FROGS. 39 



(1) The larval alimentary canal is very long, lnit slightly differentiated in its vari- 

 ous portions and filled udth ooze and silt scraped up from the objects in the pond and 

 containing many species of diatoms, blue-green and green algoe of filamentous and 

 nonfilamentous forms, small pieces of green plant tissue, and bits of fibers and other 

 nondecaying material found in ooze. As yet I have found no tadpoles containing 

 animal tissue, but as is generally known, they nibble off small pieces of flesh when 

 it is available. Insects, mites, spiders, etc., are quite universally absent from the 

 larval food; even the small water Crustacea are rarely met with. 



(2) After the fore and hind legs are both in evidence and the horny plates of the 

 tadpole mouth are shed, the tail is gradually shortened and the alimentary canal 

 shortens to become dilated anteriorly and posteriorly, forming the stomach and the 

 rectum, the latter communicating with the cloaca. During this time there is prac- 

 tically no food taken in, although the fat in the body cavity is scarcely reduced in 

 amount; evidently the material available as the result of the absorption of the tail 

 is sufficient. The epidermis is evidently shed often during the absorption of the tail; 

 for in many cases, especially in the bullfrog which is large enough for it to be more 

 apparent, epidermis is present in the alimentary canal; this tendency to swallow the 

 cast epidermis seems quite universal. Occasionally a few algal filaments seem to 

 become entangled in it and are then present in the stomach contents. 



(3) After the tail is reduced to a mere remnant with a length of 1 to 2 millimeters 

 (0.4 to 0.8 of an inch) and the mouth has increased to several times its former size, 

 the alimentary canal has become from one-sixth to one-tenth of its larval length, and 

 the young frog again begins feeding, now taking mostly animal food. The range of 

 forms consumed is very great, each species of frog apparently eating anything alive 

 and moving and yet small enough to swallow. Crustacea, Isopoda, Acarina, Arachnida, 

 and almost every gi'oup of Insecta have been found. The forms fed upon are not 

 aquatic to any extent, but feeding is done largely above and near the water. Natu- 

 rally such life as is to be found on or near the ground is most available and the stomach 

 content is largely ground beetles, plant lice, springtails (Collembola), leaf hoppers, 

 snout beetles, spiders, mites, sow bugs, ants, and small flies. Of the molluscs, snails 

 are occasionally met with, and of the vertebrates only a few have been observed, such 

 as young frogs and toads in young bullfrogs. 



The differences in selection of food may come under two categories: (a) Those 

 due to the size of the species of frog — the bullfrog, for example — can and does eat much 

 larger forms than can the smaller species, and yet it also takes the smallest mites; 

 and (b) those due to the location and nature of the ponds from which the specimens 

 were taken; for in different locations are to be found different food materials. But as 

 yet I have been able to observe no difference in the range of insects or other forms 

 eaten by each species, 



By way of summary, then, the tadpole is largely herbivorous; the transforming 

 individual does but little or no feeding, and the young frog is carnivorous, eating 

 plant and inorganic materials perhaps as a matter of accident while seizing insects 

 or other prey. 



FOOD OF ADULTS. 



Growing and adult frogs are carnivorous. To be sure, they may 

 accidentally take in vegetation or mineral matter, but tliis is an 

 incident to their pm'suit of moving prey or synchronous with the 

 sw^allowing of their own cast skins. If their prey remains quiet 

 it may escape. If om* frogs pursued their prey in the water as do 

 fishes there would be no particular need for a tongue, and some of the 

 most aquatic frogs of the world have little or no tongue as a conse- 

 quence. Practically all of the frogs of the United States are pos- 

 sessed of a good, thick, adhesive protrusible tongue, wliich is fastened 

 at its forward end. The posterior end of the tongue can be shot 

 forward and then quicldy retracted with the prey affixed or held. 



It must be remembered that our frogs do not pursue their prey 

 beneath the water; therefore it becomes evident that practically 

 all food prehension for our species takes place at or above the water's 

 surface, on the shores of bodies of water, on the banks of streams, 

 in the meadows, or in some cases in the trees. Some remain in 

 favorable position and capture the passing prey; others hop toward 



