131 



Barlow. In this instance the building of a dam resulted in the sub- 

 merging of the ground about an oak tree during several months of 

 every summer, ultimately resulting in the death of the tree. This went 

 on for several years, until the dam was washed away in a freshet, when 

 digging beneath the tree led to the discovery of the cicada larvae in 

 apparently healthy condition from 12 to 18 inches below the natural 

 surface of the ground. In both of these instances the ground may 

 have been nearly impervious, so that the water did not reach the insects 

 nor entirely kill all of the root growth in the submerged soil." 



The roots of plants, and particularly those of trees, penetrate rather 

 deeply into the soil, but finally die, leaving a large amount of organic 

 substance in the soil. As the large roots decay, animals are able 

 through the tunnels made to penetrate rather deeply and to find organic 

 food, in the shape of wood and fungi. Motter ('98, p. 225) performed 

 an interesting experiment which shows that wood buried three feet 

 below the surface and dug up after two or three months contained 

 spiders, mites, Thysanura, psocids, a beetle, and flies. Although this 

 wood was buried in a cemetery, it is not unlikely that woodland soils 

 commonly have such a fauna. Davenport ('03, pp. 22-23) nas tabu- 

 lated the habitats of many Collcmbola and shows that many species live 

 in damp soil, in sand, under bark, under stones, in caves, etc. — condi- 

 tions corresponding to the soil habitat. These insects are very sensi- 

 tive to moisture, and some are able to resist submergence in sea water 

 from twelve to sixteen hours per day. Davenport says (page 17) : 

 "During all but about six to eight hours of the day these air-breathers 

 are below the surface of the sand, during which time they must take in 

 relatively little oxygen." During certain seasons, when the soil is sat- 

 urated, such resistance must be of great value to its possessor. I know 

 of no extensive observations or experiments on the resistance of these 

 soil animals to carbonic acid, to the lack of oxygen, or to various com- 

 binations of these conditions. 



That the soil conditions in glades and forests are different has 

 already been pointed out. We have below a good example of the 

 response of a forest animal to an artificial glade or clearing. A num- 

 ber of observations have also been made on the hastened rate of emer- 

 gence of the periodical cicada where the soil has been abnormally 

 warm, as in a hothouse (Schwarz, '90a, p. 230), or where the ground 

 has been warmed by flues (Marlatt, '07, p. 90), or where a forest has 

 been burned, and possibly the heat from the fire in combination with 

 its greater absorption of heat after the fire, has caused the cicadas to 

 emerge (Marlatt, '07, p. 94). In a forest glade, made by clearing, 

 Schwarz ('90a) found the cicadas emerging when none were found 

 in the surrounding woods. Concerning this discovery he remarks : 



