150 



broken branch, a fire scar near the soil, etc. — and spreads from such 

 source. The time of year, and the method by which a tree is killed 

 will often have an important influence upon the kind of invasion by 

 animals. A tree which is killed and remains standing is not so liable 

 to rapid decay as one which lies upon the ground and becomes moist. 

 It is readily seen that there are a vast number of causes which oper- 

 ate to produce all degrees of decaying wood. A fallen hardwood 

 trunk and its stump are liable to begin decaying at the sap-wood 

 layer, just under the bark. The bark loosens; and moisture, fungi, 

 and animals mutually hasten each others' activities, and the processes 

 of disintegration. Under such bark, in the Bates woods, were found 

 the following: cjueens of the carpenter-ants (Cainponofus) estab- 

 lishing their colonies; the flat-bodied larvae of Pyrochroa; the large 

 Carolina slug {Philomyciis) ; the beetle Passalus cormitus; white 

 ants {Vermes flavipes) ; the rotten-log caterpillar {Scolecocampa 

 liburna) ; the snails Zonitoides arborea and Pyramidnla perspectiva; 

 Polydesmiis, Galerita jamis, and a Melanotus larva. These are fairly 

 representative kinds of animals of the log community at this stage 

 of development. It will be noted that the ants, the white ants, and 

 Galerita are predaceous, but that the remainder are probably sus- 

 tained largely by rotten wood, herbaceous plants, and fungi. With 

 the progressive radiate (when beginning within) or convergent (when 

 beginning without) growth of decay this animal community migrates 

 into the log or stump as its favorable habitat increases in area and 

 thickness. When this process has made considerable advance and 

 the log has become soft, the animals which began at the surface or 

 within are able to penetrate the entire log. This may be considered 

 an intermediate stage in the transformation of the log to humus. 

 This biotic community, composed of fungi and animals, commonly 

 begins its work at the surface (most frequently, in the case of fallen 

 trees, on the under side where the log touches the ground) and moves 

 progressively inward, transforming the log as it goes. In its wake 

 there follows a later stage of the transformation — the dark-colored 

 humus layer, composed of decayed wood, the dead bodies of animals, 

 and their excrement. The large number of years involved in such 

 a transformation makes it possible for many kinds of animals to find 

 this sort of habitat, — just as old artificial ponds are more fully stocked 

 with animals than newlv excavated ones. Slowlv developing ani- 

 mals are thus able to live here, the conditions prevailing being at the 

 other extreme from those suited to a life in the ephemeral fungi. 



As a fallen or standing trunk dries out, particularly upon the up- 

 per surface, if fallen, the bark often curls, cracks, and loosens from 

 the wood. In such a situation in the Cottonwood forest at Urbana, 



