AN UNUSUAL APHIS MIGRATION. 255 



necessary to refer to the usual rule of Aphis migration. It is 

 known to entomologists that these insects are viviparous and 

 wingless through the greater part of the season, when obviously 

 migration is to them a thing impossible. The periods of their 

 migration are towards the close of May or early in June, and 

 corresponding dates in September or October, the Aphides being 

 then winged. In answer to a communication of mine, the late 

 Francis Walker remarked that the time of these migrations depends 

 upon the state of the atmosphere, and in this way : so long as the 

 plants upon which the Aphides are feeding afford them an abundant 

 supply of sap they multiply rapidly, but when a change in the 

 weather checks the flow they come forth winged, and are also 

 prompted to change their quarters. There is then actually some 

 truth that on certain cloudy oppressive da3^s of spring and autumn 

 '* there is a blight in the air," according to the popular belief, for 

 it is on such days that Aphides travel. The insects are seldom 

 noticeable on any day when there is a strong wind blowing, and I 

 do not think that direction makes any difference, though the 

 statement has been made that easterly winds brings the Aphides ; 

 but I have found them on the move in winds from all parts of the 

 compass ; yet it is true that the east wind, from its unkindly 

 effects on vegetation, will bring about that check to healthy 

 growth which develops the flying brood ; nor does the Aphis 

 suff"er from this wind as do many insects ; rain, however, is 

 greatly disliked by them. As to the distance they generally 

 journey, Mr. Walker added that their migrations, he believed, 

 were short ; but the word as applied to distance is somewhat 

 indefinite ; if we have no standard of comparison, it might mean 

 a mile or a few hundred yards. I take it Mr. Walker meant a 

 distance approximating to the latter space. Yet I hope to 

 demonstrate that Aphides may and do travel, at least on some 

 occasions, a mile, and even more, though I cannot give support 

 to the theory of one naturalist that they may not only go from 

 county to county in Britain, but cross to us from the Continent, 

 or vice versa. Though if we sj)eak of flight, i. e., of voluntary 

 motion, an examination of the wings of an Aphis suggests that 

 they could sustain the insect for but a brief time ; it seems as 

 though when they make a long journey through the air they must 

 allow themselves to be simply boi'ne along by the current, which- 

 ever way it may blow. It is not probable that they form themselves 



