TECHNICAL NOTE.--Locusts or grasshoppers are common and familiar insects all over the country. The genus _Melanoplus_ includes numerous species, one or more of which are to be found in almost any locality. The common red-legged locust (_M. femur-rubrum_) of the East, the Rocky Mountain migratory locust (_M. spretus_), of the West, the large differential (_M. differentialis_) and two-striped (_M. bivittatus_) locusts of the Southwest, are especially common species. All the members of the genus have their hind wings uncolored, and the front wings marked with a longitudinal series of small dots more or less distinct, or with a longitudinal line. There is a small blunt spine or process projecting from the ventral aspect of the prothorax. If a species of _Melanoplus_ cannot be found, any other locust may be used, although there are some slight variations in the external structure of the various species. Fresh specimens killed in a cyanide bottle (for preparing see p. 463) are preferable in the study of the external structure, but specimens preserved in alcohol will do. =External structure= (fig. 38).--Note that the body of the grass-hopper is composed of successive rings or _segments_ grouped into three regions, the _head_ (anterior), _thorax_ (median), and _abdomen_ (posterior). In which region of the body are the segments most readily distinguished? Of how many segments does the head appear to be composed? The thorax is composed of three segments of which the most anterior, to which is attached the front pair of legs, differs from the succeeding two, being freely movable and bearing a large hood- or saddle-shaped piece on its dorsal aspect. To the other two thoracic segments the second and third pair of legs are attached, as are also the two pairs of wings. The remaining segments of the body compose the abdomen. [Illustration: FIG. 38.--The red-legged locust, _Melanoplus femur-rubrum_, to show external structure.] Note the smooth, rather firm and horny character of the body. This is due to the fact that the skin is everywhere covered with a cuticle in which is deposited a horny substance called _chitin_. The cuticle is not uniformly firm over the body. At the junction of the body segments in the abdomen, in the neck and between the segments of the legs, in fact, wherever motion is desirable, the cuticle is flexible, thus making bending of the body-wall possible. Elsewhere, however, it is hard and stiff, serving not only as a protective coat or armor over the body, but also affording firm places for the attachment of muscles. Insects (and all other Arthropods) have no[9] internal skeleton, but, in this firm cuticle, an _exoskeleton_.