It is quite impossible in a work like the present to deal with the classification of spiders. About forty families have been established, some of them of vast extent, the Attidae, for example, including some four thousand species. The great French arachnologist, M. E. Simon, has occupied 2,000 quarto pages in defining the families, sub-families and genera, without concerning himself with the species at all! It is, however, desirable, that the attention of the reader should be called to the primary division of the group, according to which all spiders are either _Araneae verae_ (true spiders) or _Araneae theraphosae_ (theraphosid spiders.) Now these two kinds of spider may readily be distinguished by a single easily observable characteristic, the nature of the mandibles or _chelicerae_; but it is necessary to describe the spider’s mandibles before the difference can be appreciated. Their nature is perhaps best explained by saying that each mandible is not unlike a penknife with a single small blade, rather more than half open when in use, closed when at rest. The handle of the penknife is certainly in most cases very short and thick, and the blade not really a blade at all, for it has no cutting edge, but is a “fang” or piercing instrument generally somewhat curved, and with a sharp point. The “blade” is, moreover, perforated by a tube which comes from the poison-gland, situated in the thickened “handle,” or in the spider’s head, so that poison can be forced into the wound which it inflicts. Now take two penknives with the blades half open and hold them so that they hang with the hinge downward and with the blades directed towards each other; it is clear that the blades may be made to pierce an object situated between them by moving the handles laterally, the object being attacked simultaneously on either side. This is the arrangement in the true spiders, whose jaws move sideways, though they do not always hang perpendicularly, but are more often somewhat slanted forwards. To represent the jaws of a theraphosid spider the penknives must be arranged differently. Place the handles horizontally and parallel to each other, with the blades directed downwards and also parallel. They will now work not sideways, but up and down, and both fangs will pierce the victim from above. In a word, the true spiders have jaws which can be separated or brought together, and which tend to meet in the object into which they are plunged, while the jaws of theraphosid spiders work in parallel vertical planes, and strike downwards. All the spiders which have so far concerned us are _Araneae verae_, and we have incidentally had occasion to note some of the principal families of that division—Epeiridae (or Argiopidae as some prefer to call them), Theridiidae, Agelenidae, Thomisidae, Lycosidae and Attidae.