Forced Movements, Tropisms, and Animal Conduct

Suppose now we remove some or all of the left legs from an amphipod of this kind so that it will always turn toward the left, and transfer it to water in which it must be supposed to swim in a spiral path. We will then have an organism which would become oriented in essentially the same way that Euglena does. The animal would always swerve toward the left. But, when the spiral course brings it into such a posi¬ tion that the light shines directly on the left eye, the muscular tonus of the right legs would be increased and the swerving toward the light would increase. Thus orientation would be effected in just the same way that it is in Euglena,

While these hypothetical changes that must be made in the amphipod, to make it react like Euglena, are considerable, they concern only the details. The fundamental nature of the photochemioal substances, the nature of their stimulation and the character of their connection with the locomotor organs have none of them been modified. All that has been done is to make an asymmetrical organism swimming in a spiral out of a bilateral one.^ These changes are much less fundamental than

those which we would have to imagine in order to make an amphipod orient to light by the selection of random movements. In order to bring about this latter change the whole nature of the photochemical sub¬ stances and their relations to the leg muscles would have to be modified- In the one case the required changes are all of a mechanical nature and so simple that the experiment might possibly succeed. In the other case the required changes are largely chemical, and so complex that we have no data for even imagining what ought to be done in order to bring them about (Bancroft 21 ).

The asymmetry of organisms only modifies the geo¬ metrical character of the path but not the mechanism of the reaction. The writer observed that if heliotropic animals are exposed to two equidistant lights of equal intensity they move in a line perpendicular to the line connecting the two lights.^^^’ This has been confirmed by numerous observers, e.g., Bohn on Littorina, by Parker and his pupils, especially by Bradley M. Patten on the larvae of the blowfly, by Loeb and Northrop on the motions of thelarvas of Balanus, and by others. The question arises: In which fine will an animal move when the intensity of the two lights differs ? When the animal is positively heliotropic it should cease to move in a line at right angles to the line eonnectmg the two lights but should move in a line which deviates toward the stronger of the two lights; if the animal is negatively heliotropic it should deviate toward the weaker of the two lights. When the two eyes are illuminated by two lights of different intensity, the illumination in both eyes can become approximately equal only when the eye struck by the weaker light is exposed at a larger angle than the eye struck by the stronger light. Under such conditions, the animal should be com¬ pelled to move in a straight line which, however, is no longer at right angles to the line connecting the two lights, but which deviates to an extent determined by the differ¬ ence in the intensity of the two lights. The case was

worked out quantitatively by Bradley M. Patten on a negatively beliotropic animal, the full grown larva of the blowflyThe source of light was at G (Fig. 28) (one or more Nernst lamps of measured candle power), a portion of light from these lamps passed through the screens d and d' to the mirrors M and M', set at a definite angle so that the rays were reflect<!(l to the observation point 0. The two beams of light roaching O were of the same intensity. With the means of one of the lights at a or b the animal was first caused to move aoross the field 0 at right angles to the rays reflected from the mir¬ rors M and M\ The animals first started in this direction, then came suddenly under the influence of the light ro-

fleeted by M and M'. In order to make the ratio of inten¬ sities of light from M and M' different, the observation stage 0 Tvas put at nn- equal distance from M andikf'. The larvae were made to record their trails while moving nnder the influence of two lights and the devi¬ ation of this trail from the perpendicular upon the line connecting the two sources of light M and M' was measured with a goniometer (Fig. 29). The result of the measurements of 2,500 trails, showing the pro¬ gressive increase in an¬ gular deviation of the larvae (from the per¬ pendicular upon the line connecting the two lights) with increasing differences between the lights, are given in Table I. Since the devi¬ ation or angular deflect tion was toward the

Fig. 29.—Diagram to show the method of measuring trails. The lines xy ^ and are drawn through the trails at the points reached— marked by the arrows—when the side lights were turned on. The angle of deflection from this line is measured by a protractor, P. The small figures near the arrows indicate the nuinber of wig-wag movements made when the side lights were turned on; Ist and 2nd refer to the sequence in which the trails were run. (After Patten.)

