250 THE ENTOMOLOGIST. 



among entomologists, no matter what group studied, for the 

 unusual ahundance of some special species. 



The lepidopterologists have DeilepJiila galii this year as their 

 red-letter moth. In July numerous imagines were captured in 

 many parts of these islands, extending from the south coast of 

 England to Scotland, and from Scarborough on the east coast to 

 Ireland. In recent years, with the exception of in the year 1870, 

 wlien it occurred in some numbers, in the larval state espe- 

 cially, — for instance, on the Cheshire sand-hills, — it has been 

 counted among the rarer species inhabiting the British Islands. 



All sorts of speculation are rife upon the cause of this unusual 

 appearance of Dellephila galii. The migration theory is the 

 one first set up, and may possibly be the cause of this 

 extraordinary abundance. Tliere are, however, other suggestions 

 worthy of consideration ; for instance, the long lying-over of the 

 pupffi, for some fortuitous condition of atmospheric influence, 

 suitable for the emergence of the moths. We all know how 

 the pupae of many Lepidoptera in captivity do remain alive, 

 but unemerged, for years. How much more likely are the}' 

 to remain in a state of nature, even for longer periods. 

 There, the chance of safety is greater than in the artificial 

 state of captivity. The fact of being kept too dry or too 

 damp, or accidentally disturbed when all are thought to have 

 emerged or to be dead, generally causes disturbance of Nature's 

 rules with our captive pupae. We are, perhaps, too ready to turn 

 to the migration theory, because it is one easily understood ; but 

 it does not seem wise to put every unusual appearance down to 

 the " blown-over theor}'." That migration does take place none can 

 doubt, even on great continents. Again, no one can doubt that 

 there are cases of unusual and periodical extraordinary abundance 

 of certain species ; with the converse of years of remarkable 

 scarcity. Take, for instance, tlie flights of innumerable specimens 

 of Aiinosia lAexippus on the continent of Nortli America. Tliat 

 species occurs every 3'ear in certain localities, but on some 

 occasions in such numbers as to surprise even the uninitiated in 

 those places. It cannot be migration iu such instances. 



Dellephila galii is said to occur every year on the Deal sand- 

 hills in the larval condition, but usually in very small numbers. 

 About thirty years since Mr. Boswell-Syme captured a large 

 number of the larviu there, and was then enabled to add specimens 



