124 BRITISH OAK GALLS. 



English name of gall. — "The Ovster Gall. ' 



Position of gall. — Ou the under- surface of the mid-ril > aucl its 

 offshoots. 



Manner of growth. — Gi-lalirous, glossy, ovoid, spherical. 



Colours. — Pale yellow, yellowish-greeu, pink, pale brown. 



Average dimensions of a mature specimen. — Ovoid : 

 Height, 2 mm. : length, 3 mm. ; breadth, 2 mm. 



May be sought during the months of August to October. 



Growth is complete bv the end of September. 



The typical condition of the gall is unilocular and unilarval. 



The larva pupates in the gall. The imago emerges during 

 March. "When kept under favourable conditions, imagines will 

 appear from galls, which have niatiu-ed early, during November ; 

 but from others late iu maturing the imagines do not emerge 

 until March. 



Parasites, Nos. 5, 44, 79. Inquilines, Nos. 141, 142. 



Alternate sexual generation : Spathegaster Aprilinus, Gir. 



Leaves bearing from 12 to 30 galls are frequently 

 seen. I found during September, 1901, near Hastings, 

 a twig of (^. jjeduncnhiia having eight leaves, all of 

 which were galled, the total number of galls upon 

 them beino- 207. The laro-est leaf measured 115 mm. 

 long, and had -42 galls upon it. 



The only galls with which this species can he con- 

 fused are those of IJrj/opJiantn dicisa, and Biorhha 

 reniuii. It is however more frequently met with than 

 the former, and less frequently than the latter. When 

 mature it is smaller than D. divisa, and seldom 

 spherical. Other distinguishing features are given in 

 the description of B. rennm. 



Growth is rapid, and complete in about four weeks. 



When the gall emerges from the mid-rib or its off- 

 shoots it is enveloped by a thin membrane of leaf- 

 tissue, which splits longitudinally and forms two flaps, 

 or valve-like parts, at the base of the gall ; these in 

 time wither and fall away. It is in consequence of 

 these flaps bearing a fanciful resemblance to the valves 

 of an oyster shell, that the gall received its trivial 

 name. 



Diu'ing the earliest stage of development the gall is 

 uniformly coloured, but later, numbers of little spots 



