34 HOW TO OIJSKRVK IXSKCT?. 



Three broods are usually produced in our northern states during 

 the season. The larvae of the last ones reach full size and change 

 to pupae in the autumn, Whence issue the first brood of imagoes 

 the following spring. 



The danger to the larva of the cabbage butterfly from the at- 

 tacks of birds has already been referred to. Birds, however, are 

 not its only enemies. There are certain parasitic insects related 

 to the bees and wasps that deposit their eggs on the butterfly larva 

 soon after it hatches. The young of the parasite 0:1 hatching bore 

 into the body of the butterfly larva and feed upon it until they 

 reach maturity. _ As they are careful to avoid any of the vital 

 organs of their host the latter does not die, but about the time 

 that it should pupate there issues frgm its body a number of small 

 larvae which spin cocoons on its surface. In a few days there 

 issues from these, adult parasites like those that laid their eggs on 

 the butterfly larva. Bes-ides these parasites and birds both the 

 larva and imago are preyed upon by many other insects. A bac- 

 terial disease likewise attacks and oftens destroys the larva. 



The transformation passed through by the butterfly in passing 

 from the egg to the imago is such that none would suspect from 

 an examination of the larvae, pupa, and imago that they were dif- 

 ferent stages of the same insect. Insects in which the different 

 stages are so entirely different are said to have a complete meta- 

 morphosis. Butterflies and moths both have a complete metamor- 

 phosis. In these the larva is commonly called a caterpillar. Two- 

 winged flies, whose larvae are maggots; beetles, whose larvae are 

 commonly called grubs, as well as bees and wasps all have a com- 

 plete metamorphosis. 



In certain other insects, however of which the grasshopper may 

 furnish a typical example, (see next lesson) the insect that hatches 

 from the egg , is in general form similar to the .adult. That is, it 

 possesses six legs, antennae, commonly called "horns," on its head 

 and in a division of its body into head, thtfrax', and abdomen, is 

 similar to the adult; " The chief, difference is that it lacks wings. 

 A young insect that thus resembles itsC parent is called a nymph. 

 As it grows it gft'e'ds its skin or molts at frequent intervals. At 

 each molt it gradually becomes more and more like the adult in 

 size and in the proportion of the different parts of its body. Fin- 

 ally at the last molt it changes into the imago. 



Inrects which thus resemble their parents in all stages from the 

 egg upward are said to have an incomplete metamorphosis:. Be- 

 sides crickets, cock-roaches, and other forms related to the grass- 

 hopper the true bugs and some less well known insects have in- 

 complete metamorphoses. 



