November 27, 1003.] 



SCIENCE. 



695 



feeding rests so that its moltings shall fom(! 

 to best advantage for the necessities of growth 

 is the insect that is represented by descendants 

 now-a-days. But is molting any the less a 

 reflex for that? 



Loeb, in his ' Comparative Physiology of 

 the Brain,' records certain observations on the 

 larvae of the moth Porthetria sp. with regard 

 to their regular movements up the stem of the 

 food plant, suggesting that this climbing up 

 is a positively heliotroijic reflex, resulting in 

 the (advantageous) finding of the tender new 

 leaves and buds of the food plant. I have 

 observed the traveling behavior of the larvae 

 of three species of moths, these larva? being 

 the mulberry silkworm, a geometer found on 

 lindens and an unknown species (the adult 

 was not bred) which may be called No. 3. 

 In each case only larva? just hatched from the 

 egg were used, thus eliminating any results 

 of experience or imitation. Twigs of food 

 plants were so arranged that there were for 

 each kind of larva cases in which (a) the 

 leaves were up and in the light, (6) the leaves 

 were up and in the dark, (c) the leaves were 

 down and in the light, (d) the leaves were 

 down and in the dark, (e) the leaves were 

 in horizontal plane with the twig and in 

 the light, (f) the leaves were in horizontal 

 plane with the twig and in the dark, (g) the 

 leaves and twig were in horizontal plane all 

 equally illuminated, (h) the larvse were put 

 on a part of the twig which was in the dark, 

 (i) the larvse were put on a part of the twig 

 which was in the light. These various cases 

 were easily arranged for by having a number 

 of hollow cylinders about one and a half feet 

 long, three inches in diameter, and open at 

 both ends. The twig with leaves could be put 

 into or partly into the cylinder, as desired, and 

 the cylinder put in vertical or horizontal posi- 

 tion, as wished. 



Without taking space to give in actual de- 

 tail the behavior of the tiny larvse of the three 

 different species, I may summarize this be- 

 havior as follows: The silkworms moved in- 

 differently toward light or away from it, up 

 or down, until they found food, and then made 

 an end of traveling; the linden inch-worms 

 tended strongly to travel toward darkness and 



when this direction was downward the ten- 

 dency was greatly strengthened; the unknown 

 No. 3 larvse tended obviously to travel toward 

 the light, and when this direction was upward 

 the tendency was strongly increased. That 

 is, in these three lepidopterous species, the 

 larvfe of all being leaf feeders and naturally 

 preferring tender new leaves, three different 

 conditions of reaction to light were shown, in 

 one a positive heliotropism, in one a negative 

 heliotropism and in the third sheer indiffer- 

 ence to light. This last species, the silkworm, 

 may well be looked on as having lost its 

 earlier sensitiveness to light through pan- 

 mixia — if there be panmixia — because for 

 generations the silkworm's food has come to 

 him rather than had to be gone to, and 

 there is no more nor better food up or toward 

 light than there is down or away from light 

 or sidewise and in light of the same intensity 

 as that in which he first finds himself. I have 

 reared silkworms in darkness unbroken during 

 their whole life except at moments when the 

 mulberry leaves were thrust into the dark cell, 

 and in no structural or physiological charac- 

 teristic was there any apparent difference from 

 individuals bred in bright sunlight (alter- 

 nating with unillumined nights). 



It is interesting to note the decided char- 

 acter of the negative heliotropism of the lin- 

 den inch-worms, as this is the kind of helio- 

 tropism which is distinctly unexpected of 

 larva? which have to find for themselves tender 

 fresh leaves. In a glass cylinder, lighted in its 

 upper half and darkened in its lower half, with 

 linden leaves placed at the very bottom, 92 just 

 hatched larvse were placed in the lighted half 

 at 11:20 .\.M. At 3:30 p.m. 17 larva? were still 

 in the light half, but 75 were in the lower 

 darkened half, most of them being in the 

 darkest place, that is, at the very bottom 

 under the leaves. 



Equally marked was the more familiar 

 positive heliotropism of the unknown No. 3. 

 When only two or three inches from leaves 

 put into the darkened half of a glass dish, the 

 whole group of tiny cateri>illars, certainly 

 hungry even to death, would keep steadfastly 

 in the light half. 



The behavior of these various kinds of 



