This second pair of maxillae (or labium) form then the hinder or lower boundary of the mouth. In front or above the mouth is bounded by the labrum, while the mandibles and first maxillae lie on either side of it. A median process, known as the _hypopharynx_ or tongue, arises from the floor of the mouth in front of the labium, and becomes most variously developed or specialized in different insects. The salivary duct opens on its hinder surface. It does not appear to represent a pair of appendages, but the maxillulae of the Aptera become closely associated with it. According to the view of R. Heymons, the hypopharynx represents the sterna of all the jaw-bearing somites, but other students consider that it belongs to the mandibular and first maxillary segments, or entirely to the segment of the first maxillae.
_Neck._--The head is usually connected with the thorax by a distinct membranous neck, strengthened in the more generalized orders with small chitinous plates (_cervical sclerites_). These have been interpreted as indicating one or more primitive segments between the head and thorax. Probably, however, as suggested by T. H. Huxley (_Anat. Invert. Animals_, 1877), they really belong to the labial segment which has not become completely fused with the head-capsule. It has been shown by C. Janet (1889), from careful studies of the musculature, that the greater part of the head-capsule is built up of the four anterior head-segments, the hindmost of which has the mandibles for its appendages, and this conclusion is in the main supported by the recent work on the head skeleton of J. H. Comstock and C. Kochi (1902) and W. A. Riley (1904).
_Thorax._--The three segments which make up the thorax or fore-trunk are known as the _prothorax_, _mesothorax_ and _metathorax_ (see fig. 3). The dorsal area of the prothorax is occupied by a single sclerite, the _pronotum_ (fig. 3, d), which is large and conspicuous in those insects, such as cockroaches, bugs (Heteroptera) and beetles, which have the prothorax free--i.e. readily movable on the segment (mesothorax) immediately behind--smaller and of less importance where the prothorax is fixed to the mesothorax, as in bees and flies. The dorsal area of the mesothorax, and also of the metathorax, may be made up of a series of sclerites arranged one behind the other--_prescutum_, _scutum_, _scutellum_ and _post-scutellum_ (fig. 3, e, f, g, h), the scutellum of the mesothorax being often especially conspicuous. Ventrally, each segment of the thorax has a _sternum_ with which a median _pre-sternum_ and paired _episterna_ and _epimera_ are often associated (see figs. 3, 4). The recent suggestion of K. W. Verhoeff (1904) that the hexapodan thorax in reality contains six primitive segments is entirely without embryological support.