PHYLUM AKTHROPODA — CLASS INSECTA 309 



hemimetabola naiads live an aquatic life which necessitates changes 

 and physiological adjustments not required in the adult aerial ex- 

 istence. In these orders there are greater changes during develop- 

 ment than are found in gradual metamorphosis. 



The Holometabola, in which the larva hatched from the egg bears 

 no resemblance to the adult, goes through a complete metamorpho- 

 sis. The holometabolous insects include the following orders : Tri- 

 choptera, Neuroptera, Coleoptera, Lepidoptera, Siphonaptera, Dip- 

 tera, and Hymenoptera. The larva is variously called the maggot, 

 grub, or caterpillar. It eats almost constantly since this is the 

 growth period in an insect's life history. After molting several 

 times it comes to rest and prepares for the pupal stage. The pupa 

 gradually takes on the adult form and after a few days or even 

 months, the adult or imago emerges. 



The remarkable adaptation of the immature stages of insects to 

 their food supply has undoubtedly had much to do with their great 

 success as a group. Their food habits, minute size, use of flight in 

 locomotion, and rate of multiplication, along with other distinctive 

 characteristics mentioned above, have made possible the development 

 of this dominant group. 



CLASSIFICATION 



Because of the important role insects play in the life of man it is 

 worth while to be able to recognize some of the common orders. 

 The characters most used for the separation of the orders of insects 

 are the structures of the wings and mouth parts, and the type of 

 development, or metamorphosis. The number of orders recognized 

 in this class varies considerably, depending on the authority fol- 

 lowed. It has been divided into the subclasses: Apterygota, the 

 two wingless orders Thysanura and Collembola, and Pterygota, 

 which includes all the other orders of insects. Since practically all 

 the orders fall into the subclass Pterygota, the arrangement fol- 

 lowed here is that of discussing them according to their development. 



Subclass Apterygota. — Ametabola are insects without metamor- 

 phosis. 



Order Thysanura. — The members of this order have retracted 

 mouth parts, elongated rather flattened bodies, long antennae, and 

 abdominal appendages. They are soft-bodied small insects, com- 



