196 SYSTEMATIC ZOOLOGY 



females, —the sexually mature, and others which never attain 

 sexual maturity, such as the workers in a colony of bees. 



As we have already noted, the sexes are always separate. 

 Parthenogenesis is not uncommon in several orders, occurring, 

 however, in connection with normal sexual reproduction. Some- 

 times both males and females are produced in this way, some- 

 times only males. Temporarily at least some insects may be 

 viviparous. Occasionally, too, we find a condition called paedo- 

 genesis, in which ova in female larvae develop parthenogeneti- 

 cally before the larva has become mature, and produce in this 

 way new larvae; this may go on for some time, but eventually 

 adult individuals develop. 



If insects were to be classified according to the history of 

 development, we should make three groups of them. In the 

 first group belong some of the more primitive insects, in which 

 the young leave the egg in a condition that differs from the 

 adult chiefly in size; such insects are said to develop directly 

 or without a metamorphosis. The second group includes 

 those insects in which a larva is formed, often very different 

 from the adult, without wings or with the most rudimentary 

 wings, and by a series of moults passes more or less gradually 

 into the adult condition ; such insects have an incomplete 

 metamorphosis. The third group comprises the remaining 

 insects, in which there is an additional stage. The egg develops 

 into a larva ; the larva eventually passes into a quiescent stage, 

 — when it is called the pupa, — during which it takes no food 

 and is gradually transformed into the adult or imago stage. 

 Insects undergoing such changes have a complete metamor- 

 phosis. The forms of the larva" and pupae are no less varied 

 than the forms of the imagines or adidts. 



The classification of the insects into orders and suborders has 

 undergone great changes. Certain orders have always been 

 pretty sharply defined, but others have held insects more or 

 dissimilar, so that their further study has led to a division into 

 more and more groups until we have now about nineteen orders 

 and as many suborders. For our purpose it seems sufficient to 

 divide the class into nine orders, and after taking them up indi- 

 vidually, we may briefly note the complete classification as it is 

 more or less generally accepted to-day. 



