144 SECRETS OF ANIMAL LIFE 
a prolonged larval period has been interpolated. 
Out of the egg-shell of a cockroach there comes 
a miniature of the adult, but out of a butterfly’s 
there emerges a minute caterpillar with very little 
hint of its parentage. It feeds and grows and 
molts its husk, and this logical sequence is repeated 
over and over again. The caterpillar gains strength 
and stores up nutritive reserves; it undergoes a 
remarkable metamorphosis, most of the old larval 
body breaking down and a fresh start in develop- 
ment being made on a new architectural plan. 
Eventually the winged butterfly emerges, as it were, 
by a second birth, and enters upon a phase of life 
which is preoccupied with reproduction and only 
secondarily (if at all) concerned with nutrition. 
The relatively long caterpillar period makes the 
ecstasy of the butterfly possible. A very remarkable 
achievement has resulted from the lengthening out 
of the larval phase, and in many life-histories we 
hear, so to speak, the same tune. The mayflies 
or Ephemerides are often almost diagrammatic, for 
many have two or three years of subaquatic larval 
life and two or three days (or less) of aerial and 
reproductive activity. In the sea-lamprey we find 
a somewhat similar punctuation of life—but it is 
notably improved upon. For after a long larval 
fluviatile period, sometimes of four years, there is a 
phase of vigorous adolescence and adult life in the 
sea. But the curve ends in the same way—an 
almost vertical drop after reproduction. In the 
common eel there is a greatly elongated larval 
