THE DURATION OF LIFE. 15 



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 1 , 

 and to this class I will for a short time direct your special atten- 

 tion. 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 five to six days ; but it is 

 well known that they are fed with substances 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 Ichneumonidae 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 

 Cossiis ligniperda> 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 rare intervals 

 and by the expenditure of considerable energy. Thus among the 

 dragon-flies larval life lasts for a year, and among many may-flies 

 even two or three years. 



All these results can be easily understood from well-known physio- 

 logical principles, and they indicate that the length of larval life is 

 very elastic, and can be extended as circumstances demand ; for 

 otherwise carnivorous and wood-eating larvae could not have sur- 

 vived in the phyletic development of insects. Now it would be 

 a great mistake to suppose that there is any reciprocal relation 

 between duration of life in the larva and in the mature insect, 

 or imago ; or, to put it differently, to suppose that the total 

 duration of life is the same in insects of the same size and activity, 

 1 See Appendix, note 3, p. 38. 



