I9I2] Morphology and Biology of Insect Galls 345 



equally on each side of the leaf. In the majority of cases the gall extends 

 out almost to the margin of the leaf and only the edge of the blade rims 

 its outer side. Rarely this gall is found originating from the peduncle 

 of the staminate catkin of the host. 



The very young gall of this species is densely pubescent, while the 

 well-grown specimens are usually quite smooth. In galls collected when 

 the leaves are just beginning to unfold from the bud the larval cell and 

 the outer zones of the gall are united, but very soon a separation occurs 

 and the larval cell is left rolling freely around in the outer gall. Colour 

 of mature gall green with patches of red in some places. 



An average of about three weeks elapses from the time of the open- 

 ing of the buds until the producers emerge from the galls. After another 

 week the galls are wrinkled, dried up and brown. About ten days before 

 the time of emergence of the producers the larval chambers were removed 

 from several galls and placed under dry conditions. While the time of 

 emergence' of these producers was not appreciably changed, the insects 

 in almost every case had difficulty in freeing themselves from the larval 

 cells and one wing usually remained shrunken. It would appear that the 

 outer gall during the later stages of development functions only as a 

 moist chamber for the prevention of the desiccation of the larval cell. 



The youngest galls examined were obtained from leaves that were 

 just breaking out of the bud. At this stage the larval chamber still has 

 organic connection with the remainder of the gall (Fig. 49). A well- 

 defined cambium zone, in which mitosis is taking place, divides the gall 

 wall into nearly equal parts. The parenchyma layer on the outside 

 extends from the cambium to the epidermis. It consists of small cells 

 that resemble closely those of the cambial zone. The inner half of the 

 gall, forming the nutritive layer, is composed of much larger cells ar- 

 ranged in rows radial to the larva. A canal passes from the outside into 

 the larval chamber. The epidermal lining of this canal is continuous 

 with that of the epidermis of the gall and is covered with the same class 

 of trichomes (Fig. 49). It gradually passes over into the innermost 

 layer of the nutritive zone. 



In a very short time after the opening of the buds, the larval chamber 

 is severed from the remainder of the gall. The break occurs near the 

 outside of the cam.bium zone, and separation has commenced in Fig. 50. 

 At this stage the protective layer is not yet differentiated. Soon after 

 the separation occurs it is produced, however, and the four zones of a 

 typical Cynipid gall are complete. 



The cells of the protective sheath are placed tangential to the larva. 

 There are two layers of these cells, both of which have one tangential 

 wall thicker than the other. In the outer row the thicker wall is towards 



