DESCRIPTIONS OF OAK-GALLS. 



16 



1 inillimetre in height, and with a horizontal diameter of 

 about 2 n)illinietres at the base; it is bare and green, but 

 later on it becomes yellow or yellowish brown. At the 

 corresponding spot on the under side of the leaf it appears as 



Figs. 77 ifc 78. — Galls of Gecidomyia cerris, and in section ; Galls ot 

 Cecidomyia circinans, and a specimen in horizontal section. 



a circular, slightly convex, projecting disk, of about 2 milli- 

 metres in diameter: it is very thickly covered with yellow or 

 yellowish brown outstanding, fine, but tolerably long, hairs. 

 In the interior is a larva-chamber, in which the reddish 

 orange maggot lies. When the gall contains the gall-gnat 

 larva, and not a parasite, towards the end of October or 

 beginning of November the fully ripe gall swells, so that this 

 disk opens like the lid of a box, and the maggot falls to the 

 ground, where it winters and changes to a pupa, till in May 

 the perfect gall-gnat is evolved. Should the gall contain the 

 larva of an ichneumon the lid does not open ; and in order to 

 release itself the fly bites a round hole through the side of 

 the cone on the upper side of the leaf. — G. L. Mayr. 



78. Cecidomyia circinans, Gir. — This gall may be found 

 on the under side (rarely on the upper side) of the leaf of 

 Qiiercufi cerris, often mixed with the preceding species on 

 the same leaf. It occurs as a circular or kidney-shaped disk, 

 which is about 2 millimetres high, with a horizontal diameter 

 of 5 to 6 n)illimetres, and is thickly covered with outstanding, 

 yellow or gray, hairs. In the centre of the gall, on the upper 

 side of the leaf, it exhibits an annular, mostly yellow, swelling, 

 with an extreme diameter of from 2 to 25 millimetres; within 

 this is a thin, hairy membrane, stretched horizontally, which, 

 when the gall becomes mature, opens in the middle and 

 forms a cavity : this leads to the interior at the axis of the 

 gall, and curving spirally becomes formed into a circular 

 channel, which terminates near the periphery of the orbicular 



