52 J. PERCY BAUMBERGER 



of various species. It is quite probable that mites inhabiting de- 

 caying and fermenting material, feed largely on the microorgan- 

 isms present. 



c. Association of wood-eating insects with fungi. The reason for 

 the association of Sciara and Tyroglyphus with fungi is probably 

 because of the indigestibility of the cellulose walls of the wood and 

 the small amount of protein contained in them. The composi- 

 tion of wood varies with the season and with age, species, loca- 

 tion, and tissue, so that it is difficult to make any general state- 

 ment. Haberlandt ('15) has recently studied the digestibility of 

 wood and concludes that unless the cellulose is destroyed or 

 changed, wood has little food value for mammals, as the nutrient 

 substances are inaccessible. Birch-wood was found to have the 

 highest nutritive value, giving the following analysis : water, 4.56; 

 nitrogen, 0.108 (protein, 0.675); ether extract, 0.45; nitrogen-free 

 extract, 61.56; crude fiber, 32.2, and ash, 0.46. In general it may 

 be said that, except in the Hving phloem and cambium, the protein 

 content of wood is extremely low. Carbohydrates are usually 

 abundant and wood is therefore used in the manufacture of sugar, 

 some processes yielding as high as 25 per cent. These com- 

 pounds are probably quite inaccessible to insects because of their 

 chemical and physical nature, but are readily dissolved and con- 

 verted by fungi due to their notable enzyme activities. 



The low nutritive value of wood causes the insect to either 

 lengthen its life-cycle so as to be able to extract a greater amount 

 of wood or it leads to association with microorganisms either as 

 food or as symbionts. In the first class belong the large majority 

 of heartwood-boring Lepidoptera, Coleoptera, and Hymenop- 

 tera, for example, the moth larvae Zeuzera pyrina and Sesia api- 

 formis with a two-year life-cycle ; the larvae of the beetle Saperda 

 populnea with a two-3^ear, Elaterid larvae with a three-year, 

 Melolontha \Talgaris with a four-year hfe-cycle, and Sirex (Hy- 

 menoptera) with a larval period of one year. The next step 

 which may lead to the habits of the ambrosia beetles and termites 

 might be the reingestion of material already passed through the 

 digestive tract as described by Escherich ('95) for the beetles of 

 the family Ipidae (Bostrychidae) which he beheves to be adopted 



