July i8, 1907] 



NA TURE 



275 



the fact that light from every one of the 10,000 areas 

 (each 1 cm. ^^.) which make up the large area falls 

 upon each point of the surface of the animal; the 

 intensity of the light from any single square centi- 

 metre of the area is only 0.0025 candle metre, but 

 since there are 10,000 such radiating squares the 

 total intensity is 25 candle metres " (pp. 347-8). 



In an animal having eyes that form a good image 

 the case is different. The small light, only : cm. 

 square, would form on the retina an ima.ge having a 

 very small area (.v), but the light would have con- 

 siderable intensity (y). On the retina of the other 

 eye there would be an image covering a larger area 

 (10,000 .-v), but each area (.v) would receive a light 

 intensity of only 1/10,000 r. In all probability the 

 difference between a very weak light and no light 

 at all falling on a visual element would be more 

 stimulating than the same or even a creater differ- 

 ence in the amount of the light at higher intensities. 

 If so, we should expect an animal to react more 

 strongly " to that stimulus which fell upon the larger 

 number of visual elements — that an animal normally 

 positive, for example, would be more strona'ly positive 

 to the large light than to the small, and similarly 

 that a negative animal would tend more often to 

 move away from the larger than from the luminous 

 area " (p. 349). 



A large number of experiments was made on 

 several animals, and the results, when discussed by 

 methods now in use in biometrical work, on the 

 whole bear out the line of reasoning just given. 

 With the earthworm (AUolobophora foetida) the 

 results showed that the intensity of the light is the 

 controlling factor in its movements to right or left. 

 This animal was negatively phototropic. It has no 

 eyes, but it has cells in the skin sensitive to light. 

 No image could possibly be formed. The largest of 

 the land planarians (Bipalium kewensc) has small 

 direction eyes. Numerous experiments showed that 

 animal has, to a slight extent, the power of appreci- 

 ating differences of area, as it responded by turning 

 away from the larger luminous area more oflen than 

 from the smaller. It was negatively phototropic. 

 The larva of the mealworm (Tenebrio molitor) has 

 two or three ocelli on each side of the head, but 

 nothing of the nature of lenses. It is negatively 

 phototropic. When two lights, of different areas, 

 acted simultaneously on both eyes, the responses 

 right and left were equal in number, showing that 

 " the ability of the eyes to form distinctive images of 

 objects differing considerably in size is wholly lack- 

 ing " (p. 371). The sow bug (Oiiiscus asellus), an 

 active isopod, has small eyes consisting of a group 

 of about thirty ocelli on the side of the head at the 

 base of the antero-lateral lobe. It is negatively 

 phototropic. The responses to light were of a less 

 definite character than was observed in the larvse of 

 the mealworm. It has only unilateral illumination, 

 and yet its eves have greater efficiency for the form- 

 ation of images than the larvce of the meal bue. 

 The cockroach {I'criplaneta americana) has well- 

 developed compound eves, and it is very active and 

 keenlv sensitive to differences of light and shade. It 

 was difficult to handle, and having more of what 

 may be called a restless intelligence than the 

 other animals alreadv mentioned, the results do not 

 seem quite so trustworthy. It reacts negatively to 

 direct light in an excess of about 50 per cent, of its 

 responses, but although it has relatively large eyes, 

 Mr. Cole does not think the evidence bears out 

 what one would have expected, namely, that the eyes 

 were capable of forminp' better images than those of 

 the animals already mentioned. The mourning- 

 cloak butterflv [Vanessa aritiopa) creeps and flies 



NO. 1968, VOL. 76] 



towards a source of light. It is positively photo- 

 tropic for lights varying in intensity from 2 candle- 

 power at 2 metres distance (0.5 candle metre) to 250 

 candle-power at 2 metres distance (62.5 candle metres). 

 It can discriminate between lights of different area 

 falling with equal intensity on the animal. Other 

 animals, such as the water-scorpion (Ranatra fusca), 

 the Pomace fly (Drosophila ampctophila), the European 

 garden snail (Helix pomatia), the European garden 

 slug (Limax niaxitnus), were also examined. 

 In the case of the garden snail, the inference from 

 the experiments was " that the eyes of the snail do 

 not aid greatly, if at all, in the discrimination of 

 two lights differing in area as the two used " (p. 



39i)- 



The results with the cricket frog [Acris gryllus) 

 are very instructive. It is, on the whole, positively 

 phototropic. With luminous areas of different sizes 

 but equal intensity, it turns in by far the greater 

 number of trials towards the larger of the two areas. 

 The result was the same when the skin was protected 

 and the eyes alone were left uncovered. _ After 

 section of the optic nerves, but having the skin ex- 

 posed to the light, the animal is indifferent to the 

 size of the luminous field. Still, even with the optic 

 nerves severed, the frog is positively phototropic. 

 Here light must be perceived by the skin, a result 

 in keeping with the well-known experiments of Lord 

 Lister made many vears ago on the pigment cells 

 in the skin of Rana'temporaria. Similar results were 

 found with the green frog {Rana clamata). 



Mr. Cole concludes his paper with an interesting 

 general discussion, showing that there is a correla- 

 tion between the habits of the animals and the con- 

 ditions under which they live. For example : " those 

 are creeping forms whose movements towards the light 

 take them in the direction of their food or else that 

 other conditions prevent their phototropism from tak- 

 ing them into unfavourable surroundings " (p. 407). 

 The following is very interesting :— " A query which 

 Romanes found among Darwin's manuscript notes 

 shows careful observation and puts the question very 

 clearlv. It is as follows: 'Query. Why do moths 

 and certain gnats fly into candles, and why are they 

 not all on their way to the moon^at least when the 

 moon is on the horizon? I formerly observed that 

 thev fly very much less at candles on a moon-light 

 night. Let a cloud pass over and they are again 

 attracted to the candle.' Romanes thinks the answer 

 is that ' the moon is a familiar obiect, the insects re- 

 gard it as a matter of course, and so have no desire 

 to examine it.'" Parker and Cole give a more 

 reasonable explanation. The moths and gnats react 

 to larger areas of light than to a point of more 

 intense light. Thev therefore remain near the 

 ground, on account of the bright patches of moon- 

 light, instead of flying towards the moon ; but if they 

 come near a candle, the great intensity of the light 

 at a short distance " overcomes the reactions of the 

 moonlit areas," and the insects fly into the flame. 

 Obscure the moonlight bv a cloud so as to take away 

 the patches of moonlit earth, and the insects fly more 

 readilv into the flame. 



Mr.' Cole gives at the close of his admirable and 

 sus^gestive paoer the following classification : — 



Type A. Response of eyeless /oj-mx.— Usually 

 negative; sometimes positive, and then usually to 

 verv weak light. Response to intensity only (earth- 

 worm). ^, 



Type B. Response of forms with " direction 

 eye£_LTsualIy negative (Bipalium, Periplaneta, 

 Tenebrio larvae); sometimes positive (larva of wood- 

 borer). Response wholly to intensitv. 



Type C. Response to size of luminous field.— 



