153 



from a much decayed log; unfortunately, however, he does not pub- 

 lish the list. 



Some of the animals which invade the log in its earliest stages 

 of decay continue to hold possession throughout the transformation. 

 Thus Passalus arrives early, as soon as the bark begins to loosen, 

 and remains to a late stage in the process — when the log or stump 

 can easily be kicked to pieces. The rotten log caterpillar Scolecocampa 

 has a somewhat similar history in the log. When a log reaches such 

 a condition that it looks like brown meal, and is nearly level with the 

 surface of the ground, it may during the summer become so dry that 

 it affords a favorable haunt for myrmeleonid larv^ae; probably the 

 ant-lion of Myruieleon hnmaculatus DeG., a woodland species. 



In the foregoing manner the tree trunk decays and naturally sinks 

 lower and lower, the woody fibers disappear, the debris becomes 

 darker in color, the autumn leaves, twigs, and other litter of the 

 forest gradually add layer to layer, and finally the remains of the 

 log become blended with the humus of the forest floor. Thus is com- 

 pleted one of the most important cycles of transformation to be found 

 in the forest habitat. The following diagram. Figure 17, has been 

 prepared to show the general train or succession of insects correspond- 

 ing to these changes in the conditions in trees. 



It will simplify this discussion of changes in the animal associa- 

 tions, caused either by changes in the character of the forest trees or 

 bv changes in the woodland vegetable products, to state concisely the 

 main general factors involved in these changes. To explain zoological 

 facts it is often necessary to utilize the products of the allied sciences, 

 and the student may even be forced to make some investigations for 

 himself in these fields, because these sciences may not have especially 

 treated his specific problems. All relations become of zoological sig- 

 nificance, however, when they bear upon a zoological problem. The 

 major group of causes or processes which operate in such a way as to 

 initiate changes in forests may be grouped provisionally as follows. 



1. Geological and physiographic processes: crustal movements 

 of the earth, as earthquakes; the wearing down or erosion of the 

 land, as the mowing down of forests by landslides. 



2. Climatic processes : wind storms, tornadoes, ice and sleet 

 storms, etc., which injure trees and destroy forests; lightning and 

 fires, — in brief, any climatic factor which is able to injure or kill 

 trees. 



3. The processes of competition and succession of forest vege- 

 tation ; based upon plant activities, as when an oak-hickory forest 

 is followed by a red oak-hard maple forest, or when fungi kill trees. 

 These causes are largely botanical problems. 



