19 1 2] Morphology and Biology of Insect Galls 337 



cause of cell activity, productive of sawfly galls, is the wound caused by 

 the act of ovipositing. But there is a slight possibility that secretions or 

 excretions from the developing larva may be active through the egg 

 membrane. 



The protective sclerenchyma sheath of the more advanced types of 

 galls is absent in this group, and the only protective device appears to 

 be the cuticularizing of the epidermis and the presence of tannin in the 

 cells. The cuticle has also a more important function in preventing the 

 desiccation of the underlying thin-walled tissues of the gall. 



The possible significance of the aeriferous tissue found in Pontania 

 pomum Walsh will be discussed later in this paper. 



Lenticels on galls seem to occur very rarely. They were found in 

 this group only in the one species, Pontania pisum Walsh, a leaf gall. 



The restriction of the various species in many cases to single hosts 

 seems noteworthy when the minor specific differences between the 

 members of the Salicaceae are considered. 



A series of the undescribed species of Euura on Salix serissima 

 Fernald furnished undoubted examples of cell proliferation produced 

 by the excrement of the larval producer (Fig. 72). This fact is discussed 

 in the biological section of the paper. 



Localization of Tannin-Bearing Tissue in Sawfly Galls. 



Kiistenmacher'^ has discussed the question of tannin in certain 

 Cynipid galls and Cook^^ has detected it in different stages of a number 

 of galls, but no attempt has been made up to the present to work out its 

 distribution in the family Tenthredinidae. 



Pontania pomum Walsh. 



Tannin-containing cells are abundant in the epidermis of this gall. 

 They are found also in the "Aeriferous tissue," but are not so numerous 

 there. They are plentiful in the vascular strands, but can scarcely be 

 demonstrated in the tissue next the larva. 



In the normal leaf of Salix cor data Muhl. these cells are abundant, 

 especially so in the vascular tissue and the epidermis. 



Undescribed Pontania Gall on S. humilis Marsh. 



Tannin-containing cells are very plentiful in the epidermis and in 

 six or seven rows of cells that immediately underlie that tissue. They 

 are also present in the vascular strands and in the tissue next the larva. 



In the normal leaf of S. humilis Marsh these cells are not present in 

 the epidermis of the midrib but are found in the bundle of the midrib 

 especially in the bast portion. 



