8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



" sternites." This usage is without other objections than that it leaves 

 us no specific names for the subdivisions or component minor sclerites 

 of major areas of sclerotization. Writers that adopt it seldom follow 

 it consistently. On the other hand, many entomologists find it con- 

 venient for descriptive purposes to distinguish the segmental plates of 

 the abdomen from those of the thorax as " tergites " and " sternites." 

 The use of these terms in this manner, however, is not only arbitrary, 

 but it is ungrammatical, since it is clear that the terms ending in 

 ite may be applied to metameric units of any particular group of 

 somites only if the names " tergum " and " sternum " are defined as 

 the entire dorsum and entire venter of this limited group of somites — 

 a usage which no one pretends to adopt. In general, nomenclatural 

 troubles arise not so much from a scarcity of suitable terms as from 

 a lack of consistent application of words in common use. The writer, 

 therefore, employs the terminology recommended above as the one 

 most adaptable to the needs of anatomical description. And yet, it will 

 be found that absolute consistency is not possible ; the insects are sure 

 to present some anatomical conditions that can not be made to fit 

 with any nomenclatural scheme that can be devised. Consistency 

 is said to be a jewel, but an excess of jewelry may become a burden. 



Little is known concerning the nature of the sclerotic substances 

 in the cuticula of insects, or of the procedure by which a specific area 

 of the body wall becomes continuously sclerotized. We may believe, 

 however, that minor sclerites may have been produced phylogenetic- 

 ally by the secondary subdivision of major sclerites, though in the 

 ontogeny of the individual they may proceed from separate centers 

 of sclerotization. On the other hand, it is unquestionably true that 

 primarily distinct areas of sclerotization may unite, and give no trace 

 of their independent origin in the development of the embryo or pupa. 

 In the abdomen of most adult insects, for example, the pleural 

 sclerotizations derived from the limb bases are fused with the prnni- 

 tive sterna, and each definitive " sternal " plate in such cases is a triple 

 structure, though it may lose all trace of its composite origin. 



The spiracles are important landmarks in the study of the abdom- 

 inal segments. They never exceed eight pairs in number, and while 

 one or more of the posterior pairs may be absent, the presence of a 

 pair of spiracles is often better proof of the site of a primitive seg- 

 ment than is evidence derived from the sclerites. The primary posi- 

 tion of the spiracles is a matter on which opinion differs. There can 

 be no doubt that the spiracles are subject to migration, and that in 

 certain insects they have undergone an extreme displacement ; but 

 in the more generalized segments of most insects the spiracles He in 



