162 MICHIGAN SURVEY, 1908. 



formed toward and invaded by the balsani-spnice forest conditions and 

 association. Here is a clearly defined convergent tendency, the exposed 

 ridges and the water basins both tending to become forested. Corre- 

 sponding with these environmental changes are corresponding ones in 

 the beetles. The Carabids, as previously mentioned, continue in the 

 humus from the rock openings as the water beetles may in the damp 

 moss. As the vertical extent of the forest increases and the forest 

 crown migrates upward, the intervening trunk, bark and branch habitat 

 for beetles enlarges and the leaf eating inhabitants of the forest crown 

 rise upward. This crown fauna retains, or rather continues some of 

 the characteristics found at the marginal zone, with which it retains 

 dir-ect continuity. The marginal zone of trees is likely to be birches (yel- 

 low or white) or aspens, in the rock opening succession;, and conifers 

 in the bog series, a feature which influences the beetle fauna. With a 

 dominance in the forest of Balsams. Paper Birch and White Spruce, the 

 beetles (and many other insects as well) are likely to be much in- 

 fluenced, not only by the plant food, but also by the physical con- 

 ditions associated with the forest. As one plant or forest society 

 replaces another, the unfaroi!\al)le conditions of the declining society 

 may be expected to favor insect injury as it is well known that in 

 general vigorous trees sufifer less from such attacks than those which 

 are defective. Under such circumstances as this insect injury may 

 be a useful index of succession, as well as a factor hastening it. Under 

 such circumstances the climax of insect abundance or dominance 

 may lag behind the climax of the development of the plant society 

 upon which it depends. Insects may also initiate a plant succession, 

 through a period of extreme abundance by doing damage to the food 

 plant, thus permitting the invasion of other forms. If, however, the 

 hardwoods, the yellow birch-sugar maple forest, is the climax type, then 

 the balsam-spruce-paper birch association will be succeeded by it in 

 time; and a. further change in the beetles may be expected. But here 

 also, as in the coniferous forest, a dense forest stand appears to be 

 unfavorable for the abundance and variety of beetles (as is generally 

 the case with many other animals). This scarcity of beetle life in 

 the dense forests of the Lake Superior region has been commented 

 upon by LeConte ('50, p. 201) as follows: "The whole country being 

 still almost in a primitive condition, the specimens are equally dis- 

 tributed throughout a large space; the woods will not therefore be 

 found very productive to the collector. In fact nearly all the species 

 were adjacent to small streams; or else were driven on shore, particu- 

 larly on sand beaches, by the winds and waves after being drowned in 

 the lake." 



Throughout the preceding discussion the intimate relation of the 

 beetles and the vegetation has been assumed. There seems to be 

 a good reason for this. Ulke ('02, p. 3) has well expressed this dependence 

 as follows: "Now, as about half of all the beetles depend upon plants 

 for their food, the greater variety of food plants the larger we find 

 the number of species of beetles." In this we also see why so few 

 species (relatively) are found where a climax plant society has become 

 dominant, because such societies are, as a rule, composed of but rela- 

 tively few species. At the same time it is seen why at an intermediate 



