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. III. REACTIONS TO LIGHT. 1-General features of the phototactic response. 2-The negative reaction. 3—Head and swaying movements in negative 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 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. 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 the various objects that come into its field of vision, it is certainly something more than a mere reflex machine. In organisms which are capable of a higher type of response we should at least be on our guard in attempting to explain their tropisms as due entirely to direct reflexes involuntarily performed in response to outer stimuli. The conduct of higher animals is guided in large measure by their likes and dislikes, however we may interpret this kind of behavior in physiological terms. Between such behavior and those tropisms which are the result of comparatively simple reflexes there are, no doubt, numerous intermediate kinds of conduct. It is not unreasonable to suppose that tropisms which in low forms are brought about by direct reflexes may in higher animals complicate into reactions of the pleasure-pain type while still preserving outwardly the appearance of a more mechanical mode of response. At the same time an element of direct reflex action may be retained, although closely associated with and capable of being modified by more complicated neural processes. A consideration of the experiments described in this paper will lead us, I think, to some such view. II. GENERAL HABITS. Ranatras are generally found in ponds or streams among masses of vegetation where they lie quiet the greater part of the time. Although capable under certain conditions of manifesting considerable activity, these insects are usually sluggish in their movements. Their choice of habitat is probably determined, in great part at least, by their positive thigmotaxis, since they tend to insinuate themselves between objects which afford considerable contact stimuli. Their habit of coming together to form groups is a manifestation of the same tendency. When several individuals are placed in an aquarium they mass together when at rest to form a cluster in which they are often so closely aggregated and so tangled together that those which are near the center of the group experience much difficulty in disengaging themselves. In this way they may lie for hours in an almost motionless state. The general form and dull coloration of Ranatra tend to make it inconspicuous in its natural habitat, especially as it does not reveal its presence by its movements. When lying in the water the long breathing tube through which air is admitted to the body rests at the surface. The two parts of which it is composed occasionally approach and recede from each other, moving the air between them to and fro, an operation which doubtless assists in respiration. Air is prevented from escaping when the valves are separated, by the rows of hairs which line the margins of the concave inner faces of these structures. Ranatra is carnivorous in habit, seizing its prey with its anterior raptorial limbs and holding it until it has sucked out its juices. It is quite destructive of fish eggs and frequently attacks and sucks the blood from young fishes. It is also reported to prey upon young tadpoles. DE LA TORRE BUENO1 describes the method Ranatra employs in capturing prey as follows: "When a fly attracts its attention Ranatra very slowly, almost imperceptibly, moves its fore legs, with the knife-like tarsus away from the tibia, towards its prey. When the tibiae are almost, or quite, touching the victim the movement is so sudden and quick that one is aware of it only by seeing the prey seized. Sometimes its hold is not satisfactory, and then it will let go with one tarsus, get a firmer grip with that, and then do the same with the other. Once it has the fly securely held, Ranatra slowly approaches it to its extended beak, with which it seems to touch and feel until it finds a suitable spot, and proceeds to a leisurely meal." I have usually fed Ranatras during the winter on Notonectas, or back-swimmers, as these insects were easily obtained during this time of year. The Ranatras did not pursue the back-swimmers, but as soon as their attention was attracted to the prey they lay quietly in readiness for them with their anterior limbs prepared to quickly seize the small insects should they swim sufficiently near. If a Notonecta strikes against a Ranatra the latter makes a quick 'Notes on the Stridulation and Habits of Ranatra fusca Pal. B., Canadian Entomologist, Vol. 35, p. 235, 1903. |