1918] Biology of A gromyza 11 



difference lying in the time of attack. The stimulus for the 

 formation of the gall being given while the plant is young and 

 the tissues are still forming; on the other hand, oviposition and 

 the entrance of the larva into the leaf later, when the leaf has 

 reached its main growth, does not stimulate the tissues to form 

 any swelling whatsoever, the result being a mine. 



Although no eggs were found, the plants were carefully 

 watched for mines. On June 22, 1916, very delicate mines 

 were found on the innermost leaves of the leaf-bundle. These 

 mines are first noticeable on the outer surface, under the very 

 thin epidermis of the leaf. The larvae remain very close to the 

 outer skin. The mines at first are so delicate as to be hardly 

 perceptible to the naked eye, but with the aid of a magnifying 

 glass, could be traced to the so-called egg-punctures, but no 

 signs of the eggs were found. 



In form the mine is linear, enlarging slightly as the larva 

 proceeds downward and increases in size. The mine zigzags 

 quite a little in its course, frequently the larva suddenly changes 

 to the opposite side of the leaf, so that the mine is no longer 

 visible on the upper surface, thus presenting a broken appear- 

 ance. (Plate I, Fig. 1). 



The color of the mine is white and shows plainly on the green 

 leaf, but shows more plainly on the lower part of the leaf, which 

 in the iris is of a purplish color. 



The larva proceeds downward just about as rapidly as the 

 new leaves are formed and come out of the leaf-bundle, and 

 passes on, thus being in a situation where the tissue is newest 

 and most tender. Sometimes, however, the larvas may remain 

 in the outer leaves and mine the entire length of it. This 

 is especially true of the early larvae. Plate I, Fig. 1 shows one 

 of these outer leaves with the characteristic mines in it. It 

 is not uncommon to find two or three larvae working side by 

 side in the same leaf, although each one maintains its own mine. 

 The larvae always maintain a lateral position in the mine, that 

 is with the sides of the body toward the two surfaces of the 

 leaf, always mining towards the base of the leaf; but just before 

 pupation the larva assumes a position with the dorso-ventral 

 sides towards the surfaces of the leaf, and with the anterior 

 end upwards. 



