76 THE STUDY OF INSECTS. 



head. They pertain almost exclusively to the immature stages of 

 insects ; but Stone-flies of the genus Pteronarcys retain them through- 

 out their existence. 



Tracheal gills vary greatly in form; in Corydalis they are hair-like, 

 and occur in tufts near the lateral margins of the abdominal seg- 

 ments; in the Cadd ice-worms they are thread-like, more or less 

 branched, and irregularly distributed over the surface of the abdo- 

 men ; and in certain Dragon-flies they are in the form of large plate- 

 like caudal appendages. (Fig. 88.) 



The Reproductive Organs. — The reproductive organs are situated 

 in the abdomen, as represented in Figure 80. There is a set on each 

 side of the body; but the two sets usually open by a common tube 

 near the caudal end of the body. In the May-flies and in the Ear- 

 wigs, however, the reproductive organs of each side have a distinct 

 opening. Thus May-flies are often found with two bunches of eggs 

 projecting from the caudal end of the body. 



All insects are developed from eggs ; but there are some appar- 

 ent exceptions. Thus many flies retain their eggs until after they 

 are hatched, if a proper place for laying them is not found earlier; 

 and in some flies (the Pupzpara) the young attain a considerable de- 

 velopment before they are born. In the Plant-lice (Aphididoe) there 

 is a remarkable alternation of reproduction by budding with the 

 sexual reproduction. This is described more fully in the account of 

 •that family. 



The Orders of the Hexapoda. 



{For advanced students. ) 



The Class Hexapoda, or Insects, is divided in this work into nine- 

 teen orders. This number is somewhat greater than what has been 

 commonly adopted heretofore. But we believe that in the earlier 

 classifications forms were brought together in the same order that 

 are not closely related, and that consequently the present classifica- 

 tion represents better the true relationship of the groups of insects. 



There has been some hesitation on the part of many entomolo- 

 gists in adopting this division of certain of the old orders, merely for 

 the reason that they felt that the old classification was simpler. But 

 we do not share in this feeling. It seems to us that it is easier for 

 the student to learn the characters of a large number of well-defined 

 groups than it is to learn those of a smaller number of vaguely- 

 defined groups. 



