THE LIFE CYCLE 



101 



60. Duration of life. After an animal has completed its 

 development it has but one thing to do to complete its life 

 cycle, and that is the production of offspring. When it 

 has laid eggs or given birth to young, it has insured the 

 beginning of a new life cycle. Does it now die ? Is the 

 business of its life accomplished ? There are many animals 

 which die immediately or very soon after laying eggs. The 

 May-flies ephemeral insects which issue as winged adults 

 from ponds or lakes in which 

 they have spent from one to 

 three years as aquatic crawl- 

 ing or swimming larvae, flutter 

 about for an evening, mate, 

 drop their packets of fertil- 

 ized eggs into the water, and 

 die before the sunrise are 

 extreme examples of the nu- 

 merous kinds of animals 

 whose adult life lasts only long 

 enough for mating and egg- 

 laying. But elephants live for 

 two hundred years. Whales 

 probably live longer. A horse 

 lives about thirty years, and so 

 may a cat or toad. A sea- 

 anemone, which was kept in an aquarium, lived sixty-six 

 years. Cray-fishes may live twenty years. A queen bee 

 was kept in captivity for fifteen years. Most birds have 

 long lives the small song birds from eight to eighteen 

 years, and the great eagles and vultures up to a hundred 

 years or more. On the other hand, among all the thou- 

 sands of species of insects, the individuals of very few in- 

 deed live more than a year ; the adult life of most insects 

 being but a few days or weeks, or at best months. Even 

 among the higher animals, some are very short-lived. 

 In Japan is a small fish (Solaux) which probably lives 



FIG. 53. Metamorphosis of a barnacle 

 (Lepas). a, larva ; b, adult. 



