82 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



The number of general studies of , burrowing coactions is of course 

 legion, and one of the major tasks of bio-ecology is to convert these 

 into quantitative terms, as is suggested by some of the more recent 

 researches in this field (Vorhies and Taylor, 1922; Grinnell, 1923, 

 1933; Formosov, 1928; Greene and Reynard, 1932; Greene and 

 Murphy, 1932). 



Among the birds, reptiles, and amphibians there are few true bur- 

 rowers, the so-called burrowing owl being often a tenant rather than 

 a builder, and the list comprises chiefly kingfishers, certain swallows, 

 the gopher tortoises, and a few true toads. The burrowing reaction 

 is much more common among insects and spiders, often for shelter or 

 hunting, but, among the former, especially for the deposition of eggs. 

 Notable examples are the gryllids, particularly the mole crickets, 

 locustids or acridids among the Orthoptera, tiger beetles, carabids, 

 and dung beetles in Coleoptera, many termites, and digger wasps and 

 ants among Hymenoptera. The burrowing spiders are chiefly lycosids, 

 mygalids, and agelenids, containing the true and false tarantulas, and 

 the trapdoor forms, while of the Crustacea the only considerable bur- 

 rower on land seems to be crayfish. In nearly all the above examples, 

 while burrowing plays an important part in life history and coaction, 

 significance as a reaction upon the habitat is as yet unmeasured, some 

 termites constituting perhaps the most conspicuous exception. 



Probably the disturbance reaction of greatest total significance is 

 that of ants and of earthworms in view of their great number, dense 

 aggregation, and widespread occurrence. The large earth-moving 

 worms are restricted to moist areas, but ants are important through- 

 out the globe. Perhaps no other small animal exerts such a variety 

 of influence as these small insects, though unfortunately there have 

 been few or no quantitative studies of the effect of this group. Only 

 the reactions of the mound builders are strikingly evident. Soil is 

 usually brought from a depth of six to eight feet (Fig. 17). Their 

 hills and cleared circles are a conspicuous feature of grassland and 

 desert, where they are sometimes an aftermath of disturbance by 

 cattle or rodents. 



Part of the earthworm reactions are direct; others accompany or 

 follow food coactions. As to the earthworm burrows, Darwin states 

 that these are built in two ways, either by using the pharynx as a 

 wedge to push the dirt away on all sides, or by actually swallowing 

 the soil and ejecting it as castings. The one is accomplished in a 

 short time in loose soils; the other may require a day or more in 

 compact ones. At first the castings are deposited directly on the sur- 

 face, but as the pit deepens the excreted soil is transported and fre- 



