293] NOCTUID LARVAE— RTP LEY 51 



Unavoidable differences in the physiological condition of the individuals 

 account, most probably, for the considerable variation in the "time required 

 for submergence" with different larvae in the same experiment. During 

 the rest period prior to ecdysis and some six hours before it, larvae make no 

 attempt to burrow when subjected to the test, no matter how pronounced 

 this habit may be in the species. Three individuals of Polia renigera which 

 reacted differently from the rest, failing to enter the soil, were isolated and 

 found to be parasitized by chalcids when they died several days later. One 

 larva of Agrotis c-nigrum, presenting a similar non-conformity to specific 

 habit, died of the fungus, Botrytis rileyi, some time afterward. This 

 individual revealed a marked negative geotropism, crawling up on the sides 

 of the jar and onto the thermometer, a reaction exhibited by no other larva 

 investigated. It is interesting to note in this connection that grass- 

 hoppers diseased by Empusa gryllidae and army-worms or cut-worms 

 affected by wilt present the same response, crawling always to the top of 

 some plant to die. Underfed larvae require a much longer time to bury 

 themselves than do well-fed ones of the same species, the hunger stimulus 

 seeming to partially overcome the negative response to light. Experiment 

 3 illustrates this point very clearly, the material having been kept without 

 food for twenty-four hours in the warm laboratory at a temperature at 

 which the metabolism is high. Since the larvae were very hungry, they 

 resisted the tendency to burrow for a much longer time than in the other 

 experiments, where they were well-fed. For this reason the averages given 

 in this table do not include Experiment 3. The difference in the time of day 

 when these experiments were performed bears a direct relation to the 

 hunger, since the larvae feed principally at night. In Experiment 1, per- 

 formed at 9 P.M., the slower response may be due to the fact that feeding was 

 interrupted. The difference in weather conditions prior to the performance 

 of Experiments 1, 2, and 4, undoubtedly has contributed further to the lack 

 of physiological uniformity in the material used, introducing an additional 

 source of error. 



Altho accurate data as to the relative facility with which various species 

 enter the soil can be obtained only by a long series of experiments carried on 

 under carefully controlled conditions, employing a much larger number of 

 individuals than have been available for use in our investigation, the data 

 presented afford, nevertheless, some significant indications. Since species 

 such as the arboreal Homoptera lunata or the cabbage looper, Phylometra 

 brassicae, which we know to be not subterranean, are not induced to enter 

 the soil under the conditions of the experiments and since notoriously 

 subterranean species readily manifest their ability to burrow when stimu- 

 lated by light, we are justified in applying this test in order to determine 

 whether larvae have subterranean tendencies in species with which this 

 point is doubtful. It has been determined thus that the bronzed cut-worm, 



