9]Q rFebriinry, 



During the past five years, that is, ever since the event came under my observation, I 

 have frequently thought over the matter and endeavoured to analyze the circum- 

 stances connected therewith. 



The only conclusion that I can arrive at is that the movement I saw among 

 certain moths (presumably P. gamma) on the evening of August 10th, was connected 

 with the migration of those insects from their breeding ground in the vicinity of 

 Yentnor. As the air near the earth was at the time perfectly calm, and as the moths • 

 rose upwards without apparent inclination towai'd any particular point during the 

 time they remained within sight, it is not possible to say what course they eventually 

 took. It occurs to me that the direction of their (migratory) flight would probably 

 have been influenced by any current of air they might have reached in their ascension. 



The spiral ascension of the moths could not have been due to accidental causes, 

 and, therefore, of an involuntary character. If the air had been in a disturbed con- 

 dition, I should have supposed that the action of the moths was influenced thereby 

 but as the air was absolutely still, I cannot but conclude that the peculiar upward 

 movement of tl>e moths was quite voluntary and, in fact, the initial stage of subsequent 

 migration. Again, the movement was evidently a simultaneous one of a large num- 

 ber of moths and not confined to a few individuals only. This fact would seem to 

 imply that the insects were actuated by a common influence, to seek the regions of 

 the upper air. 



Plusia gamma is always present in greater or lesser abundance in Britain. Its 

 numerical increase or decrease is without doubt (in common with that of most 

 Lepidoptera) regulated, to a very great extent, by meteorological or climatic influences 

 The wet and almost sunless summer of 1879 may have been favourable to the propa' 

 gation of P. gamma, but unless the normal numbers of the species were largelj 

 augmented by immigrants during the early summer months, I cannot think i j 

 would have occurred in such swarms as were observed in August of that yea: * 

 in the Isle of Wight and elsewhere. 



That several species of Lepidoptera do migrate is beyond contradiction, bu; 

 whether such migration is habitual with those species, or not, there is no evidence td 

 show. It has been suggested that the extensive geographical range of some specie 

 is due to their occasional migrating from place to place. I consider this not onl;j 

 possible but most probable, and I should suppose that such species possess inhereni|! 

 migratory instinct. Probably the instinct is either excited into activity or kept i:|j jl* 

 abeyance by meteorological influence, ' miii 



As far as concerns Britain, I am of opinion that unless there were occasions iIto( 

 immigration of such species as Colias Hyale, Colias Edusa, Vanessa Antiopa, Sphin\ itfl 

 convoIvuU and several others, British collectors would not have the felicity of takini «!?[ 

 those species in any part of this country. I do not go so far as to say that none c % 

 of the species specially referred to ever breed in Britian ; on the contrary, I admi' moe, 

 that they often do so (more rarely perhaps in the case of V. Antiopa). Still I thin J . f 

 that the peculiarities of our insular climate render the permanent establishment ( 

 those species, and certain others, improbable. 



Suppose that in any year a number of immigrant Colias Edusa arrived on thl 

 eastern or south-eastern coast of Britain ; during the month of June for instant 

 These immigrants would, most probably, in the course of a few days distribute thenil %,j(|^ 

 selves throughout the length and breadth of the land, and the females would in dtl^J,:{ 



'% 



