78 Mr. CuRTls's Obfervations on ^phiJeU 



large or choice colledion be viewed, when there is fcarccly a plarit 

 but what exhibits fymptoms of difeafe occafioned by vermin ? 



As the fpecies of this genus are very numerous, and afford but 

 few marks of diftinftion, Linnaeus has contented himfelf with giv- 

 ing moft of them trivial names, according to the particular plant' 

 on which they are found : a clofe attention to them will, however,' 

 diiclofe more diftimStivc chani6ters than naturahfls are aware of. 



jjph'ides are defcribcd by the heft informed authors as being gene- 

 rally oviparous and viviparous at different periods of the fame year. 

 Monf. Bonnet, who had the honour of making this difcovcry in 

 1740*, f;iys that in the fummer the females are viviparous, but 

 toward the middle of autumn they lay real eggs. De Geer obferves, 

 that the females of all the Aphides he had fecn, conftantly laid eggs, 

 intended to preferve the fpecies during winter, and that he is 

 therefore inchned to believe that the fame takes place in all JphiJes 

 whatever. From the a4th of September to the 6th of December 

 following, during which time Fahrenheit's thermometer had been 

 as low as 29, I found the Jphis falicis to be conftantly viviparous, 

 though from the inclemency of the weather very few of thefe in- 

 fers at the period laft mentioned remained on the trees, and 

 thofe few were foon after entirely cut off by the unufual cold that 

 took place, the thermometer falling to 4 degrees below o. — Other 

 Aphides are ovipaious or viviparous according to the temperature of 

 the air to which they are expofed. In very cold weather they are 

 oviparous, for this obvious reafon : the eggs are capable of refifting 

 cold more powerfully than the young. On the 2 2d of November 

 fame year as above, I found a confiderable number of eggs which 

 * had been depofited in fome auricula plants by a fmall green Aphis, 



* Or rather Monf. Trembley. See liis Letter to M. Bonnet from the Hague: Oeuvres 

 4e Bonnet. . , , 



which' 



