Huxley[218] states that crayfish avoid direct sunlight, hiding under stones during the day, and becoming active in the evening. On the other hand, they are attracted like moths to fires lighted on the bank at night, and may be scooped out by hand. Abbott,[219] giving an account of the burrowing crayfish, Cambarus diogenes, states that it is very difficult to observe the animals at work, since all their digging is done at night. It would seem from the account of Miss Hoppin, quoted by Garman,[220] that the blind crayfish, Cambarus pellucidus, is not altogether insensitive to light, for, reporting on the fauna of the caves of Missouri, she says that the crayfish are all found near the entrance to the cave, where there is considerable light. In the dark recesses there are only little white fishes. Blind fish and crayfish are also taken from the wells in the neighborhood, where the crayfish are found only in wells that are rather shallow and light; the fish, on the other hand, are only obtained from deep, dark wells.
According to the above accounts it would appear that the crayfish is negatively phototactic to direct sunlight or diffuse daylight, but positively phototactic to a light at night, and moreover, that light may influence the behavior of the animal even when the eyes have ceased to function.
The directive influence of light upon the movements of the crayfish has never been experimentally studied to my knowledge. Dearborn[221] thinks that light has no effect upon the animals. Yerkes[222] and Towle[223] have shown that Daphnia move toward the light. Bethe[224] finds that Carcinus is negatively phototactic, and also shows a tendency to hunt out corners. When the eyes are varnished with lampblack, the phototaxis disappears, but the tendency to seek out corners still remains. Bethe says that he has observed the same phenomenon in the crayfish. Keeble and Gamble[225] discovered that Hippolyte varians responds positively to light under all conditions, and Palæmon is just as markedly negative. Macromysis, however, reacted now positively now negatively, depending on the background. A black (absorbing) background called forth a positive response, while a white (scattering) background produced a negative reaction. Spaulding,[226] in studying the habits of the Hermit Crab (Eupagurus), found that it is strikingly positively phototactic. When animals are placed in an aquarium, one half of which is shaded, none of them are ever noticed inside of the dark line. Herrick[227] notes that lobsters are nocturnal, and avoid the light when placed in a tank, and Bateson[228] says that prawns and shrimps lie hidden during the day, and are active only at night. Parker,[229] in a study of Copepods, finds that the females have a strong positive phototaxis for light of a low intensity, while males show a weak negative phototaxis. To light of over 100-candle power at a distance of 10 cm. or to direct sunlight the female Copepods are negative, while the reaction of the males does not seem to be altered.
In his work on Carcinus, Bethe obtained retraction of the eye-stalks by suddenly throwing a strong light on the eye by means of a mirror. "Usually the eyes were quickly drawn in and protruded again, sometimes several times in rapid succession, like a man blinking under a sudden, strong light." When a dark object, the size of the hand, was moved just over the water, the eyes were seldom retracted, but the antennules were usually drawn in. Lemoine[230] observed that in Astacus retraction was due to touch alone, and that no light, however strong, was able to bring about such a reaction. Gulland[231] takes just the opposite view with reference to Astacus, stating that there are no setæ of any sort on the eye-stalk, and therefore it is insensitive to touch, but is withdrawn only because the animal sees the object by which the stimulus is given. If a curved needle is used, and the stimulus is applied from behind, no retraction follows. Dearborn,[232] however, working with Cambarus, agrees with Lemoine in saying, "Withdrawal of the ophthalmites into their sockets occurs only on contact with some hard object,—not from any light-stimulus of an ordinary sort." I may say in passing that in none of the following experiments on Cambarus was there ever a sign of retraction due to stimulation by light, the retraction always taking place in response to a touch-stimulus.
Lyon,[233] in his study of compensatory movements of the eye-stalks, found that when the eyes were painted with lampblack, the crayfish showed a reduction of about 10% in the compensatory movements when rotated in vertical planes, but the compensation remained the same for rotation about the dorsi-ventral axis. On rotation in the dark the compensatory movement of the eyes was found to be from 5° to 8° less than in the light.
EXPERIMENTAL
In the investigations to be described, 58 crayfish of the species Cambarus affinis were made use of, and for identification the animals were marked on the back with white enamel paint, the males receiving the even numbers from 2 to 64, the females the odd numbers from 1 to 51.
1. Reactions to White Light
The questions proposed for investigation were, (a) How does the crayfish react to diffuse daylight; (b) to reflected sunlight; (c) to direct sunlight; (d) to artificial light of different intensities? (e) What is the influence of previous conditions of exposure to light upon the reactions of the animal? (f) Do changes of temperature affect the reactions?