482 The Ohio Naturalist. [Vol. Xll, 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 enomious 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 Syrphidas 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 larva are very tenacious of 

 life, some of them existing for over three weeks with very little 

 food. 



Out-of-doors the larva 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 sur^'ive several periods 

 of cold freezing weather. 



In my experience these larvee 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 Aphis 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 May. Occasionally on curled Dock {Rumex crispiis 

 L.) and broad-leafed Dock {R. obtusijoJius L.) among Aphis 

 rumicus Linn, the first of June. One of these larvse w^as observed 

 devouring a Syrphid larva (Paragus bicolor Fab.) from the same 

 host plant. On Black Willow the larva? parasitized colonies of 

 the Willow Grove Plant-house {Melanoxanthus soliciti Harris.) on 

 the University campus the first half of October. 



