52 BULLETIN 15 7, U. S. NATIONAL MUSEUM 



" alfalfa worms " — do really serious damage. It is possible that 

 unless something occurs to check its present increase this butterfly 

 eventually may become harmful here. 



Years ago the checkered white {Pieris protodice) was very com- 

 mon here and was more or less destructive to cultivated cabbages. 

 But now it is only of casual and very uncertain occurrence. It is 

 a weaker insect than the European white {Pieris rapae), and is 

 subject to persecution by it. The relations between the checkered 

 and the European white are almost identical with those between 

 the yellow and the orange clover butterflies {G olios philodice and 

 C. eurytheme). It is quite probable that the almost complete dis- 

 appearance of the checkered white has been due mainly to constant 

 persecution of both sexes by the males of its more powerful com- 

 petitor. 



The disappearance of the other native whites {Pieris 7iapi olera- 

 cea and P. virginiensis) — weaker and less active butterflies than 

 the European white — from much of the territory formerly inhabited 

 by them in the Eastern United States may have been due to the same 

 cause. 



OBSERVATIONS ON BUTTERFLY MIGRATIONS 



Certain observations made during the course of the studies on the 

 local butterflies seem to have a direct bearing on some aspects of 

 those interesting phenomena known as migrations. 



The problem of the so-called migrations of butterflies is a very 

 complex one, involving a great number of diverse factors, both ex- 

 ternal and internal. The external factors have to do with the rela- 

 tion between the insects and their environment, while the internal 

 factors have to do with their general structure, their anatomy, and 

 their physiology — most obviously with that complex of reactions 

 which we call their habits. Though the external factors are, gen- 

 erally speaking, the same for all the species of butterflies found in 

 a given region, the internal factors, especially the habits, vary more 

 or less widely, not only in the different groups, but also often in 

 closely related species within small groups and frequently in alter- 

 native, seasonal, or other forms of a single species. 

 ' The term " migration," as applied to the linear group movements 

 of insects, is a rather unfortunate one, as it implies a more or less 

 close correspondence with the migrations of birds, with which they 

 have little in common except that they are movements measurable 

 in geographical terms. 



Bird migrations are typically movements of a very definite nature 

 from the breeding grounds to another region more or less remote 

 and back again, usually by the same route. In the case of birds the 

 round trip is completed several or many times during the life of 



