482 
The Ohio Naturalist. 
[Vol. XII, No. 5, 
It is, of course, not probable that any larva would ever norm- 
ally devour aphids so rapidly. Yet when plenty are at hand the 
number eaten by a larva during its life of eight days to two weeks 
or more must be very considerable. It should be kept in mind 
also that it is not the actual individuals eaten, alone, that deter- 
mines the amount of benefit from these insects; but the fact that 
in this way the production of enormous numbers of aphids may be 
prevented. If as Reaumour has calculated, and others have 
substantiated, one aphid may be the progenitor of over 5,000,000- 
000 individuals during her existence of a month or six weeks, we 
can see at once the important benefit that must arise from the 
destruction of one or two of these aphids early in the establish- 
ment of the colony. It is a fact that the eggs of Syrphidae are 
often deposited on the host-plant very early or even in anticipation 
of the arrival of the aphids. 
The great factor in determining the duration in the larval stage 
seems to be the abundance or scarcity of food. Indoors with 
plenty of food at hand the larval period from emergence from 
egg to formation of puparium was 8 to 9 days. When less food 
was supplied this period was extended frequently to two weeks 
sometimes as much as 20 days. The larvae are very tenacious of 
life, some of them existing for over three weeks with very little 
food. 
Out-of-doors the larvae seem to be little affected by climatic 
conditions so long as food is available. They endure very wet 
weather and I have seen them in Autumn survive several periods 
of cold freezing weather. 
In my experience these larvae may be expected wherever 
aphids or other soft-bodied insects occur in colonies. They are 
not restricted to one kind of prey. I have found them most abund- 
antly on cabbage and some other cruciferae at Columbus the latter 
half of May in an open greenhouse, and out-of-doors in Autumn 
from the latter, part of September to the middle of October; and 
on Phragmites at Cedar Point from the last week in June to the first 
week in August. 
On cabbage they are very destructive to A phis brassicae Linn. 
On Phragmites they were predaceous in large numbers on a very 
abundant unidentified aphid. I have also found them commonly 
on apple feeding on the European grain aphis ( Siphocoryne avenae 
Fab.) during Alay. Occasionally on curled Dock ( Rumex crispus 
L.) and broad-leafed Dock ( R . obtusifolius L.) among Aphis 
rumicus Linn, the first of June. One of these larvae was observed 
devouring a Syrphid larva ( Paragus bicolor Fab.) from the same 
host plant. On Black Willow the larvae parasitized colonies of 
the Willow Grove Plant-house ( Melanoxanthus soliciti Harris.) on 
the University campus the first half of October. 