Average angular deflection of the two paths of the larvas toward the weaker light Patten also investigated the question whether the same difference of percentage between two lights would give the sanae deviation, regardless of the absolute intensities of the lights used (Weber’s law). The absolute intensity was varied by using in turn from one to five glowers. The relative intensity between the two lights varied in succes¬ sion by 0, 81/3, 16 2/3, 25, 33 1/3, and 50 per cent. Yet the angular deflections were within the limits of error identical for each relative difference of intensity of the two lights, no matter whether 1, 2, 3, 4, or 5 glowers were used. Table'll gives the results.

A Table Based on the Measurements op 2,700 Trails Showing the Angular Deflections at Five Different Absolute Intensities On the writer’s theory the following explanation of tliese deviations should he given. The muscles moving tfie head of the animal to the side of the weaker illumina¬ tion, having a higher tension than their antagonists, bring ^bout a deflection of the animal toward the side of the 'Weaker light. As soon as its two photosensitive areas in tbe head—the animal has no eyes—which are not parallel, tint inclined to each other are deflected from the perpen- ciicular upon the line connecting the two lights, the photo¬ sensitive areas of the animal will no longer be struck by ■fclae lights at the same angle, but on the side of the weaker light tlie area will be struck at an angle nearer to 90° than "tbe photosensitive area exposed to the stronger light. In this way the change in angle will compensate the dif¬ ference in intensity of the two lights until the orientation of the animal is such that the compensation is complete and both photosensitive areas receive the same illumina¬ tion. The animal will then continue to move in this <3,irection.

Patten has computed the angle of the photosensitive surfaces for these animals from the angle of their orienta¬ tion under varying inequalities of illumination. This ang’le has been oomputed for the blowfly larva, using the “angu¬ lar defleetioDS ” already ascertained. The magnitude of the angle may bear no direct relation to the actual angle at which the sensitive areas are located in the body of the animal, because of the many factors which may modify the direction of the rays before they fall on the sensitive surfaces. The significant test of the hypothesis would be the constancy of the angle when computed from experimental data obtained under varying conditions.

The method of eonstmeting such an angle is shown in Fig. 30, in -which the opposing lights are assumed to be of a two-to-one ratio of intensity. The line is drawn perpendicular to the direction of the rays of light. On the line AB, construct angle BOG equal to the actual average angular deflection of the larv® at a two-to-one ratio of lights. The problem now resolves itself into the construction of an angle about OC as a bisector, which shall be of such a magnitude that equal dis-

Fia. 30.—^Diagyam for constnictinf direction of motion of larv® under influence of two lights of Afferent intensity- (After Patten.) tMLces on its oppc^site sid^ skaH have projections on the line AB o£ the ratio of two to one. Construction: Throm a point D on the line OC draw Bh perpendicular to AB, Lay oft on AB distances hx and hy^ such that hy = 2hx. From X and y erect lines perpendicular to AB, they will intersect 0(7 at / and e respectively. Bisect the line ef, and at its middle point, g, con¬ struct a line kl perpendicular to OC. From the point of intersection of hi and yy' {M), draw a line to D. From the intersection of kl and xx' {N), draw a line to JD,

Angle Meg = angle Nfg (alternate int. angles of parallel hnes, yy' and xx' being parallel hy oonstruction). Therefore triangle Meg = triangle Ngf (side and two adjacent angles being equal). Therefore triangle NgD = trisingle MgD (rt. triangles, altitude and base equal). Now by construction hx is the projection of DN on AB and hy the projection of MD on AB, and by construction hy = 2hx. The equal hnes MD and DN represent equal bilateral sensitive areas inclined to each other at such an angle, MDN, that the surface represented by MD intercepts an area of light twice as great as the surface repre¬ sented by DN, its projection on the perpendicular to the light rays being twice as great (hy = 2hx ). But the hght falling on DN is of twice the intensity of the light falling on DM, so that the total amount of light received by each of the equal areas is the same.

By this method of construction, the average angle of sensitiveness was computed for four intensity di:fferences, usmg as a basis the angular deflection of the larvae as determined by experiment. The magnitude of the angles is almost identical in all four cases.^^^ Experiments by a somewhat different method, to be discussed in the next chapter, on the positively heliotropic larvae of the barnacle show that these results of Patten are more general.

We may, therefore, say that the migration of animals to or from the hght is of the nature of a forced movement determined by the effect of hght on the photosensitive elements of the body. Unequal ihumination of symmetri- cal photosensitive elements on the two sides of the body alters the tension of symmetrical muscles, and as a con¬ sequence the animal is, when moving, compelled to chans’© its direction of motion until it is oriented in such a way to the light that symmetrical elements receive the same illumination. In this case the tension of symmetrical muscles is equal again and the animal is compelled to move in this direction.

