pubescence, or its sometimes glabrous nature, it generally has some specific importance. In one known genus only, as mentioned above, is the sternum wanting. The abdomen is exceedingly varied in form in different families ; it is united to the cephalo-thorax by a short narrow pedicle, and generally terminates with organs for spinning ; in one species, Liphistius desultor (?) (Schiodte), the spinners are placed near the anterior part of the under side of the abdomen ; and in another, Mutusca mammosa (Cambr.), one pair only are so placed, the rest occupying the ordinary position. The number of spinners varies two, four, six, or eight ; they are placed in pairs, and when a fourth pair is present it is, usually as a single one, united throughout its length (but with often a line of division visible), and occupying a transverse position in front of the rest. It is but very lately that spiders have been observed with two spinners only; see "Spiders of Palestine and Syria," 0. P. Cambridge, Proc. Zool. Soc. 1872, p. 260; Ann. and. Mag. N. II. 1870, pp., 414-417, Ibid. The spinners consist of from one to three joints, and vary greatly in size and structure, as well as in number, but their use in classification has not hitherto been as great as might have been expected ; generally they are separate, but in Tetrablemma (Cambr.) they are enclosed in a kind of corneous sheath. Those of the fourth pair when present (which they are in both sexes) are correlated, but in general only in the female sex, with the calamistrum, or double series of curved bristles before mentioned, on the metatarsi of the fourth pair of legs. The function of the calamistrum has been proved by Mr Blackwall to be the carding, or teasing and curling, of a peculiar kind of silk, secreted and emitted from the fourth pair of spinners; when thus prepared this silk becomes adhesive, from its minutely divided and elastic fibres; and, disposed about the lines of the spiders snares, it renders more easy the entanglement and capture of their prey. Immediately above the spinners is a small nipple-like prominence which indicates the anal orifice. The abdomen is covered with a continuous integument neither annulate nor segmentate ; in one species, however (before mentioned), Liphistius desultor (Schiodte), the upper side has a longitudinal series of transverse corneous plates, like those of the Phrynides ; and something of a similar kind, but less marked, is observable in Tetrablemma medioculatum (Cambr.), a very rare and remarkable little spider found in Ceylon. The epidermis of the abdomen is of various kinds and consistency in different groups, sometimes excessively thin and tender, at other times tough and coriaceous, and sometimes hard and horny, with a varied armature and clothing of hairs, bristles, pubescence, tubercles, and spines, sometimes being, however, perfectly bare and glabrous ; its colours, also, and the patterns produced by their distribution, vary exceedingly. Under neath the fore extremity of the abdomen are placed tho external openings to the respiratory organs ; these apertures consist of a narrow slit or orifice under the fore edge of round or oval scale-like plates, placed on each side of the median line ; such plates are in some spiders four im number, but for the most part only two. Other spiracular openings also exist in many spiders, both at the anterior part of the under side (fig. 28, t, t) and near the spinners (Siebold), but these are usually far less perceptible than the others before mentioned, and often discoverable with great difficulty. The ordinary pair of spiracular plates are in general very conspicuous, commonly differing in colour from the surrounding surface, and sometimes slightly pubescent. They occupy each side of the median line at tho extreme fore part of the under side of the abdomen; and between them is, in the male spider, a very minute orifice which leads to the seminal organs; while in the same situation in the female is an aperture usually rather conspicuous, always of characteristic form, and often furnished with a corneous process of greater or less development. This aperture is the vulva, and the process (when present) probably has the function of an ovipositor. The form and structure of this aperture is of great use in the determination of species, but until the female spider attains maturity no aperture at all is visible. The orifices to the breathing organs are fringed with hairs or lashes r and open and close by an internal muscular arrangement. The other pair of spiracular plates, when present, are in some cases close behind those just above mentioned, but more commonly at some distance further towards the hinder extremity of the abdomen. On the upper side of the abdomen may be seen in many spiders several impressed spots or dots, differing in number and position in different genera and species, but always placed symmetrically. These, considered by Treviranus as stigmatic openings, are probably indicated points of fixture for muscles beneath the integument, being apparently imper- forate. Smaller puncture-like marks are also often visible on the under side, and are perhaps similarly caused. The hairy clothing of the abdomen (as well as of the cephalo- thorax and other parts), variously tinted and coloured, gives the specific pattern to the spider from its diversified distribution ; and when such clothing is absent, the pattern is often equally distinct and varied, arising in such cases from a pigment in the coats of the integument itself.

INTERNAL ORGANISATION. The Muscular System of