INTERRELATIONS OF PLANTS AND ANIMALS 33 



Formation of Pupa. — After a life of a few weeks at most, the 

 caterpillar stops eating and begins to spin a tiny mat of silk upon 

 a leaf or stem. It attaches itself to this web by the last pair of 

 prolegs, and there hangs in the dormant stage known as the 

 chrysalis or pupa. This is a resting stage during which the body 

 changes from a cater- 

 pillar to a butterfly. 



The Adult. — After 

 a week or more of 

 inactivity in the pupa 

 state, the outer skin 

 is split along the 

 back, and the adult 

 butterfly emerges. At 

 first the wings are soft 

 and much smaller 

 than in the adult. 

 Within fifteen minutes 

 to half an hour after 

 the butterfly emerges, 

 however, the wings 

 are full-sized, having 

 been pumped full of 

 blood and air, and the 

 little insect is ready 

 after her wedding flight 

 to follow her instinct 

 to deposit her eggs on 

 a milkweed plant. 



Plants furnish Insects with Food. — Food is the most important 

 factor of any animal's environment. The insects which we have 

 seen on our field trip feed on the green plants among which they 

 live. Each insect has its own particular favorite food plant or 

 plants, and in many cases the eggs of the insect are laid on the 

 food plant so that the young may have food close at hand. Some 

 insects prefer the rotted wood of trees. An American zoologist, 

 Packard, has estimated that over 450 kinds of insects live upon 



HUNTER, CIV. BI. — 3 



Monarch butterfly: adults, larvse, and pupa on their 

 food plant, the milkweed. (From a photograph 

 loaned by the American Museum of Natural 

 History.) 



