200 DIPTERA OF NORTH AMERICA. 



surface of the leaf, on one of the ribs. The irritation caused by 

 the larva produces a fold or cavity in the leaf, lined inside with a 

 white pubescence. The under side of the leaf shows on the cor- 

 responding spot the swollen rib, which is pale green, bordered on 

 both sides by the same white pubescence. The galls, according 

 to their size, contain more or less Iarva3, sometimes ten or more. 

 When this deformation is very large, it involves the whole leaf, 

 which is folded in two along the midrib, the under side forming 

 the outside of the fold, and showing the swollen ribs with the 

 white pubescence in their intervals. On the 25th of May I found 

 some of the galls considerably grown, thick, and swollen ; their 

 pubescence was of the brightest white ; the larvae they contained 

 were also grown, plump, white. Other galls, on the contrary, had 

 grown but little, and appeared sickly or withered. They contained 

 no larvre at all, or their inmates appeared yellowish and sickly. I 

 suppose that some of them were attacked by parasites, as I found 

 a minute hymenopterous larva fastened to the skin of one. In 

 June some of the galls which I brought home were abandoned by 

 their inmates, which went under ground for transformation, but 

 perished soon afterwards. 



The larva of this gall is white, and has two small horny pro- 

 cesses, directed upwards, at the anal end of the body ; its breast 

 bone is truncated, heart-shaped anteriorly. 



20. 0. erubescens, n. sp. Folded margin of an oak leaf, tinged 

 with red. This deformation seems to resemble that of G. quercus 

 Lw. on the European oaks. Occurs in the spring. 



21. C. symmetrica, n. sp. Hard red gall on the leaves of dif- 

 ferent kinds of oak, small and round (between 0.05 and 0.1 in 

 diameter) when single, but more commonly assuming an irregular 

 shape by the coalescence of a number of them. 



I find them chiefly and in large numbers on leaves of Quercus 

 falcata in autumn. They sometimes invade almost the whole sur- 

 face of the leaf, and have exactly the same size and shape on both 

 its sides. The single round galls contain one larva, the compound 

 ones a number of them, depending on the size of the gall, but each 

 in its own compartment. The red substance of the crust shows 

 many cracks, when the gall is ripe, and is easily detached. Under 

 it is a harder, almost woody, yellowish substance. When the dry 

 leaves with such galls fall to the ground, the red part of the crust 

 generally crumbles away in part, partly it is found erect, forming 