It has been suggested by the anthropomorphic inter¬ preters of animal conduct that the motion of an animal to a source of light is the same phenomenon as when a human being who has lost his way in the dark is attracted by an illuminated human habitation. As Bohn pointed out, the definite path in which a positively heliotropic animarmoves when under the influence of two lights, shows that the anthropomorphic interpretation is as erroneous in this as in any other case. A human beiaj? would go to one of two illuminated houses and not toward a point between them, determined by the relative intensity of the two lights.®®

We Pave thus far said little about tlie identity of the heliotropism of plants and aninaals. Yet the two phe¬ nomena are essentially alike. When we keep positively heliotropic sessile plants and sessile animals near a win¬ dow, both will bend toward the source of light, though the mechanism of bending may not be the same in all details, the bending being produced in the case of the plant (and possibly in certain animals like Eudendrium) by imequal growth in length of the plant on the illuminated and shaded sides; while in the case of higher animals, e.g., Spirographis, it is produced by differences in the tension of the muscles on the illuminated and shaded sides of the animal. Motile plant organisms like Volvox, are driven to the source of light, owing to differences in the tension of the contractile organs on the shaded and illuminated side, and the same is true for animals like insects.

A further point of coincidence lies in the validity of the photochemical law of Bunsen and Eoscoe for the hehotropism of animals and plants. The law of Bunsen and Eoscoe says that within certain limits the chemical effect produced by light increases in proportion wdth the product of intensity into the duration of iUunaination, e.g., Effect—Kit, where i is intensity, t duration of illumination, and K a constant. This is true for the blackening of photographic paper by light, and it can be shown that the same law holds for heliotropic reactions of plants as well as animals.

Blaauw*®.^^ established this fact for the etiolated seed¬ lings of Avena sativa. These organisms were exposed to lights of a definite candle power for some time and then left in the dark. After a certain time the seedlings began to bend, becoming concave on that side which had pre¬ viously been illuminated. By varying the candle power of light {i) and the duration of illumination (#), he found that the value of it required to cause 50 per cent, of the seedlings to bend was always the same. Table III gives

Time required for different intensities of light to produce heliotropic curvatures in 50 per cent, of the seedlings of Avena the time required for different intensities of light varying from 0.00017 to 26,520 candle power to cause 50 per cent, of the seedlings to show heliotropic curvatures. As can be seen, the product it is always approximately 20. Ewald and the writer tested the validity of the law of Bunsen and Eoscoe for the heliotropic curvatures of Eudendrium. A number of stems of Eudendrium, from which the polyps had been cut off, were put upright into a trough with parallel walls, containing sea water. As soon as the new polyps had regenerated they were exposed to light of a certain intensity for a short time and then kept in the dark. In the dark the bending of the polyps in the direction of the former source of light occurred. The purpose was to find the minimum time of exposure required for a given light (40 candle power) to induce 50 per cent, of the polyps to bend to the light (Table IV).

Percentage of polyps bending toward the former source of light If we calculate from this the value of the product it for different intensities of light we find that it obeys the Bunsen-Eoscoe law (Table Y). Time required to call forth heliotropic curvature in 50 per cent, of the polyps The material varies considerably so that it is not always possible to induce 50 per cent, of the polyps to undergo heliotropic curvature. For this reason Loeb and Wasteneys repeated these experiments by a some¬ what different method.

We confined our experiments to three intensities of light by putting the specimens at distances of 25, 37.5, and 50 cm. from a Mazda incandescent lamp, of about 33 Hefner candles. The times of exposure were adjusted so that on the assumption of the applicability of the Bimsen- Eoscoe law the same effect, i.e., the same percentage of polyps bending towards the hght should be produced. Thus in some experiments the exposure for the three dis¬ tances given was 10, 22.5, and 40 minutes respectively, in others, 7,15.75, and 28 minutes, and so on. The ratios of the percentage of polyps bending toward the fight for the three distances should he as 1:1:1. Since the material differed widely in different experiments and in different dishes, it was necessary to compute the averages of a large munber of experiments.

