though other species do not. [Illustration: FIG. 174.—Development of wings in _Caloptenus spretus_: the upper row gives a lateral view of the thoracic segments, and the lower row a dorsal view of these segments; 1, second instar; 2, third instar; 3, fourth instar; 4, fifth instar. (After Riley.) _t_, tegmen; _w_, wing.] {289}[Illustration: FIG. 175.—_Caloptenus spretus._ North America. A, Newly hatched, much magnified; B, adult, natural size. (After Riley.)] Riley and Packard have given an account[221] of some parts of the post-embryonic development of the Rocky Mountain Locust, which enables us to form a satisfactory conception of the stages of development of the wings. Fig. 175, A, represents the first instar, the young locust, just emerged from the egg and colourless. Fig. 174 shows some of the subsequent stages of development of the wings, the upper line of figures giving a profile view of the thoracic segments, and the lower line showing their dorsal aspects; 1 shows the condition of the parts in the second instar, the chief difference from the first instar being the development of colour; in the third instar there is an evident slight development of the future alar organs, exhibited chiefly in the outgrowth and lobing of the free posterior angles of the meso- and metanota, as shown in Fig. 174, 2. After the third moult there is a great difference; the instar then disclosed—the fourth—has undergone a considerable change in the position of the meso- and metathoraces, which are thrust forward under the pronotum; this has become more enlarged and hood-like (Fig. 174, 3); at the same time the wing-rudiments have become free and detached, the metathoracic pair being the larger, and overlapping the other pair. The fifth instar (Fig. 174, 4) differs but little from the fourth, except in the larger size of the pronotum and wing-rudiments. The sixth—shown in Fig. 175, {290}B—is the perfect Insect, with the alar organs free and large, the prothorax much changed in form, the colour different. From the above it will be seen that the chief changes occurred at the third and fifth ecdyses, after each of which a considerable difference in the form of the Insect was revealed. In the first three instars the sexes can scarcely be distinguished, in the fourth they are quite distinct, and in the fifth coupling is possible, though usually it does not occur till the final stage is attained. The discovery that Orthoptera change their colours in the course of their development, and even after they have become adult, is important, not only from a physiological point of view, but because it throws some light on the questions as to the number of species and the geographical distribution of the migratory locusts, as to which there has existed a great confusion.