Insect migration

Insect migration is the seasonal movement of insects, particularly those by species of dragonflies, beetles, butterflies and moths. The distance can vary from species to species, but 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. The most famous insect migration is that of the Monarch butterfly which migrates from southern Canada to wintering sites in central Mexico where they spend the winter. In the late winter/early spring, the adult monarchs leave the Transvolcanic mountain range in Mexico for points North. Mating occurs and the females begin seeking out milkweed to lay their eggs, usually first in northern Mexico and southern Texas. The caterpillars hatch and develop into adults that move north, where more offspring can go as far as Central Canada until next migratory cycle.

Definition
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 kilometres 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 behaviour oriented definition proposed is "Migratory behaviour is persistent and straightened-out movement effected 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."

- Kennedy, 1985

This definition disqualifies movements made in the search of resources and which are terminated upon finding of the resource. Migration on the other hand involves longer distance movement and these movements are not affected by the availability of the resource items.

General patterns
Migrating butterflies fly within a boundary layer, with a specific upper limit above the ground. The air speeds 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. Taylor, L.R. (1974) Insect migration, flight periodicity and the boundary layer.

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.

Migration being energetically costly has been studied in the context of life-history strategies. It has been suggested that adaptations for migration would be more valuable for insects that live in habitats where resource availability changes seasonally. Others have suggested that species living in isolated islands of suitable habitats are more likely to evolve migratory strategies. The role of migration in gene flow has also been studied in many species.

Orientation
Migration is usually marked by well defined destinations which need navigation and orientation. A flying insect needs to make corrections for crosswinds. It has been demonstrated that many migrating insects sense windspeed and direction and make suitable corrections. Day-flying insects primarily make use of the sun for orientation, however this requires that they compensate for the movement of the sun. Endogenous time-compensation mechanisms have been proposed and tested by releasing migrating butterflies that have been captured and kept in darkness to shift their internal clocks and observing changes in the directions chosen by them. Some species appear to make corrections while it has not been demonstrated in others.

Most insects are capable of sensing polarized light and they are able to use the polarization of the sky when the sun is occluded by clouds. The orientation mechanisms of nocturnal moths and other insects that migrate have not been well studied, however magnetic cues have been suggested in short distance fliers.

Recent studies suggest that migratory butterflies may be sensitive to the earth's magnetic field on the basis of the presence of magnetite particles. In an experiment on the monarch butterfly, it was shown that a magnet changed the direction of initial flight of migrating monarch butterflies. However this result was not a strong demonstration since the directions of the experimental butterflies and the controls did not differ significantly in the direction of flight.

Lepidoptera
Migration in the butterflies and moths are particularly well known. The Bogong moth is a native insect of Australia that is known to migrate to cooler climates. In southern India, mass migrations of many species are noted prior to the monsoons. As many as 250 species of butterflies in India are migratory. These include members of the Pieridae and Nymphalidae.