150 Miscellaneous. 



be the starting-point of peculiar races. The following observation 

 proves not only that simple races are produced in this manner, but 

 that forms described as species, or even as actual genera, sometimes 

 acknovfledge no other origin. 



In 1852 an English naturalist, Mr. J. Thornton, indicated, under 

 the name oi Phyllophorus tesfudinatus,fin Hemipterous insect which 

 he had found on the leaves of the common maple (Acer campestre), 

 and which he regarded as the larva of an undetermined species of 

 Aphis. Subsequently, in 1858, Mr. Lane Clark also observed it, 

 and placed it, under the name of Chelymorpha phyllophora, in a 

 genus intermediate between the AphididcB and the Coccidce. Lastly, 

 in 1862, M. van der Hoeven, of Leyden, described it, also as a new 

 genus, replacing the generic names Phyllophorus and Chelymorpha 

 by that of Periphyllus, the other names being previously employed 

 to designate other genera of insects ; and our Hemipteron received 

 from the illustrious Dutch naturalist the name of P. testudo. Like 

 Mr. Thornton, M. van der Hoeven regarded it as the larva of an 

 Aphis of which the adult form was still unknown. 



These brief historical indications form a summary of all that was 

 known about this insect when we on our part undertook some inves- 

 tigations upon it, the results of which we now propose to communi- 

 cate. We first ascertained that, far from constituting a new genus 

 or even a distinct species, the Periphyllus is really nothing but the 

 larva of one of the known species of Aphides which live on the maple 

 — namely. Aphis aceris, a brown species which is to be met with 

 during a great part of the year upon the leaves and at the extremi- 

 ties of the young shoots of that tree. But, at the same time that 

 we ascertained this fact, we were set ou the track of a most unex- 

 pected discovery, constituting a new and very remarkable peculiarity 

 in the development of the animals of this group, already presenting 

 such curious phenomena in connexion with their reproduction. 



This was the faculty, become transmissible to all the generations 

 of a particular species, of engendering two kinds of individuals — one 

 normal, the other abnormal — of which the former alone, after their 

 birth, continue the course of their development, and become capable 

 of reproducing the species ; whilst the latter retain throughout their 

 existence the form which they possessed on coming into the world, 

 and appear to be incapable of propagating. Moreover these two 

 categories of individuals present such marked characters that, with- 

 out having watched their birth, and being thus convinced that they 

 are really produced by identical females, and sometimes even by one 

 and the same mother, one would inevitably consider them to belong 

 to two species, nay even to two completely different genera. Now 

 one of these is nothing but the Periphyllus mentioned at the 

 commencement of this note as having been described by the authors 

 who had observed it as a separate genus in the family of the 

 Aphides. 



Such is, in summary, the singular observation that we have made 

 upon Aphis aceris. We may now give some fuller details upon each 

 of the two kinds of individuals of which this species is composed. 



