INTERDEPENDENCE OF ORGANISMS 



39 



The primary distinction between open and closed galls 

 lies in their mode of origin. In the open gall the attack is 

 made from the outside, while in the closed gall the insect 

 enters the tissue bodily and feeds inside: ordinarily, it 



FIG. 30. Diagram of typical form of galls. a, felted; b, scroll; c, fluted; 

 d, pocket; e, covering;/, simple closed; g, nucleated, a to e are open 

 galls; /and g, closed. 



enters in the egg stage, the egg being inserted through a 

 puncture in the epidermis. In the open gall the insect 

 may be covered and inclosed by the overgrowing tissue, 

 but when inside the gall it is still outside the leaf 

 substance, and in feeding, stands upon and punctures 

 the epidermis with its piercing mouthparts. 



Felted galls (fig. 31) represent a low degree of gall develop- 

 ment. They occur mostly upon leaves, and are as a rule 

 made by mites. They usually 

 consist of a slight sacculation 

 of the part of the leaf blade that 

 is subject to attack, and the 

 malformation is mainly confined 

 to the epidermal cells, which 

 develop a wonderful growth of 

 robust plant hairs that are 

 twisted and matted together like 

 felt, whence the name. The 

 mites clamber around and feed 

 between the bases of these 

 plant hairs. 



Mantle galls represent a better development of 

 leaf cover for the gall maker: the cavity is deeper 



FIG. 31. A felted gall (a, 

 cross-section) from the leaf 

 of button-bush (Cephalan- 

 thus occidentalis) and the 

 mite (b) which causes it. 



