ORDER DIPTERA 277 



directions. It is not an unusual sight to see several such 

 blotches on a leaf. Each represents the work of a number of 

 larvae that have hatched from a single group of eggs. 

 These mines increase in size until they interfere with one 

 another and a large blotch is produced covering the entire 

 area of the leaf. Many of the larvae are naturally forced 

 to abandon their mines and form new ones in other leaves. 

 The presence of nearly mature larvae in small blotch mines 

 is an indication that these larvae have entered fresh leaves. 



The process of forming a new mine is as follows: At first 

 the larva cuts a short slit in the epidermis of the leaf. Then, 

 by inserting its mouth hooks in this slit and working them 

 back and forth, it separates the lower and upper epidermal 

 layers of the leaf. The larva then pushes the anterior end of 

 its body into the small opening that it has made. After the 

 first two segments have been forced through the opening, it 

 is only a matter of a few minutes before the larva has worked 

 its way completely within the leaf. This operation is 

 accomplished with many vigorous twists of the body as it 

 is drawn into the leaf. Larvae of the third instar bury them- 

 selves completely in the leaf in less than twenty minutes. 

 The length of the larval period varies between 9 and 16 days. 

 In warm weather the larvae mature rapidly, but in cooler 

 weather they become inactive and the larval period may be 

 prolonged for several days. 



The mature larvae escape through the epidermis of the 

 leaf but seldom make their exit through a definite hole. 

 Usually the upper epidermis becomes dry and parchment- 

 like, rupturing itself and allowing the larvae to escape. 

 In some cases, however, the larvae cut circular holes through 

 the epidermis. They fall to the ground and ordinarily 

 penetrate the soil to a depth of 2 or 3 inches. Some of the 

 puparia formed in June and July, as well as those formed in 

 September, do not transform to adults in the same year that 



