INSECTS INJURIOUS TO AGRICULTURE 357 



Eighth. — A faculty similar to the preceding which the parasite 



may possess. 5 



the eggs of the Hyponomeutas are never parasitized, and the adult generation 

 of the Encyrtus will already have disappeared while the Hyponomeutas still 

 continue to lay eggs, which will thus escape the parasite. 



Second Example. — With the Hessian fly, a small dipterous insect whose 

 larva is extremely injurious to wheat, the generations succeed one another the 

 whole year, and under particularly favorable conditions there can be five or 

 six generations in a single year. However, the time necessary for an individual 

 tc perfect its development is extremely variable, according to conditions in 

 which the pupae find themselves, and especially in regard to their position on 

 the plant — whether on the green part or near the earth or even dry stubble. 

 Some can complete their development in two weeks, while others, finding the 

 conditions of dryness exceptionally long, wait even as long as two years before 

 issuing. The hymenopterous insects, living at the expense of the insect, which 

 may appear in innumerable quantities, have, on the contrary, only two genera- 

 tions each year at the maximum, and appear only at a definite time, and during 

 a lapse of time of usually short duration. Now, since the parasites never attack 

 more than one of the developmental stages of the insect — egg or larva, according 

 to parasitic species — it results, from what precedes, that there will always be, 

 at the period of egg-laying, existing individuals of the species which will escape 

 them because they are in a developmental condition in which they are not 

 pierced; and when the generation of parasites has passed, these individuals 

 will remain unharmed and constitute the indispensable reserve for the perpetua- 

 tion of the species. It is not necessary that this reserve should exist through- 

 out the range of the plant-feeding species. On the contrary, they may be anni- 

 hilated in certain localities by a combination of climatic conditions or factors 

 of some other nature having an unfavorable influence, and it is in this way that 

 the local disappearance of certain species, of which this one is an example, is 

 to be explained. 



Third Example. — In the preceding case the average conditions, and in par- 

 ticular the relative dryness, play a part of the first rank in the determination 

 of retarded development, and the adaptation of the plant-feeding species enables 

 it to react in a more or less energetic manner to external influences. In other 

 cases the plant-feeding species has acquired a great variability in the time 

 necessary for the development of individuals. This variability, which appears 

 to be independent of average conditions, consists in reality in the fact that 

 different individuals present a variable power of reaction to identical external 

 influences. 



It is thus that, according to Boisduval ( " Essay on Horticultural Ento- 

 mology," Paris, 1867, p. 15), the chrysalids of Bombyx everia and lanestris, 

 which cause great damage in Germany, issue in a very irregular manner. " One 

 sees," says he, " moths of these Bombycids issue in September after three 

 months of metamorphosis, others in the spring of the following year; but what 

 is more astonishing is, that from the same egg laying, from the same brood of 

 caterpillars, reared under the same conditions, the moths have issued during 

 seven years in April and September — a wise foresight on the part of nature, 

 which does not wish to expose the species to sudden destruction." 



8 This factor, which favors the parasitic species, is in conflict with factor 

 No. 7. Some very remarkable examples of this have been given me by M. 

 Kunckel d'Herculais concerning egg-feeding parasites of locusts (Cantharids, 

 Clerids and Bombyliids), the development of which in certain cases can be re- 

 tarded for several years, thanks to a condition of torpidity which the larvae 



