330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



flies in a very uncertain manner, first going off on a wavy course, up 

 and down, or edging along without making much headway, then 

 perhaps holding a sustained flight for several yards, finally to bring 

 up precipitously in a tree or shrub as if the effort had taken the last 

 bit of his strength. Ordinarily the males sit by day against the 

 trunks of the trees with wings folded flat over the back, or they rest 

 on a stem or twig with the wings bent forward against the sides of 

 the body and the support (fig. 15). 



The moths of both sexes of the fall cankerworm, as those of the 

 spring canker, take no food, the mouth parts and most of the alimen- 

 tary canal being rudimentary. The posterior part of the intestine, 

 however, acts as a reservoir for the products of the excretory Mal- 

 pighian tubules and becomes filled with a semiliquid, orange-red 

 mass. In the female the eggs completely fill the body cavity, occupy- 

 ing all available space clear up to the front of the thorax. They are 

 packed in jumbled masses in eight long, thin-walled sacs, which are 

 the distended ovarian tubules. 



The moths are never very active out of doors, but they show more 

 animation on warm nights than they do on cold nights. When kept 

 in cages in a room they remain quiet all day, but with the approach 

 of evening, though the room be brightly illuminated with electric 

 light, the males begin to flutter restlessly about, and the females are 

 soon running briskly in all directions. Somehow they know when 

 night comes, regardless of artificial conditions of light and tempera- 

 ture, once more demonstrating they they are not slaves of external 

 conditions. 



By a few simple expedients a female cankerworm moth can be 

 made to give some interesting demonstrations of certain tempera- 

 mental traits she possesses. If one is placed on a table at night and 

 an electric light is held within 2 feet of her but to one side, she shows 

 at once that light has a very powerful attraction for her. She 

 immediately runs rapidly toward it. If the light is moved about 

 over the table the moth follows every motion of it, and can be made 

 to describe circles and all kinds of intricate figures until the experi- 

 menter tires of the sport. 



This idiosyncrasy of the moth for going toward the light is an 

 example of what biologists call ti^opisms. A tropism (from the 

 Greek trepein, to turn) is an innate quality in an animal that makes 

 it react in one way or another to external stimuli, without its " will " 

 having anything to do with its actions. Tropisms may be either 

 positive or negative according as the animal turns toward or away 

 from the source of the stimulus. Plants also have tropisms, as for 

 examjile, the well known habit of the sunflower for turning its 

 face toward the sun. The property of being stimulated to movement 

 by light is designated phototropism. 



