1918] Biology of A gromyza 13 



All the various pupae, those found early in the summer, as 

 well as those found in the fall, and those in the galls, were, 

 after emergence, identified as A. laterella. 



These observations then proved that this was not a case of 

 alternation of different habits in two generations, but it showed 

 that the fly is an essential leaf-miner, that the entire larval 

 stage is spent in leaf-mining; that the majority of the larvse 

 pupated at the base of the leaves where no typical galls were 

 produced; that only those larvae that developed slowly, and 

 entered the innermost leaf late in the fall at the time of cessation 

 of growth, were instrumental in bringing about the gall-forma- 

 tion. It would probably be a fair estimate to assume that not 

 over 20-25% of the larvae enter the inner leaf and form -galls. 



Just when or how the stimulus is brought about which 

 causes the gall formation has not been determined. There 

 is no sign of swelling in the fall, even after the pupa stage 

 has been reached, but as soon as the plant resumes growth in 

 the spring, the swelling occurs. 



The galls and mines are common wherever the iris occurs, 

 often nearly every plant is found to be affected. The fact 

 that the gall is formed on the innermost leaf, explains the reason 

 why only one gall occurs on the same plant. However on 

 October 23, 1917, I found several instances where two larvae 

 were present in the same plant, both apparently descending 

 down to the much coveted spot, the newly formed inner leaf. 

 In each such instance I found that the second larva remained 

 in the mine about 40-45 mm. above the other one which had 

 entered the usual place, the innermost leaf of the bundle. The 

 "upper" larva had excavated an enlarged place and transformed 

 to the puparium. These puparia, found in this position above 

 the other, are always more perfect in shape than those in the 

 gall-forming leaf. 



