1.] APPENDIX. . . 49 



the frost is very severe, they die. In the tropics the period 

 of hibernation for many animals coincides with the time of 

 maximum heat and drought. This shows that the organism 

 can be brought into the condition of a vita niinima in various 

 ways, and it would not be at all remarkable if such a state 

 were induced in certain insects by hunger. Exact experiments 

 however are the only means by which such a suggestion can 

 be tested, and I have already commenced a series of experi- 

 ments. The fact that certain beetles live without food for 

 many years (even six) can hardly be explained on any other 

 supposition, for these insects consume a fair amount of food 

 under normal conditions, and it is inconceivable that the}' 

 could live for years without food, if the metabolism were 

 carried on with its usual energy. 



A very striking example, showing that longevity may be 

 induced by the lengthening of the period of reproductive 

 activity, is communicated to me by Dr. Adler in the following 

 note : ' Three years ago I accidentally noticed that ovovivi- 

 parous development takes place in Chrysomela vartans, — a fact 

 which I afterwards discovered had been already described by 

 another entomologist. 



' The egg passes through all the developmental stages in the 

 ovary ; when these are completed the egg is laid, and a minute 

 or two afterwards the larva breaks through the egg-shell. In 

 each division of the ovary the eggs undergo development one 

 at a time ; it therefore follows that they are laid at considerable 

 intervals, so that a long life becomes necessary in order to 

 ensure the development of a sufficiently long series of eggs. 

 Hence it comes about that the females live a full year. Among 

 other species of Chrysomela two generations succeed each other 

 in a year, and the duration of life in the individual varies from 

 a few months to half a year.' 



VIII. Hymenoptera. 



Cynipidae. I have been unable to find any accurate accounts 

 of the duration of life in the imagos of saw-flies or ichneumons ; 

 but on the other hand I owe to the kindness of Dr. Adler, an 

 excellent observer of the Cynipidae^ the precise accounts of that 

 family which are in my possession. I asked Dr. Adler the 

 general question as to whether there was any variation in the 



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