1919] Repiignatorial Glands 45 



the caterpillars were placed on large sheets of blue litmus paper 

 and were then irritated at different points on the body by pinch- 

 ing them with forceps. It was difficult to see the spray on 

 account of its fine misty quality. Occasionally, by getting the 

 caterpillar in just the right position between the observer and 

 the rays of light, one could see a tiny jet or cloud of mist. The 

 effect on litmus paper, however, was instantaneous and con- 

 spicuous, for every drop that touched the paper changed it to a 

 characteristic red color. Moreover, the area covered by the 

 spray and the distance to which it could be ejected were easily 

 and accurately determined. The following experiments and 

 observations on 6". concinna illustrate the action of the glands. 



A full-grown larva was carefully placed near the center of a 

 large sheet of litmus paper. While in this position it was pinched 

 with tweezers on the fifth abdominal segment. At once a 

 fine invisible spray was thrown out around the body in a 

 posterior direction. An area surrounding the body about 

 3 inches wide and 5 inches long was colored solidly red, while 

 about the lateral and posterior borders of this solidly colored 

 area was a field covered with drops of various sizes. From 

 right to left the farthest drops were 53^2 inches apart. 



The larva was pinched again near the thorax, the main 

 pressure being exerted on the right side of the body. There 

 was an immediate spray toward the right and in a posterior 

 direction. The farthest drop posteriorly w^as 73/2 inches from 

 the caudal end of the abdomen, while the farthest drop laterally 

 was 53^ inches from the body. Figure 1 shows the pattern of 

 the spray on the litmus paper. 



An attempt was made to determine the direction of the 

 spray in relation to the point of irritation. When irritated on 

 the right side and the head left free, the head is thrown to the 

 right and the spray is directed largely to the right side of the 

 body. When irritated on the left side the movement of the 

 head is in that direction and the spray is directed mostly to the 

 left. When irritated near the posterior end of the abdomen 

 there is a decided tendency for the spray to be thrown in a 

 posterior direction. When pinched just behind the hump on 

 the second thoracic segment all of the secretion was projected 

 backward and to the left. In case of this particularly vigorous 

 larva the drops were thrown approximately 7 inches distant 

 in a lateral direction from the head and in a posterior direction 



