Illi: Ml(;i{AlI(»NS OF m TTKKKMES. 1077 



thorax ; the frontal proniinence iy ili'licnti' mul ta|icT(< gradually without 

 bt'iiig very long; they are green wiili a lateral stripe. 



The tiptries of tiiis group, t<ays Batcn (.Tourn. ent., ISfil, 24.')), 

 referring to this genus iinil its immediate allies, "are a most difficult 

 study. • . . Their specific characters are not at all trenchant; the peculiar 

 markings which may serve to distinguish well characterized examples of 

 a species are subject to become obsolete in other examples ; the species 

 again present many local varieties in dirt'erent parts of tiieir area of dis- 

 tribution." This is due, nc.) doubt in part to the prevalence of seasonal 

 dimorphism in this group, — a fact which was not known when Bates 

 wrote and which will doubtless necessitate an entire revision of the 

 species, though not in the headlong manner adopted by Pryer in his Lep- 

 idoptera nihonica. 



EXCURSUS XLII.—THE SWARMING AND MIGRATIONS OF 



BUTTERFLIES. 



. . . Before vour si<'ht 

 Mounts on the breeze the butterfly — and soars, 

 Small creature as she is, from earth's bright flowers 

 Into the Jewy clouds. 



Wordsworth.— 77ie Excursion. 



No flying insect steers its course by its tail. 



Aristotle.— Hist. Anim., vii. 



Whether or not swarming is a necessary precedent to migration among 

 insects is not known, since migration can readily be observed, and in 

 fact has been observed, only when an insect is exceptionally numerous ; 

 migration may be far more common than we imagine, and it seems to be 

 one of the conditions of the struggle for existence that each kind of 

 animal should endeavor to spread its domain. 



In a preceding excursus (pp. 376-379) we have brought together some 

 examples of the movements of butterflies en masse, but in most cases 

 these were instances of forcible removal from one point to another by 

 atmospheric agencies. It may be remarked in passing, that such mas- 

 sive movements could not have occurred but for the habit, on the part 

 of the insect involved, of congregating in such great numbers as to merit 

 the term swarming. At present, however, we are not concerned with the 

 involuntary but with the voluntary massive movements of butterflies. 



Such movements have frequently been observed and in some quarters of 

 the globe, notably in tropical regions, are claimed to be a regular and 

 annual occurrence. There is reason to believe that this may also be true, 

 on a smaller scale, in temperate regions. Hardly more than three sub- 

 families of butterflies have been observed to be concerned in such move- 

 ments, viz., the Nymphalinae and Euplocinae among Nymphalidae and 



