512 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxvi. 



The following statements sum up the principal conclusions: 

 (1) There is no reason for believing that the parts of any thoracic 

 segment are derived from more than one metamere, though the primi- 

 tive thoracic region may have been composed of more than three 

 segments, remnants of the supernumerary ones being possibly repre- 

 sented by the intercalary plates of some of the Aptera ; (2) the 

 thoracic sclerites are subdivisions of an original undivided segmental 

 wall; (3) the sclerites of the pleurum are homologous throughout all 

 the orders and modifications are brought about principally through 

 the coalescence of the pleurites; (4) the tergum consists of a primitive 

 undivided notal plate carrying the wings and, in the adult meso- and 

 metathorax of all the principal orders, except the Orthoptera, of a 

 second postnotal or pseudonotal plate developed in the membrane 

 behind the first and having no connection with the wings; (5) the 

 divisions of the notum are secondary, though similar in most of the 

 orders, and are not necessarily homologous, while modifications are 

 brought about throui^h a stronger subdivision into distinct regions 

 and even into separate sclerites. 



It is unfortunate for modern entomology that there are so many 

 species of insects. Entomologists early had to specialize as Coleopter- 

 ists, Dipterists, Lepidopterists. and so in each order a scheme of 

 anatomy and a nomenclature grew up which satisfied the needs of 

 the Avorker in that order but had no necessary connection with those 

 of workers in other groups. It is true that Andouin in 1824 worked 

 out a system of comparative external anatomy and proposed a 

 universal set of names for the sclerites. It is true also that his names 

 have been in large part employed by nearly all subsequent entomolo- 

 gists. But in the actual application of Andouin"'s names to the scler- 

 ites of the thorax, specialists in the various orders have differed widel}'^ 

 on account of their ignorance concerning the correspondence of parts 

 in different insects. Recent entomologists who have attempted to 

 enforce a uniformity of nomenclature based on a more thorough 

 knowledge of insect structure are confronted with the nonconform- 

 ing masses of literature which must form the basis of work by present 

 and future students in each order. However, even if systematists 

 never can employ a uniform system of names, it can not be denied 

 that it is best to know the true homology of the parts as far as this 

 can be determined. 



It is impossible to follow the rule of priority in selecting anatom- 

 ical terms, for the name must be descriptive of the part to which it 

 is applied. The earlier entomologists also paid little attention to 

 the duplication of parts in successive segments, but gave a separate 

 name to every piece. Andouin did aAvay with this system in 1824 

 and firmly established a nomenclature based on the belief that each 

 thoracic segment is a modification of one plan of structure. He 



