GENERATION OF INSECTS. 17 



live; for 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 coat- 

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

 hop, is nothing more than the excrement of the insect 

 {Aphis hunudi) whose propagation we are discussing. 

 •' The honey-dew," says Loudon " mostly" (we be- 

 believe always) " occurs after the crops have been 

 attacked by these insects." f Sir J. E. Smith, who 

 admits this to be the connnon cause of honey-dew, 

 contends tliat what is found on the leaves of the beech 

 is an exception; but he adduces no evidence at all 

 satisfactory in proof of its being caused by unfavour- 

 able winds J ; 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 vteather had been parched and sultry 

 ior some weeks previous, and the honey-dew pre- 

 vailed to such an extent, that the leaves of the cur- 

 rant, raspberry, &c., in the gardens, literally distilled 

 from their tips a clear limpid honey-dew, excreted 

 from the plant; lor the phenomenon was observable 

 on those plants that were entirely free from aphides, 

 and so copious vvas it, where these insects were 

 found, that had their numbers been centuple they 

 could not certainly have been the source of the sup- 

 ply. The question with me, however, was set at rest 

 by applying a lens, having previously washed and 



* Botan, Arrangement, ii., 440, 3r(l ed, 

 Encycl. of Agnculturo, p. 865, s. 5444. 

 X Intioductiun to Botany, p. 189. 

 ( Sec Liuii. Traua. vol. vi. & WiUdtuow, Piuicip. of Botany .p.34o\ 



c 3 



