Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/311

SPIDERS.] AKACHNIDA 291 horny plates, except in two instances Liphistius dtsultor (Schiodte) and (but partially only) TetraUemma mediocula- tum (Cambridge), both exceedingly rare and remarkable species. The cephalo-thorax, though undivided, yet shows by its various, and more or less strongly marked, converging grooves and indentations, that it is composed of the ordinary cephalic and thoracic segments soldered together ; and although these segments form but one undivided por tion of structure, it is necessary to &peak of it frequently by the names of its two portions caput and thorax. The caput is almost always easily traced, forming a kind of wedge-shaped portion, jammed (as it were) into the first of the thoracic segments, and giving one the idea, in most cases, of its fore part having been to a greater or less extent truncated or lopped off (fig. 28, b). Similarly the last of the four thoracic segments has the appearance of having been driven in to meet the caput, all the segments thus converg ing, on the upper side, towards a common fovea, or more or less deep central indentation (fig. 28, <?), and on the under side forming a sternum (or sternal plate), in which, however, no appearance of segmentation is generally visible beyond email eminences opposite to the articulation of the legs, with which the number of the thoracic segments, as seen above, agrees. The integument of the cephalo-thorax is either hard, horny, or coriaceous, generally more or less clothed with hairs and bristles, and sometimes, though more rarely, with tubercles and spines ; occasionally it is perfectly smooth and glabrous. The eyes, when present, are two, four, six, or eight, and are very variously, but always symmetrically, disposed on the fore part of the caput (figs. 28, 29, 30). The number and general position of the eyes form valuable characters for the for mation of genera ; while their rela tive size is strongly characteristic of species. The eyes of spiders, like those of other Arachnids, are always simple. The legs, eight in number, are arti culated to the sternal plate (mentioned above) which forms the under side of the cephalo-thorax ; in one genus. Fro. 29. Caputof Attut i i /if /"IT. showing position of eyes. however (Miagrammopes, Uambr.), found in Ceylon and Australia, no sternum properly so called exists, the legs being articulated to the continuous under side of the cephalo- thorax. Except in one or two species the legs are seven-jointed, and variously furnished with hairs, bristles, and spines. Many spiders have a kind of pad of closely set papilla3-fonn hairs beneath the extremity of the tarsi of some or all the legs, and often extending along the whole of the under side of the joint. This pad enables the spider to run over smooth vertical sur faces, acting as a kind of sucker, partly by atmospheric pressure, and partly (Mr Blackwall thinks wholly) by means of a viscid secretion from the papillse-form hair=? Eioli tirsnq pnrk with Flo> M. Capnt of WaMtnalra $ Wltn acuminate, Blackw., showing either two or three more or less position of eyes, curved or bent claws, commonly (but not always) pectinate or finely denticulated ; these (with other opposed serrated claws, in some groups) are used in traversing their webs, and as hooks to give a tension to the line of their snares by alternately pressing and straining upon them ; the spines and bristles are used also in many cases in the actual construction of the silken snares in which spiders entrap their prey. The spines on the legs of some spiders which excavate cylindrical holes in the earth (lined with silk and closed with a hinged lid) are very strong, and well suited for digging out the soil. The males of some species, again, have a curious row of short, closely set, curved, spiny bristles along a portion of the upper side of the metatarsi of the fourth pair of legs ; the use of this row of bristles (called the calamistrum) is alluded to further on. The length of the legs in spiders (both actual and relative) is very various ; and the differences between them, as well as their armature and terminal claws, furnish valuable charac ters, often generic, and always important in the determina tion of species. The falces, two in number, are articulated immediately below the fore margin of the caput ; their direction is various, ranging from a line parallel to the plane of the cephalo-thorax to one perpendicular to it ; they are in general opposed to each other, and in most cases are armed with teeth on their inner surface, especially towards the extremities ; each also terminates with a movable curved fang, which, when not in use, is (according to its mode of articulation) folded down either across the inner side of the extremity of the falx, or (as in one extensive family, Theraphosides) backwards along its length ; by means of these fangs, which are internally channelled and perforated at their extremities, a poison secreted within the caput is instilled into the wound made by them, proving, no doubt, fatal to the spider s prey, and often nearly so to human beings. Not to dwell upon the, probably exaggerated, accounts of the Tarantula, there are well-authenticated instances of recent date in regard to the very deadly nature of the poison of quite a small spider, Lathrodectus Katipo (LI. Powell), found in New Zealand (Trans. N. Z. Instit. iii. p. 56; also Id. ii. p. 81, and iii. p. 29). A larger European species of the same genus, L. ocidatus (Walck.), also bears the reputation of being venomous. The maxillae are a conspicuous, as well as an important, portion of structure in the Araneidea ; they are two strong pieces situated immediately behind the falces, exceedingly varied in form and strength, and articulated to the fore side of the sternum between the basal joints of the first pair of legs. Their real direction is always divergent, i.e., outwards from each other, but as in most cases the inner extremity is produced into an apophysis of greater or less size (sometimes following the direction of the maxilla, sometimes curved so that its extremity almost touches that of the opposite maxilla), the maxilla itself often seems lost in the part which projects from it. Owing to this, the palpus which springs from each maxilla, although really always springing from its extremity, appears more frequently to issue from its outer side ; and, in fact, it can hardly in those cases be described (without leading to some con fusion) as issuing in any other way. The form of the maxillae, together with the apparent point of palpal issue, constitutes one of the most valuable characters for the establishment of genera. The palpi, issuing from the maxilla? as just described, are two (in both sexes of many, and in the female sex of most species, leg-like) limbs, of five or (counting the basal joint maxilla to which each is articulated) six joints; in the female spider the palpi generally end with a single, and sometimes denticulate, curved claw; in some instances, however, there is no terminal palpal claw ; but in the male, the third (ntbitat) and fourth (radial) joints of the palpi are (the former often, the latter always) characterised by prominences, spiny apophyses, or protuberances, which furnish some of the strongest, as well as most tangible, specific characters of that sex ; the last (or digital) joint of

each palpus (in the male spider) is generally more or less