tllE MIGRATION AKD WSfEKSAL Of INSECTS. 237 



were as strong aijd organically capable of going on with this form of 

 reproduction as any previous generation. We do not know whether 

 this mode of continuous viviparous reproduction takes place in tropical 

 regions, but it appears quite possible. At any rate, viviparous broods 

 continue to be produced so long as a higli temperature is accompanied 

 by an abundance of food. A diminution of sap and an accession of 

 cold are suflicient to produce, almost at once, sexual forms, when eggs 

 are laid, and these in turn go over the winter. 



It would appear that, although most of the parthenogenetic progeny 

 are noruuilly wingless, individuals of any brood after the first (that 

 produced from the Avinter eggs) may become winged. These fly to 

 other localities, and found new colonies wherever suitable food may be 

 obtained. The exact cause which produces these winged forms is scarcely 

 known, but they appear to follow directly on a long-continued drought, 

 or (in the autumn) on an accession of low temperature. Any cause 

 that will check the flow of sap in the plants on which the aphides feed 

 appears to be sufficient, and hence the insufficiency of the food-supply 

 has been assumed as the actual cause of their moving from one place to 

 another. The area over which migration extends, therefore, is generally 

 identical with that affected by drought or similar circumstances. 

 The distance traversed by any particular moving body of a])hides is 

 probably not a great one, but swarms of Phorodo}i humnli and Mi/zns 

 ccrasi have been reported as crossing from Essex into Kent, and in 

 1885, Norris describes the aphides as migrating in " clouds," in 

 Hunts, from July 26th-29th. 



Continued periods of drought, then, are generally accompanied 

 with the migration of winged aphides, and continued periods of drought 

 are frequently the result of a continued east wind. In Kent, especially 

 in the hop-growing districts, Avhere aphides are frequently a terrible 

 curse, the farmer insists that the east wind brings the " fly," as he 

 terms the insects. The " fly " and the " east wind " appear together, 

 and therefore, he argues, the wind has brought the insects. As a 

 matter of fact, as we have just seen, the east wind has " brought the 

 fly," but not in the way the farmer supposes. 



The migration of aphides in this manner, and in this state, 

 appears to be purely mechanical. They rise from their resting places, 

 and the wind carries them wherever it listeth. They are borne along 

 by the very wind, maybe, that caused their appearance, and hence we 

 have an explanation of the usually erroneous suggestion made by the 

 farmers as to the connection between the aphides and the east wind. 



Sometimes these migrations, although apparently perfectly in- 

 voluntary, take place on a large scale, myriads of these tiny voyagers 

 being carried great distances, and giving rise to the "blights" of the 

 farmer. In their wanderings they are sometimes accompanied by 

 numbers of Syrphidae and ladybirds (CocciMc/M), which prey on them, 

 and help to keep them in check. 



Not only are these large flights of aphides frequently met with in 

 one's walks in fields and country lanes, but they may be sometimes 

 observed in quite unexpected localities. On one occasion the beach 

 between Bournemouth and Poole was, for some distance, ornamented 

 with a mossy-looking green track, which varied in width from one to 

 four inches. This line was left at highwater mark by the tide, and 

 extended for about a mile in length. It consisted of " millions upon 

 millions of aphides, belonging to a species closely resembling, if not 



