How insects behave 



Swarming 



Insects, like people, are gregarious at times, and seem to feel an urge 

 to congregate among others of their kind. This is a kind of reflex res- 

 ponse. When the temperature falls, the adults of a number of species 

 of insect have an impulse to move away from the light, and to press 

 themselves against some other object. This leads them to hibernate in 

 dark corners, all closely packed together, sometimes in thousands. 

 The cluster fly which infests many a North American attic during 

 the winter months is a too well-known example of this swarming ten- 

 dency of hibernating flies. 



Three kinds of swarms are familiar to most people : swarms of bees ; 

 swarms of midges on summer evenings ; and the destructive swarms of 

 locusts. 



Swarms of bees are part of the elaborate social life of bees, to which 

 we shall come later in this chapter. 



Swarms of midges are one among many instances of inseas which 

 congregate in the air for mating purposes. The swarm is normally com- 

 posed of males, and as it rises and falls in the breeze it is conspicuous 

 to us, and presumably to females of its own species. These fly in, mate, 

 and fly out again. It seems that the swarm is a convenient way of bring- 

 ing the sexes together, and shortening the time that they might other- 

 wise have to spend in searching for each other. This is particularly 

 important among insects that breed in water and have only a short 

 adult life in the air — e.g. mayflies — and in those like the winter-gnats 

 (Trichoceridae), whose aerial aaivity is Hmited to the few hours of a 

 winter's day when the temperature is high enough for them to fly. 



Swarms of locusts have been studied a great deal, because they do so 

 much damage to crops wherever they settle. Uvarov (192 1) first ex- 

 plained this behaviour by the Phase Theory. For a number of genera- 

 tions locusts behave like ordinary grasshoppers, feeding in certain of 

 the swampy grasslands of the Middle East: this is the solitary phase. 

 After a period of rapidly increasing numbers, there comes a change 

 both of appearance and behaviour, as they enter the gregarious phase. 

 They now become excited by the closeness of other individuals, and 

 the population finally moves off as a body across hundreds of miles of 

 country, to new breeding grounds. 



Locusts are bred in England regularly for experimental purposes, and 

 they can be caused to develop into the solitary or the gregarious phase 

 at will by varying the conditions under which they live, particularly the 

 extent to which they are crowded together. The phases are different, 

 not only in behaviour, but in colour and shape. The solitary grasshopper 

 is light green, the gregarious locust yellowish brown. 



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