Insects and their world 



Migration 

 It is only a step from the swarming of locusts to the migration of insects. 

 In fact, Williams (1958), who has dealt exhaustively with this subject 

 from a lifetime of experience, points out that some other migratory 

 insects, notably moths, have two phases, a dark swarming one and a 

 pale solitary one. Yet he is careful to emphasise that apparently insects 

 do not migrate just because they are overcrowded. 



The Monarch butterfly in North America is the best known insect 

 migrant. During the summer the butterflies are not crowded together, 

 but are scattered over the countryside, in the way that butterflies usually 

 are. Yet at the end of summer they assemble, and move southwards in 

 great numbers, covering perhaps a thousand miles. When they settle 

 down for the winter they are strongly gregarious, and pack themselves 

 tightly together, several thousand per tree. In the spring the butterflies 

 wake up, and fly back northwards one at a time, not as a band. 



This butterfly illustrates several characteristics of insect migration. 

 Breeding takes place at the more northerly (i.e. colder) end of the range, 

 and the movement south avoids the rigours of the winter. The stay at 

 the southern end is merely a resting stage. It is significant that the 

 butterflies assemble together for the southern journey but make the 

 return individually. Because of this it used to be thought that mass flights 

 of insects took place in one direction only, and if this had been so it 

 could not have been called migration in the true sense of the word. It 

 seemed then to be a way of shedding surplus population, and it was 

 difficult to explain how such a habit could have arisen in evolution. 



Now, however, it seems likely that every mass flight has its return 

 movement, though the returning insects, being scattered, are more 

 difficult to observe. Williams lists some thirty different species of butter- 

 fly that are known to migrate and return. Some of them, like the Mon- 

 arch, breed at one end of the range and rest at the other; some breed 

 at both ends of the range (Hke the locusts) and arrive there at the most 

 favourable season. Migrating insects other than butterflies include 

 dragonflies and hoverflies. 



Williams gives a whole chapter to discussing what might decide the 

 direction of flight. We must distinguish between the course, which is the 

 direction in which the insect points its head and tries to move, and the 

 track, which is the direction in which it actually moves over the ground, 

 under the combined influence of its own flight and the effect of the wind. 

 As insects rise higher above the ground the wind velocity generally 

 becomes greater; when it becomes greater than the maximum speed at 

 which the insects can fly, then they are carried steadily downwind 



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