THE SYRPHIDAE OF OHIO 45 



the}^ feed, and oviposition takes place at the same time. Not so easy 

 of explanation, however, is the deposition of eggs on s^xamore trees, by 

 Didea fasciata , early in autumn in anticipation of infestation by Longis- 

 tigma caryac; or of the eggs of Syrp/ins americanus early in mid-summer 

 on Phragmitcs which becomes very badly infested with aphids; or of the 

 several species on cabbage before aphids are present. In these cases there 

 are no blossoms for the attraction of the adults and the explanation must 

 fall to some tropism not connected with the feeding habits. 



It should not be overlooked that in certain cases these larv^ae destroy 

 aphids on injurious or noxious weeds (docks, thistles etc) . This, although 

 apparently a detriment, may very well be considered a safeguard against 

 too great increase of au)^ species of Aphididae. In some cases these are 

 plant-lice which also attack garden and field-crops. In others where this 

 is not known to be true, a change of food-plant for the aphid, with 

 increase in numbers, is entireh^ possible. Aphids in general are such 

 injurious insects that any of their enemies ma}- pretty safely be welcomed 

 as friends of the agriculturist. Hymenopterous parasites destroy vast 

 numbers, but it is the author's belief that Syrphid larvae are a more 

 important enemy of these pests than are the Hymenopterous parasites. 



^3) The complex ecological relations which must exist in any colony 

 of aphids, and especially' in the aphid-galls such as those of Ponphigus, 

 are worthy of careful study. 



First, there are in such colonies, the plant- lice which artj phytopha- 

 gous. Second, the ever present ants which derive sustenance from the 

 secretions or excretions of the aphids. So constant is their association with 

 the aphids that they were often of value in leading me to discover some 

 of the more obscure colonies of the latter. Their relations with the ajihids 

 is of a friendly nature, that to the host-plant, neutral. Third, the larvae 

 of Syrphidae are almost always present, preying on the aphids, killing 

 large numbers of them, and sometimes exterminating the colony. These 

 are benefactors of the host-plant. They are indirectly inimical to the ants; 

 yet I have never observed the slightest antagonism of ants toward them. 

 Fourth, Hymenopterous parasites of the aphids (especially Chalcididae) 

 are rarely absent from any colon}' of the aphids. These are, of course, ene- 

 mies of the aphids and so, indirectly, of both the ants Q.r\d Syrphidae, and 

 friendly to the host-plant. Probably the larvae of the Syrphidae devowr 

 some of the aphids which are already parasitized by the Chalcididae, espe- 

 cialh- those containing only eggs or young larvae of the parasite. If this 

 does happen it makes the Syrphidae b\- so much disadvantageous to man. 

 The aphid would, of course, be killed as effectually as it could be by the 



