160 MICHIGAN SURVEY, 1908. 



III. The Succession of Beetle Associations. 



The subject of succession is a relatively new one in entomology. The 

 only other paper treating of beetle succession known to the writer is 

 that by Shelford ('07). Our points of view are very similar but have 

 been independently conceived. Broadly speaking succession means the 

 change, in time, of the insect life at any given habitat or place. 

 Our aim is to note the changes in the composition of the beetles 

 found associated in a given hreeding habitat or region. The method 

 is first to determine what species of beetles are associated or found 

 together in the same habitat, and then to determine their mutual and 

 environmental relations, so that their laws of change may be determined. 

 Habitats and their associated insects have very rarely been considered 

 as worthy subjects for special study. Even in very excellent local 

 lists, but little attention is given to this subject. This is well exemplified 

 by Wickham's Bayfield list. In one case he says : "A peat-bog of several 

 acres in extent also proved very productive of peculiar species." But 

 unfortunately he does not indicate the kinds. 



In spite of the lack of a detailed study of the problem of beetle suc- 

 cession, however, certain general relations are apparent. We will only 

 attempt an outline of the problem as found on Isle Eoyale. This in- 

 volves an idea of the history of the island as the Lake formerly stood 

 at a much higher level, which as it fell exposed Isle Royale. We are 

 thus given, as a natural starting point, the Lake shore and beaches. 



1. The Lake Shore and its Beetle Associations. 



Topographically the shore is very diverse in its character, and all 

 stages are to be found, from a cliff to a low rocky shore and on to the 

 gravelly and sandy beach. When the shore lies at a low angle, so that 

 beach pools are developed by the waves and rain, certain water beetles 

 as Rfuintus Mnotatus at Tonkin Bay (I, 1) and Scovill Point 

 (lY, 1) find a habitat. Upon topographically older beaches, where 

 gravel and sand have accumulated, various species of Berribidium are 

 to be found, as previously listed. Such a sandy beach often contains 

 a mixed lot of beetles, and may contain examples of a lai-ge number of 

 species from all habitats, which have been tossed up by the waves. 

 It is probable that mamy of these come from a considerable distance. 

 Upon the upper parts of such a beach, where soil accumulates and' 

 annuals grow, certain flower beetles, as Leptura and Trichias are liable 

 to be found feeding. In such a soil may be expected Carabids, as 

 Pterostichus femoraUs was found upon the heath beach (V, 2) on the 

 south shore of the island. 



The transition from the upper beach to the rock openings is often a 

 gradual one; all stages of which were found preserved. 



2. Boch Openings and Associated Beetles. 



These park like rock openings and open oak ridges furnish a transition 

 from the beach into the forest conditions. They are characterized by the 

 absence of soil or the presence of only a shallow one, and by the rein- 

 deer moss and heath plant society. The shallow soil and low open veg- 

 vegetation favors the continuation of some of the Carabids found upon 