in a row in rectangular glass dishes, the steins being in¬ serted in holes made in a layer of paraffin mixed with lamp Hack as in the previous experiments. The rear side of the dish was also coated with the paraffin lamp black mixture in order to prevent reflection of light from the slightly uneven back surface of the dish. Table VI gives a summary of the results. The first three columns give the times of exposure for the three 1 Bracketed values being extreme variates are excluded from calculations of the means and probable errors.

distances of the source of light, selected, as stated, on- assumption that the Bunsen-Eoscoe law holds. On assumption the ratio of percentage bent in any two or three dishes on any one day should equal 1.0. These ratio a for each pair of distances of the source of light aro given in the three other columns of the table. The centage bending was only compared in dishes containing’ material regenerated and exposed on any one day, only in this case was there any likelihood that the mato I'i ill was in any way uniform, and since only in this caso tluo experiments were Carried on at the same temperature sm ^ I the same conditions of regeneration.

The result was that the observed ratios wero iiM 1.02:0.99; 1.02 (with a probable error of =*= 0.01) while t.l» o values calculated on the assumption of the validity of the Bunsen-Eoscoe law were as 1:1:1; i.e., the resnlts showed as great an approximation between observed aanrl calculated values as one could expect. There is a second method for testing the validity of the Bunsen-Eoscoe law, based on the use of two sourec*>s of light of equal intensity.

If it is true that the heliotropic efficiency of li^lrt 1 h determined by the product of intensity, i, into duration; o f illumination, t, we can alter this product by varying- t- stu well as by varying i Eadl had shown that the position of the eye of” tluG fresh water crustacean, Daphnia, is determined by tlic? position of a source of light,and Ewald^^® fontnfl that by exposing the eye to two different sources of llglit simultaneously the eye is put into a position determ inr*fl by the relative intensity of the two lights. When one 1 i gli t remained constant and the intensity of the other liglit wgs lowered the position of the e^e changed. He now

could show that when the duration of illumination of one eye was altered by a rotating opaque disk with one sector cut out, the heliotropic effect on the eye of Daphnia was the same as when the intensity i of the same light was reduced to an amount corresponding to the Bunsen- Roscoe law. Under the influence of two constant lights of equal intensity heliotropic animals move in a direction at right angles to the line connecting the two lights. If the law of Bunsen and Eoscoe holds the effect of a constant light should be diminished if a rapidly rotating opaque disk with one sector cut out be put in front of the light, and the diminution should be equal to the fraction of the arc of the sector. Thus a sector of 90°, which reduces, the total duration of illumination to one-fourth, should also reduce the heliotropic effect of the light to one-fourth, and the ani¬ mal should deviate from the old direction in the direction toward the light without a disk before it. If, however, we lower the intensity of the latter light to one-fourth by doubling its distance we also reduce its heliotropic effect to one-fourth, and now the animal should move again in a line at right angles to the line connecting the two lights.

The following experiments carried out by Loeb and Northrop on the larvae of the barnacle are perhaps the best proof for the validity of the Bunsen-Eoscoe law for animal heliotropism. These animals are small and can be obtained in large numbers. They were made to oollect in the comer of a dish with a little sea water and were then sucked up into a pipette ef, Fig. 31, which was blackened with the exception of the opening. When such a pipette is put into a glass dish with parallel walls whose bottom is black (by putting paraffin black¬ ened with lampblack at the bottom of the dish) the larvse will flow out in a fine stream and swim when they are positively heliotropic in a straight line toward the source of light. They thus form a rather narrow white frail on the dark bottom and it is possible to measure the angle of this

trail with the line connecting the two lights. In this way in each observa¬ tion the average trail of thousands of individuals is measured. By using one constant and one intermittent source of light and comparing the results with those obtained by two constant lights we can test the validity of the Bunsen-Roseoe law. The method of the experiments was as follows: ahed (Fig. 31) is a square dish of optical glass with blackened bottom and containing a

Fig. 31. —Method for the proof of the validity of Bumen-Roacoe law for the positively heliotropio larvaa of the baraacle. (After Loeb and Northrop.) layer of sea water. A and B are two lights, the intensity of which is ' determined by a Lummer-Brodhun contrast photometer. In front of each light is a screen with a round hole permitting a beam of light to go to the dish. The lights and the dish ahcd axe so adjusted that the I two beams of light striking the sides ah and he at right angles cross t