COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 407 



less, there are uumerous examples of positively phototropic forms whose 

 reactions to light tend to carry them always into regions of greater 

 illumination, but whose eyes are probably of little use beyond deter- 

 mining the direction and the relative intensities of lights. Here, again, 

 there appears to be a correlation between the habits of the animals and 

 the conditions under which they live, for an inspection will show that 

 these are usually creeping forms whose movements toward the light take 

 them in the direction of their food, or else that other conditions prevent 

 their phototropism from taking them into unfavorable surroundings. 

 The caterpillars of Porthesia (and probably of most other Lepidoptera) 

 may be taken as an example of the former. Although no tests have 

 been made on these forms to determine their ability to discriminate 

 between luminous areas of different size, the rudimentary condition of 

 their eyes, and the experiments with beetle larvae {cf. Tenebrio) make 

 reasonable the assumption that they respond only to the intensity 

 of the light reaching them, and not to the size of the area whence it 

 immediately comes. Under ordinary conditions, in the sunlight, the 

 largest patches of light are on the ground ; but the strongest intensity 

 is skyward, and responding to this, as soon as hatched, the young 

 caterpillars of Porthesia crawl upward and outward on the branches 

 until the reaction to food overcomes their phototropism, or they are pre- 

 vented from going farther by reaching the tips of the branches. In 

 these two ways their progress skyward is checked at the proper time. 

 In a similar manner positively phototropic snails or other crawling 

 forms are restrained by the natural conditions from continuing indefi- 

 nitely their migrations in the direction of the greatest illumination. 

 Parker (: 03, p. 462) has called attention to the fact that the surface of 

 the water forms a similar barrier to certain marine organisms (such 

 as the copepod Labidocera) which swim upward through the water 

 toward the light. 



With forms which fly, and so, being independent of solid objects upon 

 which to crawl, are not limited in the distance which they might move 

 upward, the case is entirely different. A query which Romanes ( '83, 

 p. 279) found among Darwin's manuscript notes shows careful obser- 

 vation and puts the question very clearly. 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 in the horizon % 

 I formerly observed that they 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 

 object, the insects regard it as a matter of course, and so have no 

 desire to examine it." As a result of Parker's work on Vanessa and 



