56 BRITISH APHIDES. 



wasps feed greedily upon it, bees appear totally to 

 disregard it. 



The late Mr. Alfred Smee told me that an Aphis, 

 which proved to be this Lachnus^ swarmed in such 

 thousands on his willows at Carshalton, that trees 

 thirty or forty feet high had been killed by their 

 poisonous influence. 



I have received in June specimens from Carshalton 

 and, through the kindness of Mr. Evershed, also from 

 Shere, near Guildford. My friend Mr. James Salter 

 found a cluster of them on one of his willow trees, 

 near Basingstoke, five inches long, and an inch wide. 

 The insects were ranged very closely together, side by 

 side, with all their heads turned downwards, and on 

 tlie lee side of the branch. The members of a swarm 

 usually remain for a long time motionless, but if a 

 single one is by any means disturbed it throws up its 

 hinder legs with a jerk, in a tentative manner, and this 

 movement is speedily communicated to all the indivi- 

 duals of the general mass. The action may be 

 deterrent, to prevent parasitic attacks of Hymenoptera. 

 I have attempted to represent this action in fig. 5, on 

 Plate XCIX. 



Several species of Aphides carry their wings hori- 

 zontally when at rest, like the common house-fly, and 

 some authors have considered this of generic import- 

 ance. Although doubtless the position is exceptional 

 amongst Aphides, too much value must not be placed 

 upon the circumstance, for it may be seen in several 

 Pemjphignicey and also in Thelaxes {Vacuna, Heyd.). 

 In Lachniis viminalis these two positions are indiscrimi- 

 nately adopted. The use of the conical tubercle on 

 the dorsum is not certainly known. Although the 

 apex is furnished with several minute pores, the 

 organ may be regarded as blind or imperforate. It 

 has not the function of a nectary, but probably that 

 of an odoriferous gland. 



It would appear that this Aphis has considerable 

 facility of migration from spot to spot, and that this 



