50 OHIO BIOLOGICAL SURVEY 



ENEMIES 



(i) The common milkweed (Asclepias s'p.) is a rather formidable 

 enemy of adult Syrphidac. When the latter visit these flowers for the 

 nectar their legs are caught by the peculiar and well known pollinia of 

 this plant. In collections one finds a good many specimens with these 

 pollinia clinging to their legs. But large numbers of the w^eaker-bodied 

 Sjrphidae become permanently entrapped by the flowers: thus not only 

 invalidating the remarkable specialization on the part of the flower, but 

 resulting, after long and fruitless struggle, in the death of the fly. 



vSo common is this entrapment that I found it well worth while, when 

 collecting, to visit these flowers. I have found as high as twenty indi- 

 viduals entrapped on a single head. 



(2) The parasitic insects of the super-family IclDicuuionidac consti- 

 tute a very serious enemy of Syrphidac, at least of the aphidophagous 

 species. I have reared the species, Bassus lactatorius Fabr. from larvae 

 ^w^-^w^-A&oi Syrphus americanus, Syrphus torvus, Paragus hicolor, Paras^us 

 tibialis, .lUograpta ob/iqua, and Sphacrophoria cylindrica. 



It occurs most abundantly in the different hosts from midsummer to 

 late autumn. At times fully seventy-five per cent of the individuals col- 

 lected were destroyed by this parasite. I have not reared it from speci- 

 mens taken previous to July. 



"This parasite oviposits thru 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 dipter- 

 ous puparium, usually several weeks after pupation of the host. Only one parasite 

 develops in each 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, eruciform, ovate in outline; 

 median segments largest, humped dorsally and with the posterior end smaller than 

 the anterior. As the orientation is the same as that of the pupariinn 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 the 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, of Plate IV. 



"The adult may be easily recognized V)y the following description from G. C. 

 Davis' 'A Monograph of the Tribe Bassini' (Trans. Am. Ent. Soc. XXII, p. 19, Feb. 

 '95,) who also states that it is one of the most common and wide spread species in 

 America: 'Female — length 6 mm. Head, thorax, base and tip of abdomen, hind tarsi, 

 base and lower middle of hind tibiae, black; four anterior legs, posterior coxae, 

 femora, and often tips of tibiae, tip of abdoujinal segment i, whole of 2 and 3 

 and more or less of 4, rufous; anterior orbits, mouth, tegulae, spot in front, line 



