Insects Injurious to the Elm. 



31 



•way between the bark and the main trunk. Their food being 

 the albumam, or soft white portions of newly formed wood, as 

 before stated, which lies between the liber or inner bark, and 

 the already hardened wood or duramen. The insect leaves 

 about an equally deep track in both bark and wood ; though, 

 in general, if a piece of bark be broken off, the larva of the 

 Scolytuswill come away with it. The specimen of wood, No. 1, 

 page 30, shows the tracks of a colony of Scolyti. The perfect 

 insect or beetle has the power of perforating the bark, say at 

 a ; it then com- 

 mences a tunnel, 

 till at B it forms 

 a deeper cavity, 

 which some de- 

 scribe as a turn- 

 ing place to en- 

 able the female 

 to effect her re- 

 treat, should she 

 survive the act 

 of depositing her 

 ova. I have, how- 

 ever, found eggs 

 deposited in such 

 cavities which ap- 

 pear to succeed 

 each at certain distances, and in which, as it appears to me, suc- 

 cessive batches of eggs are placed. When the eggs are hatched 

 the young larvas depart to the right and left of this main 

 channel, eating their way as they go. It will be seen that 

 at their commencement these lateral channels are very narrow, 

 the larvas being still small; but, growing as they advance, 

 the channel gradually widens, till at last it terminates, at its 

 greatest degree of breadth, in a blunt cul-de-sac. In this 

 extremity of the channel, the larva having attained its full 

 growth, sinks into the dormant period of its existence, in which 

 it undergoes its change to the perfect or winged state. This 

 takes place with only the protection of a slight husk, which it 

 constructs for itself, very inferior in structure to the elegantly 

 formed case of the chrysalids of butterflies and moths. The 

 larva gradually shortens and thickens, and the wings, legs, 

 antennas, and other members belonging to the perfect state 

 gradually develop themselves within this imperfect pupa case. 

 The beetle, when the full metamorphosis has taken place, 

 eats, or rather bores it way through the bark, and emerges 

 from the dark chambers in which the earlier stages of its ex- 

 istence have taken place, to the open daylight. Its daylight 



No. 2. — A piece of elm wood showing the tracks of 

 B. Cbalcographus and B. Topographus. 



