GENERAL 9 



In the evolution of a form of body so well adapted to leaf- 

 mining the main tendencies have been toward the loss of 

 legs, loss of color, the flattening down of the body and head, 

 the tapering of the head to a wedge, sloping forward, with 

 the thin mouth parts at its front, the recession of the rear 

 of the head into the wide prothorax with development of 

 strong muscles for moving it forward and backward, in and 

 out, and the development of heavily chitinized dorsal and 

 ventral plates on the prothorax. 



LIFE HISTORIES 



All leaf-miners hatch from eggs and in the course of their 

 development pass through larval, pupal and adult stages. 



The eggs are either deposited upon the surface of a leaf 

 or inserted into it. In the latter case the mother insect 

 makes a puncture to receive it, either by means of her jaws 

 as in the case of certain beetles, or with her ovipositor as in 

 the case of all the sawflies, most of the two-winged flies and 

 a few of the more primitive moths. In Yponomeuta the 

 eggs are laid in masses on the branches of the host and 

 the young worms must wander a considerable distance to 

 find food. The time and place of oviposition vary with the 

 species. The larvae of some of them on hatching from the 

 egg may come out upon the surface of the leaf, but in all 

 the more specialized miners they pass directly into the leaf 

 through the epidermis that the egg covers, and do not appear 

 outside. 



The larva is the real leaf-miner. All the work of excava- 

 tion falls to its share. All increase in size occurs during the 

 larval period. The larva may develop chewing mouthparts 

 capable of devouring cells bodily, or it may develop cell- 

 shearing apparatus and sap-feeding habits. In any case it 

 grows, and passes through a series of larval stages or instars, 

 each of which is terminated by a moult. At the end of each 

 instar it casts off its hard, inflexible, chitinous skin, that has 

 become too close-fitting, and grows a new one. 



