Popular Science Monthly/Volume 49/June 1896/Frogs and their Uses
|FROGS AND THEIR USES.|
DURING the last half century or more, and especially during the latter part of this time. Science has made use of a variety of natural objects and living animals in her laboratories to demonstrate the laws and facts of biology. The fundamental phenomena of plant and animal life have been taught by Huxley by placing before his students in the laboratory such material as yeast, and such types of vegetable and animal organization as protococcus, proteus animalculæ, bacteria, molds, stoneworts, ferns, bean plant, bell animalculæ, polyps, mussels, crayfish, and frogs. Foster and Balfour wrote an entire work upon the elements of embryology, using for the purpose only the egg and chick of the common barnyard fowl. For what is true of the hen's egg and its complete development is true in the main for the eggs and development of all forms of animal life, from the highest mammal to the lowest vertebrate known. In the selection of these types for work in the scientific laboratory, teachers have been chiefly guided in their choice by the accessibility of the form selected to the largest number of students of all countries, and by taking an animal of the widest geographical distribution. Thus Cuvier selected the perch as a type for the study of the structure of fishes; frogs' eggs, tadpoles, and frogs have been almost universally used, while birds and mammals have been largely studied through our investigation of the fowl, the pigeon, and the rabbit. Of course, hundreds upon hundreds of other plant and animal organizations have been most exhaustively worked out, but students always, or very frequently, date back to the manuals upon the biology of these old standbys. Gray's Human Anatomy is a very good example of one of these aids to the study of a single vertebrate species—a monograph, as it were. Some day, Mivart's Cat will hold a similar place as a work of reference. I have already referred above to the use of the frog as an animal whereby the biological student may gain much information; indeed, I do not believe there is a single animal anywhere that, for this or other purposes, has come into more general use. If a common frog be fully studied in all its phases, and a comparative study be made of its entire structure, together with its physiology and habits, the student has made a long stride toward the comprehension of life processes in general, and will find himself landed far within the domain of biology, and very liberally equipped to investigate almost any problem zoölogy may have to offer. In addition to this, ever since physiology came to be a science, frogs have been used by the researchers in that department whereby to demonstrate some of the grandest truths that men have brought to light. Especially is this true with respect to the study of the muscular and nervous systems. Physicists also use them extensively, and the medical expert in experimenting with, or detecting the presence of, a variety of poisons. The circulation of the blood, and the processes of inflammation as seen under the microscope, are now studied by thousands of students in the laboratories the world over, and they are nowhere better seen than in the web of a frog's foot. Some of the more obscure actions of the heart have been made clear by the study of the entire circulatory apparatus of this useful batrachian. The action of woorara in destroying the properties of the motor nerves has been demonstrated by Bernard upon frogs; while Matteucci, by the use of a frog's leg, has shown the contrasted action of the direct and the inverse current. The remarkable experiments of Marshall Hall and of Pflüger, on the reflex action of the spinal cord, where the most delicate animal organization was necessary, were made possible by the use of frogs; while another experimenter has worked out through them the physiological action of strychnine. Indeed, some of the most important facts. in physiology have been, and constantly are to-day being, demonstrated upon frogs; and the list of such conquests, and the light they shed upon this most useful kind of knowledge, are altogether too long to enter upon here. Much is to be learned by simply studying the action of a large, healthy frog in a tub of water. His mode of respiration under such circumstances is a lesson of itself, while the beautiful rhythmic action of his limbs in propelling himself through the water or diving beneath it is almost a complete treatise on the art of swimming. Its action upon land is quite as instructive, and the marvelous leaps and dives of one of these creatures can be studied for a long time with a very considerable degree of profit. Colton, in his Practical Zoölogy, especially invites the student to "notice how the frog sits when at rest," and I can heartily indorse the suggestion. Artists frequently miss it when
they come to represent a frog in the normal attitude of rest; and in my Scientific Taxidermy for Museums I was particular to devote an entire plate to this subject, showing the plaster cast of a large bullfrog taken from one of them in this position. But it is the camera that catches these attitudes the best of all for us, and last summer I paid very considerable attention to the photography of adult living frogs. Most of these results were as fine as could be desired, and one of the best of them is presented here as an example (see figure). If one will take the trouble to compare this with almost any of the pictures of frogs—upon direct lateral aspect—that illustrate the very numerous works upon natural history, he will be surprised to find how wide many of the latter are of the mark with respect to the attitude the animal really assumes in Nature.
