DENSITY OF POPULATION AND INSECT SURVIVAL 237 



of his experiments in this laboratory, used different species of grass- 

 hoppers, working upon the meadow grasshopper, Xiphidiuni fascia- 

 tus, more than on any other single species. Experiments were run 

 upon the effects of ether, carbon tetrachloride, ethylene chloride, 

 ethyl alcohol, and hydrocyanic acid. Two types of experiments 

 were carried out. In one set the insect containers had a direct 

 volumetric relation to the number of individuals per bottle, so that 

 if 5 insects were used, the container had five times the volume of 

 that containing but i insect; and if 10 were used, the container had 

 ten times the volume of that employed with only i animal. In 

 such experiments the volume percentage of the toxic gas was kept 

 constant. 



In the other type of experiment, all tests were run in the same 

 size bottle regardless of the numbers exposed. In these, too, the 

 volume percentage of the toxic agents was kept constant. The 

 amount of gas used was calculated to produce narcosis in most cases, 

 without death necessarily following. 



In all experiments the group survived longer; but, as might be 

 expected, there is relatively greater group resistance when the vol- 

 umes are identical than when they are proportional to the number of 

 insects. Similarly, single individuals have been found to have a 

 somewhat longer survival when placed in a smaller bottle rather 

 than a large one, when each has the same volume percentage of toxic 

 gas. The results of one of the constant volume experiments are given 

 in Table XXVI. 



The effect of exposure of these animals may be summarized as 

 follows: In the group of 30, 82 per cent survived for 24 hours; 

 in the group of 20, 90 per cent survived; in the group of 10, 90 per 

 cent survived; while of the 10 singles only 60 per cent survived 

 for the 24 hours of the experiment. In control lots under similar 

 conditions, but without the ethylene chloride gas, 97 per cent of 

 the group of 30 survived, and, all of each of the other lots lived for 

 the period of the test. Apparently the group of 30 suffered some- 

 what from the ill effects of crowding, but not enough fully to mask 

 the survival value of the group when exposed to this amount of 

 narcotic gas. 



