212 MAINS AGRICULTURAL EXPERIMENT STATION. 1916. 



outwardly directed loops, denticulated only on the outer side, almost 

 arborescent in appearance but still divisible into three pairs. The 

 interspiracular ornamentation of small, delicate, non-plumose hairs. 

 No anterior larval spiracles have been demonstrated. 



In place of the transverse folds or wrinkles on the dorsal surface, 

 it is evenly convex and beautifully reticulated with lines of papfllae. 

 I find no evidence of the segmental hairs. 



The pupal respiratory cornua are similar to those of type III being 

 much shorter than in type IV. 



ECONOMIC IMPORTANCE OF THE LARVAE. 



The principal economic importance of the larvae is four- fold, 

 two of these relationships being beneficial and two of them 

 detrimental to human interests. 



A. BENEFICIAL HABITS. 



I. As predators. By far the most important economic bear- 

 ing of Syrphidae arises from the predaceous habit of many 

 common species. Of the prey affected, aphids compose prob- 

 ably ninety-nine per cent. Coccidae and the young of Aley- 

 rodidae, Jassidae, Psyllidae and Membracidae are also attacked, 

 and occasionally even active winged insects such as Diptera may 

 be caught (55, p. 311) or the larvae of other Syrphidae devoured. 



With the vast majority of species there is no close restriction 

 to particular species of prey; although a few species such as 

 Didea fasciata and Pipiza pisticoides have been found among 

 only one or a few species of aphids. 



For the most part the service of these insects is in preventing 

 the abnormal increase of the various injurious species of aphids 

 rather than in exterminating the colonies in any particular place ; 

 although the latter result is by no means uncommon. Their 

 value is ordinarily due to their very uniform distribution and 

 constant occurrence, rather than to tremendous numbers at any 

 time or place. Nevertheless, instances of the occurrence of 

 enormous numbers of them are not lacking. Perhaps the most 

 notable of such records is that by W. G. Johnson (23, p. 97) 

 regarding the abundance of larvae during an outbreak of the 

 pea-louse (Macrosiphum pisi Kalt.) in southern Maryland in 

 1899. I quote from Mr. Johnson as follows: 



