191 2] Morphology and Biology of Insect Galls 325 



age. The cortex is approximately four times as thick as the cortex of 

 the normal stem. This is due principally to the increased size of the 

 cells. The cells of the nutritive layer are very similar to those of the 

 surrounding tissues but a well marked protective zone defines its outer 

 limits. This is clearly shown in Fig. 25. 



The cells of the protective zone present a characteristic very rare 

 in Dipterous galls, although frequently found in the Cynipid galls, 

 namely, the walls of the cells are not uniformly thickened but are much 

 heavier on the side next the nutritive zone. This unequal sclerification 

 is illustrated in Fig. 26. Crystals of calcium oxalate, that were so char- 

 acteristic a feature of this zone in C. triticoides Walsh, seem to be en- 

 tirely absent in this gall. 



Rhabdophaga strobiloides Walsh. 

 Host Salix cordata Muhl. 



"The galls are very uniform in size and form, usually top-shaped, 

 some inclining to spherical, a little oblate below and prolate above, and 

 as the female oviposits but one egg in the terminal bud of the willow 

 shoot, the galls are terminal and monothalamous. 



"The gall is a rather tightly and regularly arranged mass of from 

 70 to 80 aborted leaves, representing perhaps about i m. of the leafage 

 of a normal branch. 



"The average measurement of 200 galls was 12 mm-XiS mm., and 

 the length of the deformed part of the branch included in the gall around 

 which the aborted leaves were packed was 6 mm." — Brodie.^^ 



The leaves that constitute the principal mass of this gall do not 

 take any part in supplying the larva with food. The tissue that has this 

 function is composed of a mass of small, thin-walled cells. It terminates 

 the stem axis and the larva is in immediate contact with it (Fig. 24). 

 This tissue which really furnishes a nutritive layer seems to originate 

 from a cambium-like tissue at the base of the mass of cells. 



An important factor in the production of this gall is the practical 

 cessation of growth of the bud axis. 



The aborted leaves that compose the gall exhibit very slight ana- 

 tomical aberrations. 



Eiirosta solidaginis Fitch. 

 Host Solidago canadensis L. 



A monothalamous gall produced by the swelling of the stem of the 

 host plant. Very rarely it is found on a branch of the flowering panicle. 

 A separate gall is almost perfectly spherical in form but occasionally two 

 are produced together forming a common gall, prolate-spheroidal in shape. 



Dimensions: — Average diameter of fifteen galls, 23 mm. 



