MIGRATION AND DISPEKSAL OF INSECTS : FINAL CONSIDERATIONS. B19 



and the author also points out some generalisations suggested by the 

 facts at disposal. The species dealt with are chiefly introduced ones — 

 Pieris rapae, AiJiudiiis J'ussor, Ci'inccris a>ijiarai/i, I'/ii/toHonnis piinctatus, 

 Haeniatobia ^icrrata, Cr!/j)torli>j)ic/iiis lapathi, &c., and hence are some- 

 what outside the purview of this paper, except that a want of know- 

 ledge of these and their mode of dispersal might vitiate generalisations 

 based on observations made on native species. The general spread of 

 those species that have entered the States from Canada has been south, 

 and must be carefully distinguished from the dispersal of native species, 

 ('jj., Doryphora dcceiiilhtcata, Leptoccris triritfatus, Diabrotica lonifi- 

 coniis, which, coming up from the south, extend north and east, making 

 more or less permanent settlements under favourable conditions of 

 environment, and appear to be especially influenced by the increase of 

 food supplies derived from the spread of agriculture. Broadly, the 

 dispersal movements of insects in the Nearctic area are similar to those 

 observed in the Palnearctic, viz., from subtropical to temperate regions, 

 but, at the same time, it must not be overlooked that there is some 

 evidence of the spread of certain members of the subarctic fauna 

 southward. Schwarz has dealt (Proc. Ent. Soc. Wash., i., p. 186) with 

 the distribution of Aphodiiis vKjijies and Noniius jiiji/nun'ini, but the facts 

 at present are not numerous, and the whole question bristles with 

 difficulties when one attempts any generalisations. One can only hope 

 that the near future will give us more reliable facts on which to base 

 our generalisations. 



There are but few conclusions that can be formulated in the present 

 state of our ignorance, and, as we have already said, the different 

 phenomena included in our summary of the recorded movements 

 observed in the species of the various orders, and the different factors 

 influencing the different orders, of insects considered, make generalisa- 

 tions rather worse than useless. If we say that migrations take place 

 irregularly, and only under stress of the development of an unusual 

 abundance of species resulting in the failure of the foodplant, so that 

 there is a surplus population to be got rid of, it may be quite true with 

 regard to the occasional extensive movements of lepidoptera, but is 

 absurd in its application to the regular autumnal spring movements of 

 aphides from one foodplant to another, the annual spring swarming 

 of Apis miilifica, or the autumnal nuptial swarming of ants. We may 

 assert that, on the whole, the general trend of all dispersal movements 

 in the northern hemisphere is northerly, and in the southern hemis- 

 phere is southerly, but we are well aware, and have just illustrated 

 the point, that local dispersal movements may be in other directions, 

 whilst it seems clear that there are no return swarms that are 

 at all comparable with the going and returning journeys observed 

 in bird migrations, i.e., no return autumnal journey in a direction 

 opposite to that taken in spring. Nor are we at all clear what 

 inherent impulse is the prime factor in bringing about, in some 

 species, at far distant times, the desire to migrate and to leave 

 the place in which they are bred. On all points relating to 

 the migration of insects we want detailed facts based on accurate 

 observations. When these have accumulated we may be in a much 

 better position to discuss the rationalr of migration. At present we 

 have only attempted to bring together the observations already made 

 and to add such remarks as the facts at present seem to warrant. 



