GALLS CAUSED BY FLIES 
73 
The outer surface of the folded lobe is spotted with white 
and red. 
The gall caused by Macrodiplosis volvens consists of the 
folding upwards of that part of the margin which lies 
between two adjacent segments. The fold is never so 
broad as that of M. dryobia, and there is no marked dis¬ 
coloration. There are one to four pale orange larvae under 
each fold. The photographs in Connold’s “ Oak Galls,” 
Plate LXIII., and “ Plant Galls,” Fig. 200, depict the galls 
of M. volvens , not M. dryobia , as therein stated. 
The galls so far alluded to in this chapter are caused by 
flies belonging to the family Cecidomyidae. The other family 
of British Diptera which contains gall - causers is the 
Muscidae, to which allusion must now be made. These flies 
have the bristles of the antennae feathered. The species are 
generally large and robust (house-flies and blow-flies are 
typical examples), and gall-causers are comparatively few. 
In the Cecidomyidae we have twenty-seven genera of gall 
causers, in the Muscidae only thirteen. 
The majority of Muscid gall-causers infest plants belong¬ 
ing to the family Compositae. The genus Tephritis is perhaps 
the most extensive. Various species attack the flower head 
(capitulum), causing it to swell and remain unopened— e.g. t 
T. conura attacks the inflorescence of the Melancholy Thistle, 
and T. eluta causes the capitulum of the Black Knapweed to 
become hardened. For other species reference should be 
made to the catalogue. 
Urophora is another well-known genus. The larvae of 
U. cardui cause a very pronounced rounded or fusiform 
swelling on the stem (usually near the apex) of the Creeping 
Thistle. It is hard, glossy, green or brownish, often attain¬ 
ing the size of a walnut. The larval cells are numerous, each 
containing a single occupant. If these galls are gathered in 
autumn and kept till the following June, there will be no 
difficulty in breeding the pretty little flies. 
Trypeta bardanae attacks the Common Burdock, causing 
the seed-capsules to become swollen and deformed. Chlovops 
