3o8 Transactions of the Canadian Institute [vol. ix 



length of normal stem, owing to the shortening of the stem axis. The 

 larval cells are in the cortex of the stem at the bases of the needles. 



Dimensions: — Average length 2-6 cm. 



The abnormal development of the cortex, especially that part con- 

 tained in the wings on the stem, produces the entire mass of this gall. 

 The stimulation has increased the number of resin ducts in the cortex. 

 Several cross sections of galls were compared with corresponding sections 

 from normal stems. The average number of resin ducts in the abnormal 

 to that in the normal was in the proportion of 20 to 12. The smaller 

 accessory resin ducts are shown in Fig. 13. In every section examined 

 the additional ducts were in an irregular circle outside the normal ducts. 

 The ducts produced under stimulation were larger than the corresponding 

 normal ducts, but those that were found only in the abnormal tissues were 

 smaller than the normal structures. 



Fam. Psyllidse. 

 Pachypsylla celtidis-mamma Rile>'. 

 Host Celtis occidentalis L. 



A complicated form of pouch-gall produced in the mesophyll of 

 the leaf of the host. On the upper surface of the leaf the gall is indicated 

 by a decided depression in the centre of which is a slight elevation that 

 marks the opening of the gall. The part projecting from the lower 

 surface, is oblate-spheroidal in shape, attached to the leaf by a slightly 

 tapering cylindrical stalk. The average number of galls found on a leaf 

 is usually about ten, but in some cases much higher. The surface of the 

 gall is smooth except for a few fine scattered hairs, glaucous and greenish- 

 yellow in colour. 



Dimensions: — Height from point of attachment 6-7 mm.; width 

 7-8 mm. 



The anatomical structure of this gall shows it to be a more complex 

 type than any other of the Hemiptera discussed in this paper. 



Besides the folding of the leaf the blade has been further changed 

 in thickness and in the character of the cells. The production of the 

 greater part of the abnormal tissue is due to a wide, well differentiated 

 cambium layer, that extends right across the gall and at its margin 

 passes into the tissues of the normal leaf between the palisade and the 

 spongy parenchyma (Fig. 15). The larval chamber is lined by this 

 cambium sheath which thus functions as a nutritive layer. Bordering 

 this zone is a well developed protective tissue composed of cells with 

 uniformly thickened walls. The sclerenchyma is laminated and pene- 

 trated by branched canals, presenting the same character as that found 

 in the galls of the Cynipidse. 



