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JOURNAL OF VARIATION. 



Vol. X. No. 9. September 15th, 1898. 



The Migration and Dispersal of Insects : General Considerations. 



By J. W. TUTT, F.E.S. 



It is well-known that many insects, at certain irregular periods, 

 leave the district in which thoy come to maturity and fly to other 

 localities. Sometimes these flights extend only to a comparatively short 

 distance, at other times, hundreds of miles are covered. The terra 

 migration is usually applied to these movements of insects, in common 

 with the more regular periodical movements which are carried out hy 

 birds and fishes. 



The application of the term to the movements of insects must, 

 therefore, be considered as referring to irregular dispersal movements 

 and by no means to regular movements to and from a given locality. 

 As a matter of fact, it has never yet been shown that the progeny 

 of any immigrants, which have settled in new quarters, have returned 

 to the home of their ancestors, although it has been suggested as 

 probable in the case of certain locusts, and also in the case of one 

 butterfly, Anosia archijrpns. 



It is possible that some winged insects have greater powers of 

 dispersal over the greater part of the earth's surface than any other 

 highly-organised animals. The power to fly immense distances ; the 

 ability of many species to go without food in the imago state for long 

 periods of time ; the lightness of their bodies, from which cause winds 

 and storms may aid them in flying great distances, often, indeed, over 

 hundreds of miles of ocean ; all these tend to give them a power of 

 dispersal probably unequalled even among birds. 



That insects migrate over vast distances is well known. Thoy 

 have been found at sea, flying strongly, hundreds of miles from land. 

 Ilawk moths and other Lepidoptcra frequently fly on board ships as 

 thoy approach the shores of tropical countries, and rest on ships in the 

 open ocean. Flocks of fragile insects, e.(j., Deiopeia 'pulchella and 

 Tcrias lisa, have been known to cover hundreds of miles without rest 

 of any kind. Dragonflies, beetles and Diptcra have also come on 

 board ship when far from land, and the power of locusts to cross wide 

 tracts of sea is well known. Instances of insects being met with at 

 sea will be given when the subject is considered in greater detail. 



Besides the remarkable powers of flight exhibited by many insects, 

 it is evident that almost all tho causes, some more or less accidental, 

 that tend to the dispersal of the higher animals, would also be 

 favourable to their distribution, in some cases, in a much higher 



