120 



determination of the homologies of the other principal 

 tracheae. 



In most orders of insects the longitudinal veins can be 

 distinguished from the cross-veins by the fact that the cross- 

 veins are not preceded by tracheae. 



In some of the many-veined insects, as Odonata, the 

 cross-veins, as well as the longitudinal veins, are preceded 

 by tracheae ; there being, in these insects, a great multiplica- 

 tion of tracheae. 



On the other hand, in the Trichoptera, Diptera, and most 

 Hymenoptera, a great reduction of the tracheal system has 

 taken place. It is not well, therefore, for the student to 

 begin his studies of this subject with members of either ot 

 these orders. 



In those orders where a specialization of wing- veins by 

 addition has taken place, the accessory longitudinal veins are 

 preceded by tracheae. 



Finally, it should be remembered that it is not safe to base 

 conclusions upon the study of a single insect ; a large series, 

 representing as many genera and families as is practicable, 

 should be investigated. 



