TABLE V.

THE INFLUENCE OF SOUNDS ON FROG WITH EIGHTH NERVES CUT.

Bull Frog Operated May 17, 1904.

Amount of reaction Amount of reaction to auDate of to tactual stimulus ditory and tactual stimuli

Cutting of the eighth nerves renders the frog irresponsive to sounds which markedly influence the tactual reactions of the normal animal. We may therefore conclude that the reactions with which we have dealt in this investigation are due to stimulation of certain sense organs of the ear, and that the use of the word hearing in connection with them is appropriate.

I.

VII. SUMMARY AND CONCLUSIONS.

Observation of frogs in their natural habitat shows that they are stimulated by sounds. The sense of hearing apparently serves rather as a warning sense which modifies reactions to other simultaneous or succeeding stimuli than as a control for definite auditory motor reactions.

2. Experimental tests prove that sounds modify the frog's reactions to visual and tactual stimuli. When the sound accompanies the visual or tactual stimulus it serves to reinforce the visual or tactual reaction, but when given alone it never causes a motor reaction.

3. The sound of an electric bell occurring simultaneously with a tactual stimulus markedly increases (reinforces) the leg reflex of green-, leopard- and bull-frogs. If the sound precedes the touch by 1" it is without effect on the reaction; if the interval is not longer than .35" it usually causes reinforcement, whereas for an interval of from .4" to .9" there is partial inhibition of reaction. According to its temporal relation to

another stimulus, an auditory stimulus may either reinforce or inhibit the reaction appropriate to that stimulus. What may be called reinforcement-inhibition curves for auditory stimuli are presented in this paper.

4. The green frog responds to sounds made in the air whether the tympana be in the air or in water. There is some evidence that the influence of auditory stimuli is most marked when the drum is half submerged in water. The influence of sounds upon tactual reactions is evident when the frog is submerged in water to a depth of 4 cm.

5. Sounds varying in pitch from those of 50 to 10,000 vibrations per second effect the frog. The most striking results were obtained by the use of an electric bell with a metal gong. With this sound in connection with a weak tactual stimulus a maximum reaction of the leg may often be obtained even when either stimulus alone causes no perceivable reaction.

6. Sounds modify the reactions of the frog after tympana and columellae are removed. Cutting of the eighth cranial nerves causes disappearance of the influence of sound. It is clear then that the reactions to sounds are really auditory reactions and that the sense of hearing in the frog is fairly well developed, although there is little evidence of such a sense in the motor reactions of the animal.

7.

Those portions of this investigation which were carried out in the spring months show marked influence of sounds for both males and females, whereas experiments made during the winter indicate a much diminished sensitiveness to auditory stimuli in both sexes, but especially in the male.

THE REACTIONS OF RANATRA TO LIGHT.

By S. J. HOLMES.

Contributions from the Zoological Laboratory of the University of Michigan, No. 100.

With Six Figures in the Text.

CONTENTS.

I.

II.

INTRODUCTION.
GENERAL HABITS.

III. REACTIONS TO LIGHT.

1-General features of the phototactic response.

2-The negative reaction.

3-Head and swaying movements in negative phototaxis.
4-The effect of contact on phototaxis.

5-The effect of temperature on phototaxis.

6-Phototaxis leading to fatal results.

7-Inhibition of phototaxis by other activities.

8-The effect of hemisecting the brain.

9-The effect of covering the anterior half of the eyes.

10-The effect of covering the posterior half of the eyes.

11-The effect of destroying or covering one eye.

12-Reactions of specimens with only a small part of the lateral sur

face of one eye exposed.

13-Phototaxis as modified by experience and habit.

14-Formation of habits of turning.

IV. GENERAL CONSIDERATIONS ON THE PHOTOTACTIC RESPONSE.

I. INTRODTCTION.

In endeavoring to ascertain the way in which animals of various kinds orient themselves to the rays of light I have experimented with quite a large number of species in the hope of finding forms in which the exact mode of response would reveal itself. Animals vary greatly as regards both the definiteness. of their reactions to light and the ease with which their movements can be followed. Among creatures of small size such as: the Copepoda, Cladocera and Ostracoda, it is almost impossible

to observe the precise movements concerned in orientation, and in many larger forms the rapidity, irregularity, or indefiniteness of their light reactions renders the same difficulty almost equally great. In studying the reactions of animals to light we are naturally confronted with the question as to how far the movements involved are the result of choice, or something analogous thereto, and how far they may be explained as the result of reflex responses to photic stimuli. If they mainly fall into the latter category we are led to inquire just what these reflexes are and how they produce the particular kind of behavior observed.

In

It is a quite commonly accepted hypothesis that the phototactic reactions of organisms are effected by the action of light directly or indirectly upon the tension of muscles concerned in locomotion. In nearly all insects and in a large proportion of other arthropods this tension, if it exists, must be brought about through the central nervous system, since the opacity of the integument prevents any appreciable direct effect of light upon the musculature. In most arthropods phototactic impulses are set up by means of light entering the eyes, and not as in many lower forms through the stimulation of the integumental nerves; this is shown by the fact that when the eyes are blackened over or destroyed responses to light no longer occur. most animals it is not possible to observe any effect of light upon muscular tension, although there is considerable indirect evidence that such an effect is produced. AS RADL' has remarked, it is difficult to explain the fact that an insect with one eye blackened over moves about in a circle except on the assumption that light affects unequally the tension of the muscles on the two sides of the body. Such circus movements are comparable to those which take place in a vertebrate animal upon the destruction of the semicircular canals in one side of the head. After this operation there is produced a marked difference in the muscular tonus of the two sides of the body and, as a consequence, the animal, instead of going in a normal manner

'Untersuchungen über den Phototropismus der Tiere, 1903.

veers continually toward the weaker side. A small difference in the muscular tension of the two sides of an insect body which would be sufficient to cause the creature to orient itself to the rays of light might not be patent to direct observation, especially if the movements are rapid or irregular, as they frequently are. There are several forms, however, in which the effect of light upon the muscular tone is quite clearly manifested, but none more so than in the common water scorpion, Ranatra fusca. In many ways this species is admirably adapted for the study of phototaxis; it is of large size, its long slender legs move in a slow and deliberate manner so that one can observe just how each action is performed; it may be readily kept for a long time in the laboratory, shows no signs of fear when being experimented with, and reacts to light with a remarkable degree of precision. For an investigation of the modus operandi of the phototactic response Ranatra is probably not equalled by any other known form.

It is especially advantageous to study phototaxis in some such organism if we wish to ascertain how far the reflex theory of orientation will carry us. If orientation is the result of comparatively direct reflexes we are better able to determine their precise mode of action. If a more involved type of reaction occurs there is a better opportunity afforded for proving its existence, and, perhaps, ascertaining something of its nature. It does not follow that because we can construct a theory to account for orientation by means of direct reflexes that the process necessarily takes place in so simple a manner. Between the stimulus and the reaction there may be processes of a complicated nature whose existence is not ordinarily betrayed by any outward and visible sign. No one would consider a dog's following the scent of a rabbit a matter of simple chemotaxis. While it is not a process requiring conscious ratiocination, it is doubtless one involving psychic operation of considerable complexity. The possibility should be borne in mind that many of the tropisms of insects may be less simple and direct reactions than is commonly supposed. If a bee finds its way to its hive over miles of woods and fields guided by its memories of