LIFE HISTORY OF INSECTS 329 



invertebrate animals. In some species the instincts, especially 

 those connected with providing for their offspring, simulate care- 

 ful and accurate reasoning, and it is difficult not to look upon 

 them as animals endowed with a high degree of intelligence. 



Life History. — As regards life history, three different types 

 can be recognized among insects. In the primitive order Thy- 

 sanura alone there occurs " direct development " in which the 

 newly hatched insect is nearly a miniature of its parent, and 

 merely increases in size. The two common types of development 

 are by incomplete and complete metamorphosis. Insects which 

 have an incomplete metamorphosis are those which differ more or 

 less from their parents when hatched, but which gradually assume 

 the parental form with successive moults or sheddings of the 

 skin. The young or " nymphs " of such insects invariably lack 

 wings, and often have other characteristics different from their 

 parents. In such insects as lice, in which the wings are absent in 

 the adult, there is very little difference except in size between 

 the young and adult forms. Insects which have a complete 

 metamorphosis are those in which, as in butterflies, the newly 

 hatched larva is totally different from the parent, and does not 

 gradually assume the parental form. Instead, it retains its worm- 

 like larval characteristics until full grown and then transforms, 

 during a resting and more or less quiescent period of relatively 

 short duration, into the adult form. This transformation, which 

 may amount to nothing short of a complete remodeling of the 

 entire body and all its organs, is sometimes accomplished in an 

 amazingly short time. Many maggots transform into adult flies 

 in less than a week, and some mosquito larvae transform into 

 perfect mosquitoes in less than 24 hours. 



The length of Ufe of insects in the larval and adult stages 

 varies with almost every species and with environmental con- 

 ditions. The larval stage may occupy a small portion of the 

 life, as in the case of many mosquitoes and flies, or it may con- 

 stitute the greater part of it. There are some mayflies, for 

 instance, which live the greater part of two years as larvae but 

 exist as adults not more than a few hours. As a rule male in- 

 sects are shorter lived than the females ; the length of life of the 

 latter is determined by the laying of the eggs — when all the 

 eggs have been laid the female insect has performed her duty 

 in life and is eliminated by nature as a useless being. The result 



