58 CHAPTERS ON THE NATURAL HISTORY 



most safe way to get rid of the pellets, besides being the least 

 troublesome, and the method by which they are the least likely 

 to roll back into the burrow. 



Why crayfish build at all, I think, is due to the fact that they 

 must have a safe place to lay and hatch their eggs, and also a 

 place to escape at times from their enemies. That it does not 

 always effect this latter will be seen from Audubon's account 

 of the towers of C. diogenes which he figured in the plate with the 

 White ibis (Vol. vi., p. 57). He says: "The crayfish often bur- 

 rows to the depth of three or four feet in dry weather, for before 

 it can be comfortable it must reach the water. This is generally 

 the case during the prolonged heats of summer, at which time 

 the White Ibis is most pushed for food. The bird, to procure the 

 crayfish, walks with remarkable care toward the mounds of mud 

 which the latter throws up while forming its hole, and breaks 

 up the upper part of the fabric, dropping the fragments into the 

 deep cavity that has been made by the animal. Then the Ibis 

 retires a single step, and patiently waits the result. The crayfish, 

 incommoded by the load of earth, instantly sets to work anew, 

 and at last reaches the entrance of its burrow ; but the moment 

 it comes in sight, the Ibis seizes it with its bill." It would ap- 

 pear from this that Audubon did not believe that the crayfish 

 towers were structures erected upon some architectural design ; 

 and his observations further go to prove that even the burrow 

 does not always protect the unfortunate crustacean from its 

 enemies. I do not place much credit in this story of Audubon's, 

 however, for to fill a burrow having a " depth of three or four 

 feet " with little bits of mud, from a mound rarely higher than 

 ten inches, would take an ibis the best part of two hours; then 

 it is not at all likely that all the pellets would go the bottom 

 and fill the chamber there existing, even were the burrows per- 

 fectly straight and vertical, which they very rarely or never are. 

 To say the least of it, Audubon, as a naturalist, was very imagi- 

 native sometimes, and I am strongly inclined to think that this 

 ibis story is simply another example of it. 



In this locality (Washington, D. C.) the burrowing crayfishes 

 remain under ground during the winter, at points safe from 

 freezing. This they do in a torpid condition, coming out early 

 in the spring, after the cold weather is well by ; and it is soon 

 after this that we begin to notice the appearance of their towers 

 in their accustomed places. They feed, as Huxley says, upon 



