42 OHIO BIOLOGICAL Sl'RVEY 



about the nest, especially about tlie superficial galleries and though hard and dry, must 

 contain considerable initriinent. They are probably eaten not only by the Microdou 

 larvae but also by many other Synoeketes." 



It seems possible that this habit might have been derived in several ways. (See 

 Fig. C.) On page 37 the idea is suggested that it might have arisen from a former habit 

 of preying on the immature of the nests, and, if so, would be related to the aphidoph- 

 agous group. The question arises whether there might be any significance in the fact 

 that the ants are so constantly associated with aphids and so, incidentally, with the 

 aphidophagous vSyrphid larvae. This might have afforded the original opportunity 

 for the entrance of Syrphid larvae to the ant-nest. 



On the other hand it seems probable that this group may have been derived from 

 the non-rat-tailed portion of Group IV. That is, from living on decaying vegetable 

 and animal matter, the larvae may have changed to eating waste materials in the nests of 

 Hymenoptera. There would be an especially natural step from Group III in the case of 

 3IicrodoH, if these larvae are found to live on the wood in any case. And it will be 

 remembered that Adlerz from observations in Sweden concluded whether rightly or not 

 that the larvae live on the moist and tender wood forming the walls of the ant galler- 

 ies in pine stumps, although they were also found in burrows in the dry bark. 



BIOLOGICAL AND ECOLOGICAL RELATIONS AND 



ECONOMIC IMPORTANCE OF THE LARVAE 



According to the above outline of larval habits the following are the 

 principal biologic, ecologic and economic relations which would arise: 

 ( i) To the insects (chiefly aphids) on which certain species prey. 



(2) To the plants on which their prey is feeding, and on which they are conse- 



quently found. 



(3) To the other insects which are concerned in such associations as aphid colo- 



nies and aphid galls. 



(4) To their own (Hymenopterous) parasites. 



(5) To plants on which certain larvae feed. 



(6) To plants within the tissues of which certain larvae live. 



(7) As scavengers. 



(8) To such colonial insects as ants and bumblebees in the nests of which cer- 



tain species live. 



(9) To the predaceous enemies of both the terrestrial and aquatic forms. 



(10) If the report be true, to those insects on which the larvae live as internal 

 parasites. 

 ( I ) This first relation is in the nature of a clo.se predatism on small 

 insects, chiefly colonial Homoptcra such as Aphididac and to some extent 

 /assidae, Aleyrodidac and Mcmbracidae. It may also in some cases be 

 considered obligate parasitism on the colonies. These forms are depend- 

 ent on one or another kind of small, soft-bodied insect for their food. 



My own studies have determined eight species of Syrphidae with the 

 aphidophagous habit in the State. The larger number of the.se are verj' 

 common. Some as Didea fasciata and Syrphiis xanthostornus, are appar- 



