1916] The Sleep of Insects 253 



facing north. The next day was cold and none were to be 

 found in the field. 



The above notes on the position of sleeping P. protodice were 

 for only October. We thought that the insects' behavior may 

 have been influenced by the autumn weather, and it seemed 

 desirable to know their sleeping behavior during the entire 

 summer. The notes below give this data for the following year. 



A hillside gently sloping toward the north was covered in 

 the early summer with horseweed, wild tansy, ironweed, poison 

 ivy, dock, etc. In the center of this variety of vegetation was 

 a patch, about fifteen feet across, of the white milkweed in 

 flower. On these plants at about 5:30 p. m. were hundreds of 

 these white butterflies feeding, flying about, and a few pairs 

 mating. "What an opportunity," thought I, "to return at 

 twilight and see how the insects orient themselves in reference 

 to wind, light, etc., when preparing to sleep, with sufficient 

 numbers in a limited area to make the data conclusive. " 



At 7:25 I returned to this spot, but where I looked for 

 hundreds to be asleep on the plants upon which an hour before 

 they had been so exuberantly drinking in all the goodness of 

 life, I found but two fluttering from flower to flower, sipping 

 here and there, and soon these too disappeared. They had left 

 off their gregarious habit of the day and each one had individ- 

 ually sought his own resting place for the night. A few of them 

 were found on near-by bushes, iron-weed, horse-weed and low 

 grass, but these were only a small part of the splendid flock; 

 most of them had flown to some distant place. The nearest 

 butterfly was ten feet from the food-plant, and not one rested 

 on the milkweed. 



Since so few were in the immediate vicinity, they never, 

 occurred more than one on each plant, and they all rested in a 

 vertical position, with head up. In the question of their 

 relation to the wind we have two points of environment to 

 consider: some of the butterflies rested on low plants and grass 

 in places protected from the wind, and others rested on or near 

 the tops of tall plants, in places exposed to the wind. A gentle 

 breeze was blowing from the east at this time ; when we examined 

 the butterflies at rest on the high plants in the full sweep of the 

 wind we found them with the ventral side of the body facing : 



East, 22; North, 1; West, 0; South, 1. 



Northeast, 3; Southeast, 1; Northwest, 2; Southwest, 0. 



