CHAPTER XII 

 THE PTERYGOTA. THE EPHEMERIDA 



The group Pterygota includes practically all our common insects 

 and is the main branch of the class Hexapoda, the Apterygota though 

 of equal rank, being a mere twig in size, in comparison. 



As a whole the Pterygota are characterized by the presence of wings, 

 though as already indicated, many of them for one reason or another 

 have lost these structures. 



Almost all characters present in insects may be found in this section 

 without referring to the Apterygota: practically all the pests and all the 

 beneficial forms belong here; and their differences are so great that 22 

 orders have been established as subdivisions for them. 



The earliest writers on insects did not regard these differences as of 

 great importance, and called the groups families or gave them even lower 

 rank. More recent workers, however, have regarded them as of greater 

 significance, some students of the subject being inclined to recognize more, 

 rather than fewer orders, and it is not at all unlikely that time will finally 

 bring a general acceptance of 26 or 28 such groups instead of the first seven 

 established by Linne, or the 22 here treated. 



THE EPHEMERIDA 



The Ephemerida, May-flies or shad-flies as they are often called 

 (Fig. 38), are insects of medium or small size. The adults have delicate 

 bodies and gauzy, fragile wings, the latter usually with many cross-veins. 

 The fore wings are much larger than the hind ones, which in some cases 

 are absent, and the former are in general, rather strongly triangular in 

 outline. When at rest they are held vertically above the body. At 

 the end of the abdomen two or three long threads, each composed of 

 many segments and often called caudal filaments, are usually present, 

 the lateral ones being cerci corresponding to those in the Thysanura. 



The mouth parts of the adult May-fly are of the chewing type, but 

 so poorly developed that it is doubtful if they are ever made use of. 

 In some cases they are even rudimentary. The reproductive organs 

 differ from those in all the other groups, the ducts being not united on the 

 middle line below, but opening separately to the outside — apparently 

 the retention of a very primitive condition. The early stages are passed 

 in the water, the nymphs breathing — at least after the first few molts — 

 by tracheal gills. These are delicate, usually wing-like in form, and are 

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