16 THE BIOLOGY OF THE FROG chap. 



most of them are confined to the Western States. About the 

 only species which occurs east of the Mississippi, with the 

 exception of Bufo quercicus, which ranges from North Caro- 

 lina to Florida, is B. lentiginosus, which is widely distributed 

 and quite abundant. 



The Hylidae, or tree frogs, form an extensive and widely 

 distributed family. The tips of the toes are furnished with 

 small adhesive disks which enable the animal to climb up 

 the trunks of trees. Many species are able to climb up a 

 vertical surface of smooth glass. This is rendered possible 

 not so much through the suction of the disks as by a sticky 

 secretion which is produced by the glands of the skin at 

 these points. 



Male tree frogs are usually able to make a noise which is 

 astonishingly loud for creatures of so small a size. In Hyla 

 and its allies the vocal sac of the male is capable of great 

 distension, and when fully inflated, becomes much larger 

 than the head. The voice is heard most often in the breed- 

 ing season, but it may also be heard during most of the 

 summer, especially about dusk. The note of the tree frog 

 is often regarded as indicative of approaching rain. It 

 is heard frequently immediately before a shower. The 

 integument of the creature is easily affected by changes 

 in moisture. Situations where the air is damp are always 

 preferred, and it is not unnatural that their song should 

 be heard when the atmosphere approaches the point of 

 saturation. 



A great many species of tree frogs have a remarkable 

 power of changing their color under different external con- 

 ditions. When among green leaves their color is usually 

 green, but when on the bark of trees or on the ground their 

 color may change to a brown or gray. 



The North American species of this family north of 



