122 Annals Entomological Society of America [Vol. XI, 



abdominal spiracles typically occurs in the eighth abdominal 

 segment of adult insects, but is sometimes concealed if the 

 posterior segments are retracted; otherwise, it is in the seventh, 

 and the arrangement in this particular is quite uniform through- 

 out an order. As, for example, there are always eight pairs of 

 abdominal spiracles in the Orthoptera but only seven pairs in 

 the Lepidoptera, with the possible exception of Hepialus. The 

 fact must not be lost sight of that in the larva of this form 

 eight pairs of functional spiracles are present, and that these are 

 rudimentary in the pupa. The caudal appendages of the 

 abdomen, the genitalia, are designated in this paper by the 

 Roman numerals I, II, III and IV, beginning with the cephalic 

 pair. No attempt has been made to apply the names given by 

 systematists to these parts. 



With the foregoing general principles in mind, some account 

 of the details of the work may now properly be considered. 



Method of Prepar.\tion. — The method of preparing 

 insects for a study such as the following is very simple. They 

 were softened by leaving the entire insect, the abdomen only, 

 or merely the posterior end, whichever was desired, in a five 

 per cent solution of caustic potash for periods varying from 

 twelve to twenty-four or thirty-six hours, depending upon the 

 hardness and freshness of the specimens used. If pinned 

 specimens were used, they were first boiled in water for a short 

 time. They were then washed in running water, and studied 

 under a binocular dissecting microscope. A compound micro- 

 scope was also used for studying mounted balsam preparations 

 of the smaller parts. 



Discussion and Tables. — The arrangement of this section 

 of the paper will be as follows : The Apterygota will be taken up 

 first, and next the Pterygota, beginning in each case with the 

 generalized forms. There are also included tables showing the 

 sclerites and appendages present in the species studied. The 

 presence of a sclerite or a pair of appendages is indicated in the 

 tables by a plus sign, and their absence by a minus sign. When 

 either is rudimentary, an asterisk is added to the plus sign. The 

 abbreviations t and s stand for tergum and sternum respectively. 



a. Thysanura. — It is obvious from the figures of both 

 Lepisma and Machilis that the problem is complicated in the 

 Thysanura rather than simplified, as would seem natural. It is 



