hegxer] 



GALL INSECTS AND INSECT GALLS 



201 



stegosaurus one-twelfth the linear dimensions of the type specimen 

 now on exhibition in the museum, and issued another pamphlet 

 describing same. 



Thus protected why should the stegosaurus worry about war 

 or the high cost of living ? 



Gall Insects and Insect Galls 



Robert W. Hegner 



Zoological Laboratory, University of Michigan. 



Everyone who has spent much time in doing field work with 

 animals or plants has noticed the numerous abnormal enlargements 

 that are present on the stems, twigs, and leaves of many plants. 

 Those who have attempted to find out what is inside these growths 

 have discovered, on cutting them open, that they contained either 

 voung or adult insects. The older naturalists did not allow these 

 plant growths, or galls, 

 to escape examination, 

 but in spite of their 

 investigations and the 

 more detailed re- 

 searches of recent 

 years, there are many 

 things we do not 

 know about them. 



The Willow-Cone Gall 

 One of the most 



Fig. i. 



(After 



The Willow-Cone Gall. 

 Beutenmiiller) 



abundant of the insect galls is the willow-cone gall (Figure i), 

 a structure that occurs so regularly on certain willows as 

 to be considered by most observers a normal part of the willow 

 plant. The relations between this gall and the insects that 

 inhabit it are among the most remarkable in nature and no better 

 example can be found for the purpose of demonstrating the com- 

 plexity of the web of life than this common plant growth. 



The egg of the willow-cone gall-fly is inserted by the mother into 

 the bud sometime in April or May. What stimulates the plant to 

 produce the gall is not known definitely, but several of the theories 

 that have been advanced will be discussed later. By midsummer 



