INSECTS INJURIOUS TO AGRICULTURE 353 



complete their development within the eggs. The phytophagous insect 

 is then killed within the egg, and the plant thus completely escapes 

 its depredations. 



Much more frequently it does not immediately stop the growth of 

 its host; it introduces its egg into the phytophagous insect and in the 

 more or less advanced stages of its development, either during the 

 embryonic period (Encyrtus fuscicollis, divers Platygasters), or fre- 

 quently during the larval or nymphal period (Ichneumonids, Bra- 

 conids, etc.). The phytophagous insect which carries in its interior 

 the larva of the parasite, continues to grow and to feed upon vegeta- 

 tion, and is killed by the parasitic insect only when the latter has 

 reached its full development, and when the host has done all the 

 damage it is capable of doing during its existence. The benefit 

 accomplished by the parasite is manifest only in the following genera- 

 tion, and consists in the suppression of the descendants that would 

 have been mothered by the phytophagous larva?, if they had been able 

 to develop until their transformation into adult insects. 



Whether they belong to one or another of these catagories, the 

 predatory and parasitic insects play a regulating role that is useful and 

 remarkable. When, on account of cultural conditions or climatic 

 circumstances or other influences, the phytophagous species tends to 

 increase beyond the average, it thus furnishes conditions eminently 

 favorable to the multiplication of the parasitic species, and that in 

 its turn causes the phytophagous form to decrease. 



In a very interesting, but insufficiently known work, Bellevoye 

 and Laurent (1897) have shown that it is not necessary that the 

 parasite should have a greater fecundity than the phytophagous species 

 in order to bring the latter back to its normal condition when it has 

 exceeded it. As paradoxical as is this assertion, with a fecundity 

 simply equal and even inferior, it may rapidly reach the point of 

 annihilation, if other factors and other conditions do not interfere to 

 interrupt this action. All other things being equal, nothing prevents 

 the development of the parasites, so that by their work a greater and 

 greater quantity of the plant-feeding species are destroyed each year. 

 In order to state this fact precisely let us, with the authors just cited, 

 take as simple an example as possible, that of an invasion of the cater- 

 pillars of Bombyx. Suppose that at a given period the proportion of 

 parasitized caterpillars is one fourth, and that the parasites have placed 

 a single egg in each caterpillar. Of 8 chrysalids, 6 will give out 

 Bombyx and 2, parasites. We will suppose that of the 6 moths there are 

 3 males and 3 females; and of the 2 parasites, 1 male and 1 female. 

 Let us suppose that the fecundity of the parasitic species is equal to 

 that of the host species, and that the number of eggs laid by a female 

 of each of the two species is 100. 



vol. Lxxn. — 23. 



