234 LEAF-MINING INSECTS 



mine is obscured. This primary mine is often very short, 

 not more than an eighth to a quarter of an inch long. Some 

 of the linear mines that are very narrow at the beginning 

 and gradually enlarge, are called serpentine mines. Other 

 mines are intermediate between the linear and the blotch 

 types. The linear-blotch mines start as linear mines and 

 suddenly change to the blotch type. They differ from the 

 trumpet mines of the Lepidoptera in that the linear part 

 does not enlarge gradually to form a blotch, but makes an 

 abrupt connection with the enlarged part. 



There is little difference between the blotch and the 

 linear-blotch mine. In the former the primary linear mine 

 becomes obscured by the blotch mine, while in the latter 

 the original linear mine remains evident, and is usually of 

 some length. The linear-blotch mines are produced by 

 solitary miners. This perhaps explains their formation, 

 since a single larva can mature in its mine without enlarging 

 it sufficiently to cross the original linear tract and so 

 obliterate it. 



The mines can be further grouped according to their 

 position on the leaf, the number of larvae within a mine, 

 the arrangement of the frass, the position of the puparium, 

 and the exit holes made by the larvae as they escape from 

 the leaf. 



Considerable variation is shown in the position of the 

 mine on the leaf. The larvae of most species show a prefer- 

 ence for the upper surface. A few species are found only on 

 the lower surface. Some species, of which Cerodontafemora- 

 lis, Agromyza parvicornis, and Agromyza later ella are good 

 examples, alternate between the upper and the lower surface 

 of the leaf. 



In a strict sense, most Dipterous leaf-miners are solitary, 

 because they hatch from eggs laid singly on the surface or 

 within the leaf, and as the larva emerges from the egg it 

 must start its own mine. Since the eggs are often laid 



