96 On the Development of Periphyllus testudo. 
began to show through the integument. After this, in the 
beginning of September, I found Periphylli changing their 
skins. 
After this first moult they agreed exactly with the second 
life-phase of the ordinary larve, while a fortnight later they 
had attained their perfect state, in which they were all wing- 
less. These nurses began immediately to propagate by light- 
yellow ordinary larvæ, which at the end of September changed 
their skin for the last time, and remained wingless like their 
mothers. The larve born from these nurses agreed very 
nearly with those of the preceding generation, and became 
developed before the middle of October into winged male and 
wingless female imagos, which quickly paired, after which 
the females attached about eight brownish-yellow eggs to the 
bark of the maple tree. These eggs slowly became black, 
and in the beginning of February 1870 will again furnish the 
first generation of Aphis aceris (Linn.). 
aving reached the end of my memoir, I will briefly run 
over the results to which the investigation described has led. 
In the first place, it is proved that Periphyllus testudo (V. 
d. Hoev.) is not a distinct species, but a peculiar larval form 
of Aphis aceris, occurring in the earliest period of its life. 
cond place, that this larval form is not, as has 
hitherto been supposed, incapable of further development, but 
that it is merely subjected to a long-continued cessation of 
development in its first stage, by which the multiplication of 
the above-mentioned species of Aphis is greatly limited. 
In the third place, that it is produced only by the genera- 
tions in which both winged and wingless individuals occur. 
In the fourth place, that it is produced both from the winged 
and wingless nurses, in association with the ordinary larval 
form which undergoes a rapid development, and with indivi- 
duals which form a sort of transition to the latter. And 
in the fifth place, that in the successive generations the Peri- 
phylli continually increase in number, whilst the number of 
the ordinary larve diminishes, and, indeed, to such a degree 
that the fourth generation (that is to say, the third from which 
Periphylli are born) only produces a few ordinary larve. 
From this I think we may conclude that nothing but Peri- 
Phylli are produced from the fifth generation, which, in my 
case, unfortunately died. Should this actually prove to be 
the case, it would serve especially to prevent any very great 
increase of this species during the summer. 
