May, 1911.] Two Species of Syrphidae. 339 



ber. The middle of September none were to be found. On 

 October 10, 1910, four larvae of this species were collected from 

 S3"camore. Eight days later one of them pupated. I have not 

 determined accurately the duration in the larval stage. 



The larvae of Didea fuscipes live in the colonies of the large 

 aphid, Longistigma {Lachnus) caryae Harris which appear so 

 abundantly in fall on the under sides of the lower horizontal 

 branches of the Sycamore (Platanus occidentalis L.). I have also 

 found the larvae on a Basswood tree (Tilia americana L.) affected 

 with these plant lice. They are apparently closely restricted in 

 food-habits to the body fluids of this one kind of aphid and may be 

 expected wherever Longistigma caryae occurs with any regularity. 

 They are rather sluggish and probably often spend their entire 

 lifetime among the particular group of plant-lice in which they 

 hatch. 



When feeding the larva seizes an aphid with the hooks of its 

 mouth-parts. The body-wall is punctured and the juices, which 

 alone are eaten, are slowly sucked out leaving the body-wall 

 shrunken and crumpled. These dried-up skins can frequently be 

 found on the branches where larvae have fed. It is ni}^ belief 

 that these flies destroy large enough numbers of the aphids to be 

 of considerable economic importance in keeping them in check. 



The excrement of the larva is dark purplish in color and leaves 

 conspicuous blotches on the white sycamore bark. The moist 

 excrement seems to be of use in helping the larva to cling to the 

 surface of the bark. 



I have discovered no habits of protection in the larval stage 

 more than that derived from the surrounding colon\^ of aphids. 

 They are certainly not conspicuous when so located. The loca- 

 tion on the under side of the twigs is no doubt a protection from 

 the weather and from some birds; but this is, I think, entirely 

 incidental to the similar location of their prey. The covering of 

 spines and especially the conspicuous bristly prominences may 

 be defensive. 



I have found no particular enemies of this stage. 



PUPA. 



The pupa is concealed in the hardened, slightly inflated, 

 sub-cylindrical, last larval skin, within which the changes to the 

 adult form take place. As the larva approaches metamorphosis 

 it attaches itself usually to a somewhat protected place on the 

 under surface of the limb. The anterior segments are retracted, 

 the skin becomes inflated filling out the wrinkles characteristic of 

 the larva. It rounds out anteriorly and dorsally, the point mid- 

 way between the fourth and fifth segments coming to lie at the 

 anterior pole, the mouth being shunted backward on the ventral 

 side 