When a frog is at rest in this sitting position he presents us with a number of external characters that are very interesting to study. It will be noticed that the little apertures forming the nostrils open and shut alternately, while at the same time the mouth is closed, and the rising and falling of the skin covering the throat show that a pumping operation is going on. This is just exactly what is taking place, and the air pouring in through the nostrils has to be swallowed in order to be conveyed to the lungs. There being no ribs, the chest can not enter into this respiratory act, so a frog can be easily suffocated by prying its mouth open for a time. The skin in these creatures also forms a very important part of the respiratory apparatus, and a frog can be killed with ease by tying him out in the hot sun, for the cutaneous surface must be kept continually moist in order to have its functions preserved. This is insured in very dry weather by its power to absorb a quantity of water which is stored away for use in an internal nonurinary reservoir, from which receptacle it is excreted over the surface of the body. When one suddenly picks up a frog during the long, dry months of summer, it often voids a quantity of this clear water in a succession of jets. The large, round eye of the bullfrog is peculiar in some respects, for, if we tickle its corneal surface with some light object, as a straw, it will be noticed that the thick upper lid, covered as it is by the common integuments, has very little movement, while on the other hand, as the animal rotates its eyeball inward and beneath this, there at the same time passes up over the organ the thinner, somewhat transparent, lower eyelid. This shield, entirely covering the ball, as it does, reminds one of the structure seen in birds, and called the nictitating membrane. As soon as the irritation is withdrawn, the animal again opens his eye, which, by the way, with its truly beautiful iris, is, in my opinion, one of the most elegant structures seen in Nature. Posterior to and below the eye we meet with a flat, oval area, also covered by the skin, which is the tympanum of the ear. One might possibly mistake this for a thin flat bone in the skin, but this latter tissue in frogs is perfectly smooth and is completely devoid of either scales or osseous plates. There is an American genus of frogs, however (Ceratophrys), a few representatives of which form an exception to this rule. If we puncture the eardrum in the frog, it will be found that a fine pig's bristle may be passed by a natural passage through the opening made into the mouth. It goes through the Eustachian tube, a canal which is also present in man and other vertebrates, permitting, as it does, the vibrations of the tympanum. The fact that the frog is devoid of any neck; that its spine makes a hump near the middle of its back; the characters of its two pairs of limbs; and other external features, are all too well known to the intelligent observer and reader to require special description here. There is another thing we must notice, however, and its presence is not generally observed nor appreciated. If we watch carefully at the distal end of the backbone, upon either side of it, it will be seen that the skin pulsates at those points with sufficient force to make it apparent to the eye. These pulsations are performed by the posterior pair of lymph hearts. Now, the lymph hearts have nothing to do with the circulation of the blood as performed by the heart, but they, on the other hand, pump the lymph contained in the large lymphatic vessels into the veins. There are two pairs of these lymph hearts—the pair just noticed and an anterior pair, which are below the margin of the shoulder blade, upon either side, and near the lateral processes of the third vertebra. They are muscular organs endowed with the power of contraction, and are extremely important ones in the internal economy of the frog.
In a brief essay, such as I am now writing, it will by no means be practicable to enter upon the extremely interesting subject of the internal structure of the frog. Even to touch upon this ever so lightly would require a small volume to print it. Not a few books are in circulation now devoted largely to the anatomy of these animals, and others no doubt will appear from time to time. A few years ago the distinguished British naturalist, Prof. St. George Mivart, devoted an entire treatise to The Common Frog, and it is truly a most instructive work. In it he describes a number of different kinds of frogs, but what gives the book its special biological significance is that he discusses the life history of these tailless batrachians, and their anatomy and physiology, with a variety of other forms that are either closely affined to them or more or less remotely connected. In summing up, Prof. Mivart shows the differences existing between a frog and a fish, a frog and a reptile, a frog and a bird, a mammal, and so on; and indeed what a frog really is, and he claims it to be "a tailless, lung-breathing, branchiate vertebrate, with four limbs typically differentiated, undergoing a complete metamorphosis, and provided with teeth along the margins of the upper jaw." This last character is one that distinguishes the frogs from the toads, while from other batrachians the frogs are at once separated by the absence of a tail.
