1887.J 37 



for many years. Certain undefined local causes have undoubtedly 

 combined to make the species in question (^A. cratcegi) extinct in 

 Britain at the present time. This seems to have previously occurred, 

 and yet it became common again. What explains these periodic ap- 

 pearances so well as immigration ? What I venture to suggest is 

 this — that the insect will remain extinct in Britain until the general 

 causes (whatever they may be) which induce migration in insects act 

 in such a manner as to bring a wave of immigrants to our shores 

 again. Whether the species will then establish itself for a time de- 

 pends on local conditions. 



Migration or immigration is, in my opinion, due to one set of 

 causes (which we may well term general), and the establishment 

 (permanent or otherwise) of the species is due entirely to other con- 

 ditions (local), viz. :— those of the atmosphere, climate, distribution of 

 food-plant, &c., of those localities which the immigrants take posses- 

 sion of. 



Mr. Goss compares Aporia cratcegi with the genus Melitcea. The 

 larva of A. cratcegi is gregarious, as are to a certain extent the species 

 in the genus Melitcea. Here, to my mind, the similarity ends. He 

 remarks that "A. cratcegi is capable of soaring a great height when 

 pursued or alarmed," and "is a powerful flyer." These facts separate 

 it at once in its habits from the sluggish Melitcece, which sham death 

 in the net, and, if alarmed, often fall to the ground. But even sup- 

 posing this species to be generally sluggish, why should it not be 

 migratory ? We must remember that the swarms of insects are not 

 acting under normal, but under very abnormal conditions. It is not, 

 I presume, normal for any insect to travel thousands of miles out to 

 sea, except at special times, and driven probably by a strong migratory 

 instinct. 



Mr. Goss also compares A. cratcegi with Colias JEdusa, C. Hyale, 

 Vanessa cardui, Sphinx convolvuli, and other migratory and cosmo- 

 politan species, and suggests that A. cratcegi cannot be placed in the 

 same category as these. Certainly not ! but because A. cratcegi is not 

 cosmopolitan, is this a proof or a probable proof that it is not migra- 

 tory ? Is Urania Leilus cosmopolitan, or Timetes Chirofi ? Tet we 

 have proof positive that these are migratory loithin certain limits 

 (vide "The Naturalist in Nicaragua," by Thos. Belt, F.G.S., or the 

 extracts of the late Mr. Newman, Ent., vol. vii, pp. 60 and 61). I 

 believe A. cratcegi extends over almost as wide an area in the Old 

 World as these species in the New. A. cratcegi is found more or less 

 throughout Europe and Western Asia. I see no reason myself why 



