84 
gator. An accurate count of every individual within each net bag was 
kept and, as a matter of course, before the transplanting could take 
place every square millimeter of leaf surface which was to be inclosed 
was gone over with a lens to make sure of the nonpresence of other 
individuals. This course was followed with the succeeding generation, 
and on the appearance of the winged generation the Plum was by no 
means abandoned. Winged individuals were confined for days upon 
this plant to see whether they would deposit young upon its leaves and 
to see whether these young would fix themselves and procreate. 
The simultaneous appearance of winged individuals absolutely indis- 
tinguishable from those upon Plum upon the Hop and the failure of 
the confined offspring of plum migrants to settle in a normal manner 
obviated the necessity for a direct observation upon the actual passage 
through the air of winged individuals from Plum to Hop, although, as 
a matter of fact, the return migration was readily observed from the 
greater number of individuals. Taking up the same course of obser- 
vations upon the Hop, individual leaves were carefully cleaned under 
the lens, marked and stocked, and inclosed with netting. Every morning 
the young were counted and removed, some destroyed and some estab- 
lished upon cleaned leaves, and so the insect was followed day after 
day throughout the entire summer, the exact number of generations 
found, the exact intervals of the winged generations, the exact point 
at which the sexual individuals appeared, the exact number of offspring 
of a considerable number of females of each individual ascertained, and 
the return flight to Plum observed. 
Here the return migrant females were carefully watched in the same 
manner as before until they gave birth to the true sexual females, the 
number of molts was counted, the number of offspring ascertained, and 
the latter were isolated, and watched almost every minute to full growth. 
In the meantime the development of the male, which is winged, was also 
watched on Hop in similarly isolated individuals. Its migration to Hop 
was in turn followed and a certain number were introduced into the 
receptacle containing the isolated females. Thus the birth of the ovipa- 
rous females was observed, the latter were isolated, and egg-laying 
watched. 
These observations were necessarily carried out in a single locality 
and almost entirely by Mr. Pergande. Independent and fully cor- 
roborative observations were also conducted by Mr. W. B. Alwood 
simultaneously with his remedial work. I myself examined the progress 
of the work during a field visit and Dr. Riley took the field on several 
occasions to satisfy himself of the satisfactory progress of the investi- 
gation. So far for a single locality (Richfield Springs, N. Y.). The 
bulk of the work was done there. But it was necessary with so wide- 
spread a species to study its life-history at other points. Prof. Riley 
was able to do this personally in the hop fields of England and the 
south of France. Prof. Osborn was sent to the hop-growing regions of 
