Page:The Scientific Monthly vol. 3.djvu/243

 INSECT MIGRATION 237

Unmistakable as these mountain barriers are, however, both they and the lake and ocean obstacles, already noted, seem comparatively slight disturbances when one considers the general regularity of the north and south vallevs and coast-lines in North America which are the natural lanes of travel. So, in this country, true insect migration, as well as that of birds, falls into comparatively simple, unobstructed channels. In Europe, on the contrary, the east-and-west position of the great mountain ranges, as well as the ever-varying direction of the coast- line, offer peculiar difficulties to the insect flights which, therefore, become deflected and confused. These conditions alone are sufficient to account for the great varieties of direction reported for dragonfly swarms on that continent. In fact, they may also account for the apparent failure of a true interpretation there; for only the study of our more direct, but corresponding, activities on this continent has seemed to throw a considerable light upon the problem. Butterfly migrations in the old world, too, have shown such varied behavior that the idea of regular seasonal movements rests upon far less definite data than has been obtained in this countrv. Nevertheless, as the writer has pointed out in a previous article,^ the recorded instances of dragonfly migrations in Europe remarkably coincide with certain flight-ways of migratory birds; so there, as well, persistent scrutiny of certain conti- nental highways should shed an additional light upon the other insect migrants that travel there.

The more they are studied the more impressive and thought-pro- voking do these parallel behaviors become, until one seeks to flnd some complementary light which may be shed by one creature on the activi- ties of the other. For the hitherto almost unnoted spectacle of both life-orders flooding along the same great continental highways quite naturally gives rise to thought and conjecture. Indeed, it is not im- possible that the insects, being perhaps the first historic travelers along the highways, attracted the insect-eating birds, resulting in an habitual adherence to those routes, where a constant food-supply could be secured during their long flight to the south. As a matter of fact an early report of a dragonfly swarm passing over Dresden states that "starlings, blackbirds and sparrows accompanied the insects and threw themselves upon them with great eagerness." Still, excepting for hawks, king- birds, the purple martin and, to a lesser extent, the swallows, dragon- flies are not habitually chosen for food. "Monarchs," moreover, are notoriously distasteful to birds. So, as regards these two insects at least, such a theory finds little support.

Xevertheless, there may be even other forms of insect life which habitually use the great highways^quite different insects which wovld he desirable for food. In the writer's opinion there are considerable

i*'Do Insects Migrate Like Birds?'' Harpers Magazine, Sept., 1915.

�� �