_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. _Legs._--Each segment of the thorax carries a pair of legs. In most insects the leg is built up of nine segments: (1) a broad triangular, sub-globular, conical or cylindrical haunch (_coxa_); (2) a small _trochanter_; (3) an elongate stout thigh (_femur_); (4) a more slender shin (_tibia_); and (5-9) a foot consisting of five _tarsal segments_. The fifth (distal) tarsal segment carries a median adhesive pad--the _pulvillus_--on either side of which is a claw. The pulvillus is probably to be regarded as a true terminal (tenth) segment of the leg, while the claws are highly modified bristles. Numerous bristles are usually present on the thighs, shins and feet of insects, some of them so delicate as to be termed "hairs," others so stout and hard that they are named "spines" or "spurs." In the relative development and shape of the various segments of the leg there is almost endless variety, dependent on the order to which the insect belongs, and the special function--walking, running, climbing, digging or swimming--for which the limb is adapted. The walking of insects has been carefully studied by V. Graber (1877) and J. Demoor (1890), who find that the legs are usually moved in two sets of three, the first and third legs of one side moving with the second leg of the other. One tripod thus affords a firm base of support while the legs of the other tripod are brought forward to their new positions. [Illustration: After Marlat, _Ent. Bull._ 3, n.s. (U.S. Dept. Agr.). FIG. 3.--Thorax of Saw-Fly (_Pachynematus_).