Mar., 1912.] 
Life-Histories of Syrphidae III. 
483 
On these plants the position of the larvae is determined by that 
of the aphids. On cabbage they are largely on the under side of 
the outer drooping leaves but may be found well in among the 
more compact leaves of the head. On Phragmites they are mostly 
on the upper, but also on the under side of the long linear leaves; 
on Rumex spp. chiefly among the flower spikes, and lower leaves; 
and on apple and willow on the outer small and tender twigs. 
Parasites. 
I have noted one very bad enemy of Syrphus americanus — the 
Iehneumonid parasite, Bassus laetatorius Fabr. I have reared 
this species from larvae and pupae not only of S. americanus but 
also of Paragus bicolor, Paragus tibialis, Allograpta obliqua, and 
Sphaerophoria cylindrica. 
It appears most abundantly on S. americanus especially during 
midsummer, July and August, on Phragmites and again in Septem- 
ber to November on specimens from cabbage. At times I have 
found fully 75% of those collected were destroyed by this parasite. 
I have not reared the parasite from specimens taken previous 
to July. 
This parasite oviposits through the body wall of the larvae, the 
eggs hatch and the larvae develop without preventing the formation 
of a more or less complete puparium by the host. Within the 
puparium the development of the larval parasite goes on at the 
expense of the Syrphid. The latter is entirely devoured and the 
parasitic larva reaches in size nearly the capacity of the puparium. 
Pupation then takes place and the adult emerges by gnawing a 
small irregular hole in the anterior end of the dipterous puparium 
about 3 or 4 weeks after pupation of the host. Only one parasite 
develops in each host individual. 
The larvae when full grown measure about 4 mm. in length by 
1.8 mm. in height, by 2 mm. in width. They are plump, whitish, 
erusiform, ovate in outline; median segments largest, humped 
dorsally and with the posterior end smaller than the anterior. As 
their orientation is the same as that of the puparium it will be seen 
that the full grown larva fits very nicely, in size and shape, the 
puparium of the host. There are 14 body-segments clearly 
shown; the only conspicuous appendage is a U- or V-shaped 
chitinous piece in the region of the mouth. Sketches of a larva 
and a pupa are given as Figures 58, 59. 
The adult may easily be recognized by the following descrip- 
tion from G. C. Davis’ “A monograph of the Tribe Bassini” 
Trans. Am. Ento. Soc. XXII, p. 19, Feb. ’95, who also states that 
it is one of the most common and wide spread species in Europe 
and America. 