In the United States we have at least fifty or sixty different species and subspecies of frogs and tree frogs, belonging to a number of different genera. The typical genus is the genus Rana, and to it belongs our common bullfrog (Rana Cateshiana), it at the same time being the species that furnishes the legs so much enjoyed by many epicures. When Huxley gave his biological chapter on these animals, he used the Anglo-continental types, R. temporaria and R. esculenta, the latter form being the one used for the table in Europe. One of the most interesting species that have been described is the one discovered by Mr. Wallace in Borneo a number of years ago. It has been popularly called the "flying frog," from the fact that it has toes of great length, and these are fully webbed to the tips. If the animal wishes to descend from the top of a high tree it has only to make the leap, and by spreading out its toes it converts its feet into four veritable parachutes, and thus this little aërial batrachian reaches terra firma in safety. Among the most curious types are the tree frogs; and Gibson says these "are readily distinguished from all others by having the ends of their toes dilated into knobs or disks, generally provided with a sticky secretion, by means of which they can cling to the leaves and branches of trees. They are small, elegant, and exceedingly active creatures, the males possessing loud voices, of which they make copious use during the breeding season and on the approach of rain." Frogs have from remote times been regarded as weather prophets, and at the present day, in some parts of Germany, the European tree frog (Hyla arborea) is used as a barometer. A few of them are placed in a tall bottle provided with miniature ladders, the steps of which they ascend during fine weather, seeking the bottom again on the approach of rain.
Anatomical structures of a variety of kinds are characteristic of different species of frogs, having to do with the voice organs. So it is that many croak, some chirp, and some almost bellow. Many emit noises most disagreeable to all ears, while others give vent to sounds that under some circumstances are quite enjoyable. Darwin says, "Near Rio de Janeiro I used often to sit in the evening to listen to a number of little Hylæ [tree frogs], which, perched on blades of grass close to the water, sent forth sweet, chirping notes in harmony." This, however, is not the case with another species that occurs in Surinam, also a tree frog, endowed with an extremely disagreeable voice, and, what is worse, they congregate together in great numbers, and then, when they unite in their piping, they have been known to drown the orchestra of the Paramaribo theater.
Frogs live principally upon insects, and these they capture with their peculiarly formed tongue. This organ is soft and extensible, being at the same time covered with a viscid secretion. Anteriorly it is closely attached to the floor of the mouth, while behind it is to a large degree free. The free part is thrust forward when the frog desires to capture an insect, and the latter sticking to the tongue, it is with marvelous rapidity whipped back to the frog's throat and swallowed. Frogs will also seize some of their prey in other ways, and for this purpose they use their jaws and teeth, which latter are to be found on the palate and upper jaw. I have frequently taken them with hook and line, the former being baited only with a small piece of red flannel. So far as I know at present there is no species of frog that has come to the knowledge of science that possesses a poison gland or apparatus. Some of them, however, secrete from their skin pungent and disagreeable secretions. These are protective in a way, and prevent other animals from preying upon them. In a species of Hyla I met with upon the island of Cuba this secretion was so strong as to bring water to the eyes, and upon one occasion when handling one of these creatures and then rubbing my eyes the lids of the latter swelled tremendously, and this condition was only reduced after proper treatment lasting over three or four days. There is another tree frog (Hyla micans) in which this secretion of the body is slimy and profuse, and it at the same time possesses luminous properties, which probably also serves to protect the animal from its enemies. There are many beautiful examples of protective mimicry to be seen among frogs, especially among the brown or the green tree frogs, or other arboreal forms that are mottled and shaded with greens, grays, and browns. These species usually feed at night and are still all day, being detected only with difficulty, as they rest upon leaves, limbs, or rocks. Some are brilliantly colored, but they are nonedible varieties, and so their high-colored skins serve them as a protection.
Fossil frogs first occur in the Tertiary, but they become more abundant in the Miocene period.
The region of the delta of the Yukon is described by W. H. Dall, in his paper on Alaska as it Was and Is, as remarkable for being the breeding place of myriads of waterfowl, some of which are peculiar to the Alaskan region. Nearly a hundred species gather there, and one of them comes all the way from north Australia, by the coasts of China and Japan, to lay its eggs and rear its young in this spot.