Insects injurious to Spruces and other Conifers. 187 



"cylindrical bark-beetles,'' appears to be the most destructive, 

 especially the genera Hijlurgus, Tomiaus, Scolytus, Xyloterus, Bostri- 

 ehus, Xyhborus, Crypturgus, etc., as variously described by authors. 



736. They are very small, but occur in immense numbers, and by 

 burrowing under the bark, and in the newly formed layer of sap- 

 wood, effectually girdle the trees and destroy them. There are 

 also various span-worms and larvae of caterpillars, saw-flies, bud 

 and leaf-lice, and other insects that feed upon . the leaves and 

 buds. 



737. The Balsam Fir suffers from the same long-horned beetle that 

 proves so destructive to the pines and spruces {Morwhammuscmifusar), 

 and from a saw-fly (class 6), and from the larvae of various moths 

 and butterflies. 



738. The Hemlock suffers from borers in the wood, and inch- 

 worms upon the leaves, but seldom to much extent. The same may 

 be said of the red and white cedars, the junipers, the larch, and 

 other conifers. 



739. Very many of these destructive insects have their insect 

 enemies, some of them being carnivorous beetles, others mites that 

 fasten upon them, or ichneumon flies, that lay their eggs in their 

 bodies. It has also been noticed that certain insectivorous birds, 

 and especially woodpeckers, will appear in unusual abundance 

 in spruce and pine woods infested with bark-boring insects. They 

 will even follow the logs of the mill-yards, and persist in their 

 search for this food wherever there is opportunity. By protecting 

 these birds we may therefore derive much benefit from their aid in 

 the destruction of these injurious insects. 



CHAPTER XVIII. 



PROCESSES FOE INCEEASINR THE DURABILITY OF TIMBER OR FOR 

 IMPROVING ITS QUALITY. 



740. In vegetable, as in animal organizations, life itself is gener- 

 ally suflicient to maintain the elements against decay while it lasts, 

 although in both it inevitably tends to dissolution, with age, if not 

 prematurely destroyed. In most kinds of wood the inner portion, 

 or heart-wood, is more solid and durable than the newer and softer 

 sap wood near the outside. It is generally thought that this mature 

 wood is no longer a living part of the tree, but simply the old fibers 



