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reproduction continually going on during mild weather. Aphidius 
may also be present either as invisible undeveloped overwintering 
larvae within the living bodies of its host, or it may be present as 
mature larvae or pupae in the dead and dried "cocooned" bodies of 
the same. Besides this, in the light of recent studies of Aphidius by 
Mr. Viereck, the same may be true with reference to its occurrence in 
a considerable number of other common species of aphidids, inhabit- 
ing a great variety of vegetation, in the same neighborhood, upon 
which this same species of Aphidius is parasitic. Thus, it is per- 
fectly clear why, with Toxoptera swarming in the fields, and the 
parasite present, about 10 days, with the temperature ranging from 
40° or 50° to 60° or 70° F., is sufficient to enable the latter summarily 
to suppress the invasion. The abruptness with which this change is 
brought about is easily explained by the fact that a parasitized female 
Toxoptera produces young during only a comparatively few days after 
being parasitized, although she may survive several days longer, 
especially if the weather be cool enough to retard the development 
of the parasite. 
In the North the situation is usually quite different, as parasites 
can not begin their work here to any extent until after the eggs have 
hatched, and the stem mothers and their offspring have appeared in 
the fields, thereby furnishing host insects. The overwintering of 
immature Aphidius larvae in the bodies of the host is in the North 
ordinarily precluded by the absence of living host individuals during 
severe winters, although mature larvae may winter in the dead bodies 
of the host as in the South. Stem mothers are probably never 
present in great numbers and considerable time is therefore neces- 
sarily required for their offspring to become excessively abundant. 
For this reason parasitism, over the section where the host insects 
pass the winter in the egg, begins later, and, at the start, proceeds 
necessarily much slower than in the South, but on the other hand 
Aphidius, unless the winter be an exceptional one, must of necessity 
winter over in the "cocooned" bodies of its numerous hosts, as 
mature larvae or pupae, and would therefore promptly respond to 
the warm days of early spring, although delayed somewhat by 
low temperatures that might not retard the host insects. 
There is one point in connection with parasitism by Aphidius that 
must be always kept in view, particularly to the southward, in order 
that mistakes and misstatements may be avoided regarding its 
actual occurrence in any particular locality. While the larva is 
contained within the still living body of its host its presence there is 
not easily detected. Indeed it is not until the larva becomes nearly 
full grown that it can be detected even by an expert. Therefore, 
in the light of what has previously been stated concerning the situa- 
tion in milder latitudes, there may be millions of living larvae 
