142 



the habits of Balaninus (Cf. Chittenden, '08). On a previous page 

 mention is made of the habit of the May-beetles (Lachuostcrna) de- 

 foHating oaks. 



The invertebrate animals of the forest crown are largely insects, 

 and for this reason some of the treatises on forest insects, and on cer- 

 tain families of Lepidoptera, make excellent manuals for this as- 

 semblage. Thus Packard's "Forest Insects" ('90) and his mono- 

 graphs on the arboreal bombycine moths ('95; '05; '14) are very 

 essential. In his "Forest Insects" the various kinds of insects are 

 grouped according to the kind of tree and the part of the tree which 

 they inhabit, and thus one can readily find what is given concerning 

 those living upon or in the foliage, buds, fruits, twigs, etc. A some- 

 what similar arrangement is found in Felt's "Insects Affecting Park 

 and Woodland Trees" ('05, '06). The crown community varies with 

 the kind of trees composing it, as many kinds feed upon a relatively 

 small number of food plants, on allied kinds of plants, or on those of 

 members of the same plant association. The herbivorous species are 

 influenced in variety and abundance by the kind of vegetation ; their 

 predaceous and parasitic associates, however, are only indirectly in- 

 fluenced in this manner. 



5. The Tree-Trunk Community 



In an earlier section attention was called to the equable conditions 

 in tree trunks, and to the available moisture in the food of wood- 

 eating insects. The outer growing part of the tree contains the great- 

 est amount of water, insoluble starch, soluble sugar, and other food 

 materials; the heart-wood, on the other hand, is dead and contains 

 only a small amount of water (see Roth, '95, p. 29). In view of these 

 relations it is but natural that the outer parts of living trunks should 

 be subject to attack by more animals than are the drier and less nour- 

 ishing inner parts. We should expect that young animals would thrive 

 best in the layers of the outer, moister wood, not only on account of 

 the softer wood being less difficult to chew, but also on account of 

 its greater nutriment and the larger supply of water in these layers. 

 The inner parts are thus protected not only by the outer layers, but 

 also by the general inability of many animals to digest dry wood. 

 Many of the insects which live in wood, particularly in dry wood, re- 

 quire several years to attain maturity. This gradual rate of develop- 

 ment seems to be due in part to the slowness with which metabolic 

 water is produced by the growing larvae. There are many cases re- 

 corded in which developing larvae have apparently been delayed in 

 maturing for many years by living indoors and in dry wood. Weis- 

 mann ('gi, p. 48) has published an interesting case of Buprestis splen- 



