Mar., 1912.] Life-Histories of Syrphidae III. 483 



On these plants the position of the larvas 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 

 Ichneumonid parasite, Bassus laetatorius Fabr. I have reared 

 this species from larvse and pupae not only of 6". americanus but 

 also of Paragus bicolor, Paragus tibialis, Allograpta obliqua, and 

 Sphaerophoria cylindrica. 



It appears most abundantly on 5'. americanus especially during 

 irddsummer, 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 larvas, the 

 eggs hatch and the larvae develop without preventing the fonnation 

 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 arid wide spread species in Europe 

 and America. 



