I.] THE DURATION OF LIFE. 15 



nothing to be found in existing zoological text-books upon the 

 duration of life in animals ; and even monographs upon single 

 classes, such as the Amphibia, reptiles, or even birds, contain 

 very little on this subject. When we come to the lower animals, 

 knowledge on this point is almost entirely wanting. I have not 

 been able to find a single reference to the age in Echino- 

 dermata, and very little about that of worms, Crustacea, and 

 Coelenterata ^ The length of life in many molluscan species 

 is very well known, because the age can be determined by 

 markings on the shell-. But even in this group, any exact 

 knowledge, such as would be available for our purpose, is still 

 wanting concerning such necessary points as the degree of 

 fertility, the relation to other animals, and many other factors. 



Data the most exact in all respects are found among the 

 insects'", and to this class I will for a short time direct your 

 special attention. We will first consider the duration of larval 

 life. This varies very greatly, and chiefly depends upon the 

 nature of the food, and the ease or difficulty with which it can 

 be procured. The larvae of bees reach the pupal stage in live 

 to six days ; but it is well known that they are fed with sub- 

 stances of high nutritive value (honey and pollen), and that they 

 require no great effort to obtain the food, which lies heaped up 

 around them. The larval life in many Iclinemnonidae is but 

 little longer, being passed in a parasitic condition within other 

 insects ; abundance of accessible food is thus supplied by the 

 tissues and juices of the host. Again, the larvae of the blow-fly 

 become pupae in eight to ten days, although they move actively 

 in boring their way under the skin and into the tissues of the 

 dead animals upon which they live. The life of the leaf-eating 

 caterpillars of butterflies and moths lasts for six weeks or 

 longer, corresponding to the lower nutritive value of their food 

 and the greater expenditure of muscular energy in obtaining 

 it. Those caterpillars which live upon wood, such as Cossus 

 Hgniperda, have a larval life of two to three years, and the same 

 is true of hymenopterous insects with similar habits, such as 

 Sirex. 



Furthermore, predaceous larvae require a long period for 

 attaining their full size, for they can only obtain their prey at 



^ See Appendix, note 4, p. 55. " Ibid., note 5, p. 55. 



2 Ibid., note 3, p. 38. 



