8 ANIMAL LIFE AND HUMAN PEOGRESS 



tlie activity of the principal actors. Most of these are beetles 

 of various kinds and sizes, but always small. 



Perhaps the commonest and most destructive of all the 

 grain-pests are the two weevils, Calandra oryzae and Calandra 

 granaria, easily distinguished from other beetles by the long 

 snouts characteristic of all the weevil family. Calandra 

 oryzae has well-developed wings concealed beneath the horny 

 wing-cases or elytra, but it so rarely uses them that one 

 wonders why they have been preserved, especially as Calandra 

 granaria, which has precisely similar habits, seems to get on 

 very well without them. The almost microscopical egg is 

 deposited in a hole made in the surface of the grain by the 

 female insect, and in a few days a tiny grub is hatched which at 

 once bores its way into the interior. Here it feeds and grows 

 into a rather shapeless, pulpy larva, which exhibits very httle 

 activity ard presently passes into the pupal state. Finally the 

 perfect insect emerges and bores its way out of the hollowed 

 grain to enter upon an active existence of some considerable 

 duration. The entire period of development, from egg to 

 perfect insect, under favourable conditions occupies from 

 three weeks to a month. In one of my own experiments the 

 number of adult weevils {Calandra oryzae) increased more than 

 three-and-a-half times in less than five weeks at a temperature 

 of about 31° C, the Indian summer temperature. Assuming 

 a threefold increase per month it is easy to calculate that in 

 six months the ntmaber of weevils would increase 729 times, 

 and as the adults are rather long-lived and feed voraciously 

 on fresh grains, it is obvious that an enormous amount of 

 damage can be done in the space of a few months.^ 



Of only less importance, apparently, is the small grain-beetle 

 known as Rhizopertha dominica. The habits of this insect 

 are very similar to those of the two weevils, but the egg is 

 deposited outside the grain and the young larva has to depend 

 for its food-supply upon the soft frass and damaged grains 

 due to the activities of the parents. In fact the adult insect 



^ In an experiment made since this lecture was delivered a single pair 

 of Calandra oryzae gave 1501 adult weevils in 111 days, at a temperature 

 of 25°-28° C. in a moist atmosphere — a multiphcation of 750 times. 



