MIGRATION AND DISPERSAL OF INSECTS : FINAL CONSIDERATIONS. 315 



Migration and Dispersal of Insects : Final Considerations. 



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



Nothing would have given us greater satisfaction than to have 

 discovered in our enquiries into this subject an analogy between the 

 migration of insects and the migration of birds, but we can find none. 

 The latter is regular, systematic and purposeful, the former appears to 

 be spasmodic, irregular, uncertain, and undertaken solely on account of 

 the absolute necessities of the time, yet there is one point in which an 

 analogy appears to exist. Migrating birds from the tropical and 

 subtropical areas north of the equator go north, those from the tropical 

 and subtropical countries south of the equator go south ; so undoubtedly 

 do the insects, and one is tempted to suspect that the primary motive was 

 common to both. It has been suggested that, as the present northern 

 subarctic regions in comparatively recent (Miocene) geological times 

 had a subtropical climate, and possibly fauna and flora, the 

 slowly changing climatic conditions drove certain members of the 

 fauna (and flora) southward very gradually during the winter, but 

 that they regained the lost ground in summer and returned to their 

 old haunts to breed, and that thus an attempt was made to 

 maintain the old ground. But each successive winter drove them 

 just a little further than its predecessor, and the distance to return 

 in summer became greater ; and then, when, later, the climatic 

 conditions became gradually ameliorated, it is supposed they 

 attempted and still attempt slowly to regain the lost ground. 

 There are many objections to this being considered as initiating a 

 migrating tendency such as that observed in many insects. It may be 

 accepted as forming a basis for the acquisition of a habit of following- 

 up annually the region that best afforded food at each season, but it is 

 diflicult to understand how such a dispersal tendency for food purposes 

 could be changed into a migrating tendency on a large scale, especially 

 in such short-lived animals as insects. On the other hand as an 

 element in dispersion within given limits, one sees that climatic 

 conditions may come in. As a matter of fact, a species occupies a 

 certain area, some a greater, others a less, usually in a number of 

 isolated habitats. Every species spreads, or attempts to spread, each 

 year beyond its normal limits, but is cut back ruthlessly by climate, 

 etc., to its normal area. Seasons, however, vary, and, according to the 

 conditions, the species may be pruned more severely, in some seasons 

 on its northern, in others on its southern, limits. If it happen that 

 the pruning is done more markedly on the northern or southern side 

 for several seasons together, there will be a swinging of the central 

 point of the habitat north or south as the case may be, whilst on the 

 whole the average remains stationary. The species may be looked upon 

 as being secure at this central point and tries to spread in all directions, 

 but is continually pressed at all sides to keep to its true territory. If 

 now the secular change of climate above hinted at came in, so that 

 there was an excess of pruning done on the north, and if there were 

 none done on the south margin, as is assumed, the species would 

 gradually shift southwards, but, as far as actual spreading or migration 

 is concerned, the species really does nothing at all difierent from what 

 it did before and can acquire no habit of migration thereby. The fact 

 is we are scarcely yet in a position to speculate on the stimulus that 



