386 APPLIED BIOLOGY 



are many moths with cocoons). Also, there is an advantage 

 in that the larvae live under conditions different from the 

 adult insect; for example, numerous larvae feed on foliage 

 which the adults never frequent except for laying eggs. 

 These are some of the apparent advantages which have come 

 to insects with a complicated life-history consisting of four 

 stages, egg, larva, pupa, and adult ; and it certainly has 

 been worth while for insects to develop complete metamor- 

 phosis in place of such direct development of eggs into 

 miniature adults as occurs in the simplest wingless insects 

 and also in the not distantly related centipedes. No one 

 knows why and how insects have acquired such complicated 

 life-histories; but the known facts have convinced all en- 

 tomologists that they all once had life-histories even simpler 

 than the grasshopper now has, and that there has been 

 developed greater complexity by introduction of larva and 

 pupa which have certain advantages in that they adapt 

 insects to peculiar conditions of life, as suggested above. 



Parthenogenesis. A peculiarity of the eggs of some insects 

 is that they develop without fertilization. This is called 

 parthenogenesis. Throughout the summer months the in- 

 dividuals of certain plant-lice are all females, and their un- 

 fertilized eggs develop into females. There may be many 

 such parthenogenetic generations in a summer. In the 

 autumn both males and females develop from unfertilized eggs, 

 and fertilized eggs are produced which survive the winter 

 and develop into females in the spring. These begin a new 

 series of parthenogenetic generations which continues until 

 the next autumn. 



Honey-bee drones (males) develop from unfertilized eggs, 

 and the workers (undeveloped females) and " queens " 

 (mature females) are from fertilized eggs. Thus mating a black 

 drone and a yellow " queen " will result in half-blood or 

 hybrid workers and young " queens," but yellow drones. 



Other cases of parthenogenesis are found among insects and 



