90 Mr, CuRTis's Obfervations on Aphides. 



When plants afTume a fickly appearance, or are disfigured by 

 difeafe, from whatever caufe the difeafe may arife, they are faid to be 

 blighted. Blights originate from a variety of caufes, the chief of 

 which are unfavourable weather and infeifls. 



Two opinions prevail very generally in regard to blights : the 

 one, that the infects w^hich are the caufe of them are brought from a 

 diftance by eafterly winds ; the other, that they attach themfelves 

 to none but plants already fickly. Neither of thefe opinions, as far 

 as I have obferved, is founded in fa6t. I am induced, from the nu- 

 merous obfervations I have made on infe£ls for a feries of years, (in 

 purfuing the cultivation of plants) to confider the Aphis as by far 

 the moft general caufe of the difeafes diltinguifhed by the name of 

 blights. Other infects, it is true, more efpecially the larvce of fome 

 of the Leptdopieray as thofe of the PhaUence toririces^ disfigure and do 

 infinite mifchief to plants, by rolling and curling up the leaves. But 

 thefe for the moft part confine themfelves to certain trees and 

 plants. Their ravages alfo are of fhorter duration, being confined 

 to the growth of one brood, and they are alfo lefs fatal. It would 

 be no difficult matter for me to fill a volume with obfervations, to 

 which I have been an eye witnefs, of the injuries which plants 

 fuftain from infeds ; but that would be foreign to my prefent pur- 

 pofe, which is to (how that the Aphh is the grand caufe of thefe 

 difeafes, and to place the modus operandi^ or the manner in which 

 they effecl this bufinefs, in its true light. 



We are fully aware that certain gregarious infe^ls may at parti- 

 cular times rife up in the air, and, if fmali and light, be impelled by 

 any wind that may chance to blow at the time; and on this prin- 

 ciple we account for that lliower of Aphides defcribed by Mr. White 

 to have fallen at Selborn. But certainly this is not the mode in 

 which thofe infe6ls are ufually difperfed over a country. The phas- 

 nomenon is too unufual, the diilribution would be too partial ; for 



Aphides^ 



