258 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



areas of marsh land where frogs abound. Frogs are used more 

 than any other vertebrate in the laboratories of schools and 

 colleges, not only because they are easily obtained but because 

 in their structure and habits one may find illustrations of so 

 many of the fundamental facts of zoology. The tree-frogs or 

 tree-toads, family Hylidce, are more closely related to the 

 toads than to the frogs. Their toes are usually provided with 

 adhesive disks which enable them to cling to the trunk of a 

 tree or other perpendicular surfaces. Their vocal sacs are 



FIG. 118. A western garden toad, Bufo halophilus. (Reduced.) 



large, and they make a noise out of all proportion to their 

 size. Hyla versicolor, the most common of these tree-frogs, 

 is green, gray or brown above, and has the power of slowly 

 changing from one color to another so as to produce a re- 

 markable harmony between the frog and its surroundings, 

 thus making it almost invisible to its enemies. This change 

 in color is brought about by the expansion or contraction of 

 certain pigment cells in the skin. 



The structure and habits of the toads, family Bufonidce, 

 have already been discussed (Chapter II). There are 

 about fifteen species in the United States and less than one 

 hundred species in other lands. Some of the exotic species 



