52 BRITISH INSECTS 



known " woolly aphis" or " American blight," 1 secrete 

 a waxy substance in the form of delicate threads or 

 flakes, and live beneath it. Others, of which the 

 " currant-blister aphis " 2 may be instanced, give rise 

 to galls (i.e., abnormal growths) on leaves, stems, or 

 roots. In all cases the infested plants are weakened 

 by the continual sucking out of their sap ; while in 

 many instances the functioning of the leaves is 

 arrested, owing to their stomata, or pores for gaseous 

 exchange, being blocked by the sticky discharge of 

 the insects. This substance, called " honey-dew," 

 was formerly thought to be excreted through two 

 tubes, or cornicles, which are situated on the backs of 

 many aphides ; but it is now known to issue from 

 the anal opening of the alimentary canal. 



The two remaining families of the series Dimera 

 are the " jumping plant-lice" 3 and the " snow-flies." 4 

 The former have three ocelli (true aphides have none), 

 and are further distinguished by their habit of jump- 

 ing. The common apple-sucker 5 is often abundant 

 in orchards, where it does much mischief. Adult 

 snow-flies resemble tiny white moths, whose bodies 

 and wings are covered with a fine powder or bloom. 

 One species is sometimes found in enormous numbers 

 on various kinds of cabbage, and when the plants are 

 shaken, the insects rise and float in the air like 

 miniature snowflakes whence the popular name. 

 The early stages of the life-history are peculiar, and 

 have not been fully investigated. It is known, how- 

 ever, that the young insect secretes a scale, and that 

 it remains beneath this protection, fixed to the under- 

 side of a leaf, until the metamorphosis is complete. 



1 Schizoneura lanigera. 2 Rkopalosiphum ribis. 



3 Psyllidcc. * A hurodidtz. 5 Psylla mali. 



