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. dryohia, 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. dryohia, 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 Tepkritis is perhaps 

 the most extensive. Various species attack the flower head 

 (capitulum), causing it to swell and remain unopened — e.g., 

 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. 



Urophara 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-capsutes to become swollen and deformed. Chlorops 



