3lS The entomologist's record. 



Southern America. These so-called migrating swarms usually iiy 

 from the northwest to southeast, and it has been suggested that this 

 is due to the fact that at the time of migration the vegetation of the 

 Pacific slopes is parched, whilst the forests of the Atlantic slopes are 

 green. But the fact that the butterflies fly year after year from the 

 same direction shows that, wherever they come from, they must 

 lay their eggs, at least in part, before migration. Besides, the 

 Pacific slopes are certainly not to the northwest of these areas, 

 and the butterfiies are not suspected of crossing the Rocky Mountains or 

 the Andes. The migrating species in Nicaragua, referred to by various 

 writers, must come from the forests of Honduras and Guatemala, 

 and those in the Amazon valley from the countries to the north 

 and northwest of that river. It appears, therefore, that these 

 so-called migrations are at the most only local dispersal move- 

 ments from regions that regularly produce these species in excess. 

 Such at least is our impression after full consideration of the 

 facts. Bates and others have noticed that many of these flights 

 are composed entirely of males, an important fact, if true, that puts the 

 movements of these species in quite a different category from 

 those of such species as (Julias cihisa, Pi/rainns rardiii and Anoda 

 avchippiis, in which both sexes are equally aijundant. 



It must be considered as quite outside the province of this paper to 

 show how the presence of man has, in some instances, led to the 

 extermination of animals and plants, and how, in certain cases, 

 his presence has led to the spread or wider distribution of other 

 species. This is especially the case with those insects which feed 

 upon cultivated crops. ■ Just as the spread of many plants (especially 

 cultivated plants) has caused the spread of birds attached to 

 them, so the spread of plants has, more or less, influenced the 

 spread of insects. The introduction of non-indigenous timber 

 trees into Britain from various districts, has added a large 

 number of insects to our fauna, and the sustained extension of the 

 area over which Anosia anhiiipus has spread is dependent upon 

 the growth of the Asdepiaa. It would appear, too, that the well-known 

 Colorado beetle feeds in America on the native Solanum roi^tratinii and 

 that, wherever the beetle, owing to the cultivation of the potato 

 [SuIaniDii tnbcrustnii), has spread more rapidly than its native foodplant, 

 the crops have been injured ; but as the native foodplant spreads the 

 potato is left for it and but little harm is done. There can be 

 little doubt that the introduction of the potato into its haunts has 

 enabled it to spread very considerably. The Coliads revel among our 

 cultivated clover crops, and endless other cases might be cited. 



The question of the migration and dispersal of insects has hereto- 

 fore received nothing but the most haphazard treatment, and it is only 

 quite recently that the broader questions underlying dispersal move- 

 ments have been at all carefully considered. Already there are signs 

 that scientific methods will be applied in the near future to this subject 

 and that careful observations and reliable statistics will furnish data for 

 useful generalisations bearing on the subject. A paper by Webster 

 {Ilc/it. Knt. Soc. Out., 1901, pp. 63 ct acq.), entitled "The trend of 

 insect diffusion in North America," illustrated by maps, sketches the 

 paths by whicli some insects, once unknown in North America, but 

 now common, have made their way to their present areas of habitation, 



