The Blattidae, or cockroaches, are an extensive family of Insects, very much neglected by collectors, and known to the ordinary observer chiefly from the fact that a few species have {221}become naturalised in various parts of the world in the houses of man. One such species is abundant in Britain, and is the "black beetle" of popular language; the use of the word beetle in connexion with cockroaches is, however, entomologically incorrect. One or two members of the family are also well known, owing to their being used as the "corpora vilia" for students commencing anatomical investigation of the Arthropoda; for this purpose they are recommended by their comparatively large size and the ease with which an abundant supply of specimens may always be procured, but it must be admitted that in some respects they give but a poor idea of Insect-structure, and that to some persons they are very repulsive. The inflexed position of the head is one of the most characteristic features of the Blattidae; in activity it is partially released from this posture, but the mouth does not appear to be capable of the full extension forwards that is found in other Insects that inflex the head in repose. The labium is deeply divided, the lingua forms a large lobe reposing on the cleft. The maxillary palpi have two basal short joints, and three longer joints beyond these; the labial palps consist of three joints of moderate length. The under-surface of the head is formed in large part by the submentum, which extends back to the occipital foramen. [Illustration: FIG. 119.—Under-surface of _Periplaneta australasiae_. _c_, Coxae.] The front of the head is the aspect that in repose looks directly downwards; the larger part of it is formed by the clypeus, which is separated from the epicranium by a very fine suture angulate in the middle; there is a large many-facetted eye on each side; near to the eye a circular space serves for the insertion of the antenna; close to this and to the eye there is a peculiar small area of paler colour, frequently membranous, called the fenestra, and which in the males of _Corydia_ and {222}_Heterogamia_ is replaced by an ocellus. The antennae are very elongate and consist of a large number of minute rings or joints, frequently about 100. The head is not inserted directly in the thorax, as is the case in so many Insects; but the front of the thorax has a very large opening, thus the neck between it and the head is of more than usual importance; it includes six cervical sclerites.