1017.] 



89 



the grains. Younger larvae cannot penetrate sound grains, but the least 

 abrasion of the hard outer skin enables them to enter. The larva then excavates 

 a cavity in the seed and pupates therein without forming any cocoon. In the 

 notorious Calandra oryzae the egg is laid in an excavation in the grain made 

 by the female, and the larva has no need to seai'ch for a suitable jDlace of entry 

 to its food — a fact which may be correlated with its legless condition ; the 

 whole of its larval and pixpal existence is passed, weevil-fashion, in a single 

 grain. The damage done by A. undulatus is on the whole more slow and steady. 

 It has a great capacity for withstanding adverse conditions, but is handicapped 

 by its slower rate of reproduction. The other two species are far more easily 

 affected by adverse conditions, but under favovirable circumstances can do 

 great damage in a short time. 



Tiirning to the part of the paper which deals with chemical questions : 

 tests were made of the effect of dry gases — cai'bon dioxide, hydrogen, 

 and nitrogen — on the larva of A. undulatus and the adults of R. dominica 

 and C. oryzae, it being thought that, if such measures were effective, wheat 

 might be stored in sealed bins, into which siich gases could be introduced. 

 But it was fovmd that the insects can survive for long periods in an inert 

 condition, from which, if a supply of oxygen be restored to them, they can 

 resuscitate. The time taken to produce actual death was spoken of as "the 

 lethal period," and this decreases as the temperature rises, and also as the con- 

 centration of gas becomes less dense. In an atmosphere of dry carbon dioxide 

 at 30" C, the lethal period of A. undulatus was from 63 to 89 hoiirs; of 

 C oryzae, from 30 to 51 hoiirs ; and of R. dominica, from 14 to 50 hours (p. 234). 

 In the experiments with hydrogen and nitrogen respectively, the lethal period 

 was also often of considerable length. Moreover, the period varies not only with 

 temperature and gas-density, but also with the season of the year, and in the 

 case of different individual insects. On the whole, A. undulatus seems the most 

 resistent to all the gases, R. dominica the next, and C. oryzae the least resistent 

 (p. 238). 



The interpretation of these phenomena is very suggestive. In Chap, iv are 

 reviewed various sviggested explanations of the nature of the process of respira- 

 tion, and by a series of experiments the writers come to the conclusion that, in 

 the case of these insects at least, respiration is at the root an euzymic process 

 due to the presence in the cells of an enzyme capable of bringing about oxida- 

 tion. A number of larvae were killed and ground to pulp in such a way as not 

 to destroy the cell substance, and it was found that certain chemical manifesta- 

 tions of the respiratory process still continued. When the living insects are quite 

 cut oft' from atmospheric oxygen, the enzymic action may enable them to con- 

 tinue respiration by making use of the chemically-combined oxygen in their 

 own bodies — i.e., by breaking down reserve-substances (fats) and oxidising 

 other compounds with the oxygen thus set free. This is regarded as the ex- 

 planation of the long survival of the insects in the gases used. The wide 

 difference in length of the lethal period in individual insects indicates that its 

 duration is regulated by the degree of enzymic activity of the insect at the 

 time. Such enzymic activity may also control the supply of energy at such times 



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