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 
Sycamore. 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 my 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 colony 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 
