INJURIES BY ROUNDHEADED BORERS. 



343 



deposited a minute wormlike larva (fig. 19, c) issues therefrom and 

 immediately begins boring into the bark with which it finds itself in 

 contact. The larva usually proceeds directly to the inner bark, or 

 cambium, immediately next to the wood. Here the larva mines and 

 feeds until it reaches a certain growth, when it makes preparation for 

 a change called pupation. The entire growth of the insect is attained 

 in the larval form. 

 Usually, before it at- 

 tains full growth, how- 

 ever, the larva mines 

 either into the solid 

 wood or into the outer 

 corky bark and digs out 

 an elongate oval cell, 

 in which it will soon 

 pupate. From the far- 

 ther end of the pupal 

 cell the larva, as a gen- 

 eral thing, extends the 

 mine almost to the sur- 

 face of the tree or log, 

 in order to facilitate its 

 emergence into the open 

 air when it has gone 

 through its changes in 

 the pupal cell to the 

 adult or beetle form. 

 This work completed, it 

 retires to the pupal cell 

 and awaits the change 

 to the pupal form. 

 Finally the outer skin 

 comes off and the insect 

 lias an entirely different 

 form and appearance 

 (fig- 20, d). It is now 

 a pupa. The length of 

 time passed in this form 

 is variable with the 

 species and with the local conditions, the pupa resting perfectly 

 quiescent in its cell during this period. At length another change 

 takes place and the insect is in the adult or beetle stage (fig. 20, 5). 

 At first the beetle retains the white color of the pupa and ^arva, and 

 the outer tissue of the body is quite soft. But gradually the color 

 turns darker and the outer tissue becomes hard and chitinous. When 



FIG. 19. Work of the western larch bark-borer (Te- 

 tropium velutinum). Sections of bark of western 

 larch : a, Cluster of eggs deposited under overlap- 

 ping scale of outer bark, the overlapping scale, in 

 this instance, having been removed ; 6, inner surface 

 of bark with newly started mines ; c, small larva, a 

 few days old. Slightly enlarged. (Original.) 



