April, 1904.] Galls and Insects Producing Them. 1 3 1 



ber of specific external characters ? Let us first review the struc- 

 tural characters of the leaf galls, since these galls show the most 

 uniform line of development. Considering Neuroterous irregu- 

 laris the gall of greatest simplicity, we can formulate the following 

 diagram : 



C. papillatus. 



A. confluentus. 



N. irregularis — C. tumifica. 



-H. centricola. 

 -A. inanis. 



-D. palustris. 



-A. petiolicola. 



In N. irregularis the zones are not so well developed as in 

 C. tumifica. In C. tumifica the zones are perfect, but in contact. 

 In C. papillatus the protective and parenchyma zones are sepa- 

 rated, but connected by long parenchyma cells. In H. centricola 

 and A. inanis the protective and parench3mia zones are connected 

 by fibro-vascular bundles. In A. confluentus the}^ are connected 

 both by fibro-vascular bundles and b}- parenchyma cells (Fig. 

 121). In D. palustris the parenchyma and protective zones are 

 not connected. In A. petiolicola the zones are in contact, but 

 the tissues are yevy dense, due to location in the petiole of mid- 

 rib of the leaf. 



If galls become independent structures they are undoubtedly 

 subject to the same laws of natural selection as any other group 

 of organisms, or if they be considered as parts of the plant they 

 must also be subject to the same laws of natural selection as any 

 other part of the plant on which they live. How, then, have 

 these laws affected the gall ? It may be a protective coloration 

 against birds and rodents, and other insects, but this cannot be 

 very important since mau)^ species of galls are verj^ con.spicuous. 

 Furthermore, animals make but very little use of galls for food. 

 So far I have observed other animals using galls for food but 

 once and then birds were tearing open the large galls of Pemphi- 

 gus vagabundus and eating the insects. The tannin which devel- 

 ops in such abundance in all galls as the}" approach maturity is 

 probably a great protection against insectivorous animals. 



The greatest insect enemy with which the gall insect has to 

 contend is the great number of parasites. The size, shape and 

 character of the epidermal covering of the gall may be a protec- 

 tion against this numerous eneni}-. The thickness of the gall and 

 the density of the ti.ssues, especially the protective zone, is an 



