1.] APPENDIX. . 51 



It is evidently, therefore, a general rule that the duration of 

 life is directly proportional to the number of eggs and to the 

 time and energy expended in oviposition. It must of course 

 be understood that, here as in all other instances, these are not 

 the only factors which determine the duration of life, but many 

 other factors, at present unknown, may be in combination with 

 them and assist in producing the result. For example, it is 

 very probable that the time of year at which the imagos appear 

 exerts some indirect influence. The long-lived Biorhiza 

 emerges from the gall in the middle of winter, and at once 

 begins to deposit eggs in the oak buds. Although the insect is 

 not sensitive to low temperature, for I have myself seen ovipo- 

 sition proceeding when the thermometer stood at 5° R., yet 

 very severe frost would certainly lead to interruption and 

 would cause the insect to shelter itself among dead leaves on 

 the ground. Such interruptions may be of long duration and 

 frequently repeated, so that the remarkably long life of this 

 species may perhaps be looked upon as an adaptation to its 

 winter life. 



Ants. Lasius flavus lays its eggs in the autumn, and the 

 young larvae pass the winter in the nest. The males and 

 females leave the cocoons in June, and pair during July and 

 August. The males fly out of the nest with the females, but 

 they do not return to it ; 'they die shortly after pairing.' It is 

 also believed that the females do not return to the nest, but 

 found new colonies ; this point is however one of the most 

 uncertain in the natural history of ants. On the other hand 

 it is quite certain that the female may live for years within the 

 nest, continuing to lay fertilized eggs. Old females are some- 

 times found in the colony, with their jaws worn down to the 

 hypodermis. 



Breeding experiments confirm these statements. P. Huber^ 

 and Christ have already put the life of the female at three to 

 four years, and Sir John Lubbock, who has been lately occu- 

 pied with the natural history of ants, was able to keep a female 

 worker of Formica sanguinea alive for five years ; and he has 

 been kind enough to write and inform me that two females of 

 Formica fit sea, which he captured in a wood together with ten 

 workers, in December 1874, are still alive (July 1881), so that 



^ ' Recherches sur les moeurs des Fourmis indigenes,' Geneve, 1810. 



E 2 



