GENERATION OF INSECTS. 17 



the caterpillars, he avers, never attack wild hops 

 which grow in stony places, because they cannot 

 get at the roots.* It appears to us, however, that 

 there can be little doubt that the sweet syrupy 

 coating, called honey-dew, found on the leaves of 

 the hop, is nothing more than the excrement of 

 the insect (Aphis humuli) whose propagation we 

 are discussing. ' The honey-dew,' says Loudon, 



* mostly' (we believe always) ' occurs after the crops 

 have been attacked by these insects. 'f Sir J. E. 

 Smith, who admits this to l)e the common cause of 

 horjey-dew, contends that what is found on the leaves 

 of the beech is an exception; but he adduces no evi- 

 dence at all satisfactory in proof of its being caused 

 by unfavourable winds;! while the undoubted fact of 

 its being the excrement of aphides in so many other 

 instances^ weighs strongly against him, 



A novel theory of honey-dew has just been pub- 

 lished by Mr John Murray, who ascribes it to an 

 electric change in the air. ' Last summer,' he says, 



* we investigated the phenomenon with great care: 

 the weather had been parched and sultry for some 

 weeks previous, and the honey-dew prevailed to such 

 aft extent, that the leaves of the currant, raspberry, 

 &.C, in the gardens, literally distilled from their tips 

 a clear limpid honey -dew, excreted from the plant; 

 for the phenomenon was observable on those plants 

 that were entirely free from aphides, and so copious 

 was it, where these insects were found, that had 

 their numbers been centuple they could not cer- 

 tainly have been the soiu'ce of the supply. The 

 question with me, however, was set at rest by ap-^ 

 plying a lens, having previously washed and dried 



* Botan. Arrangement, ii, 440, 3cl ed. 

 ■I Encycl. of Agriculture, p. 865, s. 5444. 

 t Introduction to Botany, p. 189, 



§ See Linn. Trans, vol. vi, and Willdenow, Princ. of Bota- 

 ny, p. 343. 



VOL. TI. 2* 



