ECOLOGY OF ISLE ROYALE, 71 



At otlaer times were collected on rock beaches ants, (No. 15) Gam- 

 ponotiis herculeanus L., carrying away dead caddice flies, and as acci- 

 dental visitors a Ciinhex americann- I^ach (No. 106). a butterfly 

 (No. 107), Basilarchia artheinis Dru., and a running spider (No. 103), 

 Lycosa pratensis Emer. 



In connection with the rock beaches may be mentioned the beach 

 pools {Fig. 5), which are depressions in the rock filled with water by high 

 waves. They are naturally most abundant on flat or gently sloping 

 beaches, and their permanency varies with their size and depth, affecting 

 evaporation, and with their height above the lake, affecting the frequency 

 with which they are filled. In those which are permanent are found 

 shells, Limnaea emarginata Say (No. 58), and Plan-orhis parvus Say 

 (No. 59), and a few insects, Rhantus Mnotatus Harr. and Gorixa sp. 

 (73, 74, 75). The water beetles and water boatman are strongly stereo- 

 tropic, staying on the bottom or in, crevices, and leaving it only to dart 

 quickly to the surface for air. 



YII. The Cladonia Clearing and Jack Pine Ridges. 



The elevated position of the rock ridges and their physiographic rela- 

 tion to the uplands are the two chief factors determining the succession 

 of biota upon them. In response to the rapidity of drainage, and the 

 slowness of soil formation the first plant life to invade the rock beaches 

 is a lichen association composed to a large extent of Cladonia rangiferina, 

 which carpets the rock to a thickness of 1 to 3 dm. With it are associated 

 various xerophilous shrubs and herbs, but no trees. Consequently the 

 insolation is strong, and after rains that water not removed by surface 

 drainage is soon evaporated. The soil consists only of those thin de- 

 posits formed by the disintegration of the underlying rock and the 

 decay of the vegetation, and is held in place by the tufts of lichens. 

 Such natural clearings in the forest are frequent near the lake {Figs. 

 6, 7, 9), either on gentle slopes but little above the lake and consequently 

 of late origin, or upon the devated rock ridges {Figs. 8, 25, 26), where 

 they are of much greater age. Their shape and size varies naturally 

 with the topography. 



In these Cladonia clearings has been developed a very characteristic 

 faunal .association, rich in species and in individuals, and especially dis- 

 tinct in the number and variety of insects. The fauna may be con- 

 veniently divided for discussion into three groups, aerial, terrestrial, and 

 subterranean. Since the latter is the most nearly fixed in habit, it may 

 be described first. 



1. Subterranean Fauna. In the shallow depressions and crevices of 

 the roc\i}{Figs.7,25,26),Sire thin soil deposits supporting a dense growth 

 of various plants, especially the Cladonia lichens, the bearberry, and 

 dwarf juniper. Ants are frequent, running over the surface and ex- 

 cavating below it, but they make their nests only in the deeper crevices 

 or under the densest growth of plants Avhere the depth of soil is suffi- 

 cient to allow them to make their excavations and to conserve the mois- 

 ture supply. In the crevices they are usvially 1 dm. or more below the 

 surface. Camponatus herculeanus L. (22), Myrmica rubra L. (61), and 

 Leptothoraoo canadensis Prov. (63) are the species generally represented. 

 The nests are more frequent near the margin of the rock clearings, 



