It is well established that locusts of the migratory species exist in countries without giving rise to swarms, or causing any serious injuries; thus _Pachytylus cinerascens_—perhaps the most important of the migratory locusts—is always present in various localities in Belgium, and does not give rise to swarms. When migration of locusts does occur it is attended by remarkable manifestations of instinct. Although several generations may elapse without a migration, it is believed that the locusts when {294}they migrate do so in the direction taken by predecessors. Their movements are to a large extent dependent on the wind, and it is said that they make trial flights to ascertain its direction. When on the wing probably very little muscular effort is necessary. Their bodies contain elastic air sacs in communication with the tracheae, and at the time of flight it may be presumed that the body is comparatively empty, food being wanting, and the internal organs of reproduction, which occupy a large space when in activity, yet undeveloped, hence the sacs have full room for expansion, as explained on p. 283. Thus the Insects exert but little effort in their aerial movements, and are, it is believed, chiefly borne by the wind. Should this become unfavourable it is said that they alight and wait for a change. The most obscure point in the natural history of the migratory locusts appears to be their disappearance from a spot they have invaded. A swarm will alight on a locality, deposit there a number of eggs, and then move on. But after a lapse of a season or two there will be few or none of the species present in the spot invaded. This appears to be partly due to the young locusts dying for want of food after hatching; but in other cases they again migrate after growth to the land of their ancestors. The latter fact is most remarkable, but it has been ascertained by the U.S. Entomological Commission that these return swarms do occur. [Illustration: FIG. 176.—Portions of body of _Caloptenus spretus_ to show some of the air-sacs. (Modified from Packard.) A, Dorsal aspect of anterior parts; B, lateral aspect of posterior parts of body; _a_, enlargements of tracheae in head; _b_, pair of large sacs in thorax; _c_, sacs on the tracheal trunks of abdomen; _s_, spiracles.]