94 M. C. Ritsema on the Origin and 
17th Junein that year *.  Asthe result of their investigation, 
they state that Periphyllus testudo, Van der Hoeven, of which 
neither a new genus nor even a new species is to be formed, is 
nothing more than an abnormal infertile form of Aphis aceris, 
Linn., and that this form is produced from individuals iden- 
tical with those from which normal larve, capable of further 
development and of reproduction, originate; nay, they some- 
times even ascertained that the same mother produced both 
forms. 
one specific type. 
In the spring of 1868, whilst still unacquainted with the 
investigation just noticed, I had made some passing observa- 
tions upon the same subject. The results of these investiga- 
tions, continued and completed in 1869, will be stated in order 
in the following pages. 
s early as the beginning of February, I observed upon a 
tree of Acer pseudoplatanus (Linn.) young larve of a species of 
plant-louse; and on closer examination of a small branch, I 
also found on the bark, especially in the axils and at the base 
of the buds, the glistening, black, oval eges from which they 
had issued. 
* This memoir is translated into English in the * Annals and Magazine 
of Natural History,’ ser. 3. 1867, vol. xx. pp. 149-152, and is transferred 
from this without change, but with the addition of a figure of Periphyllus 
, into Hardwick's ‘Science Gossip’ for Sept. of the same year. 
t Comptes Rendus, February 4, 1867, 
