38 



GENERAL BIOLOGY 



Animal galls. — Animal galls are less diffuse. Under the 

 stimulus of the attack of the insect in feeding, the tissue 

 grows rapidly, producing more food: moreover, around 

 the point of attack it grows and shuts in and covers and 

 protects the gall maker. Furthermore, it continues to 

 grow and shape itself into symmetry, its final form often 

 resembling a fruit. More remarkable still, it often de- 

 velops unpalatable substances (such as tannin) in its walls 

 and sharp spines upon its surface, and thus protects its 

 enemy the gall maker, from being eaten. 



Most animal galls are small, but a few of them, such as the 



aphid gall of the cottonwood 

 shown in figure 29, grow large 

 enough to become when numer- 

 ous, a feature of the winter land- 

 scape. This one is formed not 

 about a single aphid, but about 

 an aphid colony; and its irregu- 

 larity is doubtless due in part to 

 the grouping of individuals in the 

 attacking aphid flock. 



The commoner forms of ani- 

 mal galls are these: 



ffelted 



open 



mantle 



scroll 



pocket 



fluted 



gall 

 gall 

 sail 



covering gall 



Fig. 29. Winter aspect of aphid 

 galls on a cottonwood tree. 



closed 



simple 

 nucleated. 



The differences between these forms are indicated in the 

 following diagram, (fig. 30). 



