252 LEAF-MINING INSECTS 



lent life history study of this miner may be found in Webster 

 and Parks (1913), from which we have taken our account. 



The female inserts her eggs beneath the upper epidermis. 

 They are pale, white, oval, about 0.25 mm. long and can 

 frequently be seen through the upper epidermis. The lar- 

 vae commence feeding immediately after hatching. The 

 mine is at first very small and thread-like gradually widen- 

 ing with the growth of the larva. The larva is not able to 

 enter a fresh leaf and must obtain its nourishment from a 

 single leaf. The mine is typically linear, considerably 

 curved but never greatly elongate. Often, however, a 

 blotch is produced on the leaf. Frost (1923) mentioned its 

 peculiar habits when mining the leaves of Nasturtium. The 

 mines on nasturtium are always short and of the linear or 

 serpentine type. Seldom more than one or two mines are 

 found on a single leaf. When full grown the larva invari- 

 ably retreats a short distance from the end of its mine and 

 makes a slit parallel with the sides of the mine through which 

 it escapes. 



Webster and Parks tells us that the number of generations 

 depend upon latitude and seasonal conditions. In North 

 Indiana during the season of 1912 there were six generations. 

 The larvae may transform within their mines or escape as 

 mentioned above and form their puparia outside their mines. 

 Apparently in arid and semi-arid regions the larvae trans- 

 form within their mines while in humid sections they aban- 

 don their mines to transform. In Florida the larvae may 

 continue their feeding throughout the winter. Even in 

 Arizona in mild winters larvae have been found mining in 

 the leaves until Christmas. The larvae apparently feed until 

 cold weather prevents their activities and naturally large 

 numbers are killed by the cold and never transform. 



The two winged elm leaf -miner, Agromyza ulmi Frost, is 

 abundant in New York and Pennsylvania. It is an inter- 

 esting leaf-miner because it is one of the few Diptera which 

 mine the leaves of woody plants. 



