44 OHIO BIOLOGICAL vSURVEY 



these insects; but the fact that in this way the production of enormous 

 numbers ofaphids is 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 establishment 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."* 



Assuming an equally voracious appetite, or anything like it, for the 

 hundreds of larvae of this and other species with the same habit to be 

 found on any ordinary farm, it is easy to see that here we have a very 

 important check for the enormous rate of reproduction among plant-lice. 



(2) The aphidophagous larvae of Syrphidae doubtless often save the 

 life of the plants on which their prey lives. Aphids with their enormous 

 rate of reproduction can easily cause the death of their host-plants, espe- 

 cially if infection is begun early. The vSyrphids, if the}' do not extermi- 

 nate the aphid colon\-, often check them sufficiently to allow the host- 

 plant to continue growth to maturity. 



A nice balance must exist here between these three organisms. If the 

 Syrphids are to mature, a sufficient number of aphids must be produced. 

 But if too many aphids are produced, the host-plant may be killed, the 

 aphids, and in turn the Syrphids, exterminated or forced to migrate to 

 another plant. It seems possible that the habit of the adults of aphidoph- 

 agous species of scattering their eggs, ma}^ be of importance here, in that too 

 many Syrphids are kept from preying on a single aphid colon}^, in which 

 case the available food would be exhausted before an}- of them reached 

 maturity. 



In many cases there must al.so be a nice adaption of the life-cycle of 

 the Syrphid to the length of the life of the aphid colony. Many aphids 

 spend only a certain part of the year on one host-plant and then mi- 

 grate by certain individuals to other plants. Migration of the larvae 

 to any distance is out of the question, and apparently in some cases 

 natural selection has worked out the proper length of life of the larval 

 stage to meet the.se requirements. 



The life-cycle must be begun at the proper time on the right hosts as 

 well. Eggs for the first spring generation of Syrphus amcricaniis are 

 deposited, among other places, on leaves and twigs of apple at blo.s.som- 

 ing time; where are already developing colonies of the European Grain 

 Aphis. The adults are attracted about the trees, on the blos.soms of which 



•The Author, Ohiu yaliintlisl, Volume XII, Number 5, pp. 481, 4S2, March 1912. 



