Insect migration

Insect migration is the seasonal movement of insects, particularly those by species of dragonflies, beetles, butterflies and moths. The distance can vary with species and in most cases, these movements involve large numbers of individuals. In some cases, the individuals that migrate in one direction may not return and the next generation may instead migrate in the opposite direction. This is a significant difference from bird migration. All insects move to some extent. The range of movement can vary from within a few centimeters for some sucking insects and wingless aphids to thousands of kilometers in the case of other insects such as locusts, butterflies and dragonflies. The definition of migration is therefore particularly difficult in the context of insects. A behavior-oriented definition proposed is Migratory behavior is persistent and straightened-out movement affected by the animal's own locomotory exertions or by its active embarkation on a vehicle. It depends upon some temporary inhibition of station-keeping responses but promotes their eventual disinhibition and recurrence. This definition disqualifies movements made in the search of resources and which are terminated upon finding the resource. Migration involves longer distance movement and these movements are not affected by the availability of the resource items. All cases of long-distance insect migration concern winged insects. Migrating butterflies fly within a boundary layer, with a specific upper limit above the ground. The airspeeds in this region are typically lower than the flight speed of the insect. These 'boundary-layer' migrants include the larger day-flying insects, and their low-altitude flight is obviously easier to observe than that of most high-altitude windborne migrants. Many migratory species tend to have polymorphic forms, a migratory one, and a resident phase. The migratory phases are marked by their well-developed and long wings. Such polymorphism is well known in aphids and grasshoppers. In the migratory locusts, there are distinct long and short-winged forms.