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 larve, 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 
~ey a cat or toad. A sea- 
mone, which was kept in an aquarium, lived sixty-six 
‘3. Cray-fishes may live twenty years. A queen bee 
; kept in captivity for fifteen years. Most birds have 
lung 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 
8 
Fig. 538.—Metamorphosis of a barnacle 
(Lepas). «, larva; 6, adult. 
