THE vSYRPHIDAE OF OHIO 



43 



ently very closely restricted to one species of prey. Others, notably 

 Syrphus ainericamis, Sphaeropnoria cylindrica and Allograpta obliqiia are 

 found on a variety of plants preying upon at least several species of apliids 

 or other insects, even attacking larvae of other species of Syrphidac. 

 By the species which I have studied, perhaps the greatest benefit is being 

 wrought by the destruction of vast numbers of cabbage aphis (Aphis 

 brassicae), although that arising from the destruction of such insects as 

 the European Grain Aphis, (Siphocoryne avcnae) Longistigma, Pemphigus, 

 Colopha, etc., is not b}" any means to be despised. 



They kill the individual at once; but the colony may exist for a con- 

 siderable time, replacing those lost or even increasing its number by 

 reproduction. Sometimes the colony is entirely exterminated by the rav- 

 ages of the Syrphidae. There seems to be a sufficient margin between the 

 amount of food which the larva must have to successful!}^ accomplish 

 pupation, and the larger amount which may be taken and stored as surplus 

 fat, to make the extermination of the host-colony possible in some cases, 

 without the loss of the Syrphidae. Theoretically under these conditions 

 the complete extermination of aphids might result. 



Several larvae of one species and frequently two or three species are 

 often to be found in a single aphid colony. Hardly one colony in five 

 will be examined which does not have in its midst one or more larvae. 



"In order to determine something of the capacity of these insects for 

 devouring plant lice and hence their degree of economic importance, I tried 

 feeding the larvae of Svrphus aniericanus on cabbage aphids (Aphis 

 brassicae Linn, j) The aphids were touched to the mouth of a larva which 

 had not been kept from food. A four-day-old larva devoured the first 

 aphid in 4.5 minutes, a second, third, fourth, and fifth, smaller than the 

 first, in 2, 1, i, and 0.5 minutes respectively. The .sixth, a larger one, 

 was retained for 3 . 25 minutes. These were very thoroughly eaten, all the 

 viscera and body fluids being picked and sucked out. After this the lice 

 tendered were not eaten so closely, but killed, a seventh in 2 minutes, an 

 eight in i . 75 minutes, and a ninth in 1.5 minutes. On another occasion 

 the same test was made with an older larva which devoured a dozen or 

 two before my patience became exhausted. The tests were sufficient to 

 establish the voraciousness of the appetites of the.se larvae. 



"It is, of course, not probable that any larva would ever normally 

 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 mu.st be 

 very considerable. It should be kept in mind also that it is not the ac- 

 tual individuals eaten, alone, that determines the amount of benefit from 



