290 ENTOMOLOGY 



derived from a large luminous area and that from a small one, even when 

 the light from these two sources is of equal intensity as it falls on the 

 animal. These butterflies usually fly toward the larger areas of light. 

 This species remains in flight near the ground because it reacts positively 

 to large patches of bright sunlight rather than to small ones, even though 

 the latter, as in the case of the sun, may be much more intense. 



V. antiopa retreats at night and emerges in the morning, not so much 

 because of light differences, as because of temperature changes. On 

 warm days it will, however, become quiet or active, without retreating, 

 depending upon a sudden decrease or increase of light. 



The maggots of the muscid Phormia regina are, as the author has 

 observed, negatively phototropic until full grown, when they become 

 positively phototropic for an hour or less, leave the decaying matter in 

 which they have developed and wriggle along the ground toward the 

 sun; or if the sunlight is diffused by clouds, wander about aimlessly, but 

 a.t length bury themselves in the ground to pupate. Here the positive 

 phototropism just before pupation is adaptive, as it is in the case of 

 sexually mature ants, which make a nuptial flight into the sunlight when 

 they have acquired wings. The swarming of the honey bee is likewise a 

 case of periodic positive phototropism, as Kellogg has observed. 



Though adaptive in their results, these phototropic reactions can 

 scarcely be said to be performed on account of their usefulness. They are 

 performed anyway, and may result harmfully, as when they lead a moth 

 into a flame or, to take a more natural example, when they expose an 

 insect to its enemies. 



Phototropism and thermotropism, either together or singly, as Wheeler 

 suggests, may explain the up and down migration of insects in vegetation. 

 "On cold, cloudy days few insects are taken because they lurk quietly 

 near the surface of the soil and about the roots of the vegetation, but 

 with an increase in warmth and light they move upwards along the 

 stems and leaves of the plants, and, if the day be warm and sunny, es- 

 cape into the air." 



Thermotropism. Ants are strongly thermotropic; they carry their 

 eggs, larvae and pupae from a cooler to a warmer place or vice versa, and 

 thus secure optimum conditions of temperature. Caterpillars and cock- 

 roaches migrate to regions of optimum temperature. 



In thermotropism it appears that the direction of heat rays has little 

 or no effect as compared with differences of intensity. 



Tropisms in General. Other kinds of tropisms are known, for 



