1916] Thoracic and Cervical Sclerites of Insects 37 



HISTORICAL REVIEW. 



Several theories have been proposed to explain the probable 

 formation of the thoracic segments and almost every writer 

 seems to have a different view of the subject. This has led to 

 the formulation of many different theories, some of which are 

 merely the unsupported statements of their respective authors, 

 who have failed to give reasons for their views. Such theories 

 apparently have no evidence to support their presentation and 

 hence cannot be accepted, especially when other writers have 

 advanced differing theories established upon firmer grounds. 

 Some investigators have based their ideas of the thorax upon 

 the study of a limited number of insects, whereas they should 

 have studied at least a representative number from different 

 groups, in order to obtain a general conception of the thorax. 

 Others who have studied a large number of insects have failed 

 to recognize the relation that exists between the thoracic 

 sclerites. This has given rise to incorrect interpretations of the 

 formation of the thorax and to great confusion in the homologies 

 of the thoracic sclerites in the same and in different insects. 



The earlier entomologists wrote very little about the insect 

 thorax. Knoch, 1801, termed the prothorax "Collum" and the 

 meso-, and metathorax, "Pectus." He may have recognized 

 that the thorax was composed of three segments, which are 

 topographically grouped into two parts, the collum and pectus, 

 or he may have considered that the thorax was actually com- 

 posed of only two segments, viz. : the collum and pectus. 



Chabrier, '20, Kirby, '28, and later Verhoeff, '02-'04, 

 although aware that the thorax was composed of at least three 

 segments, thought that it might be divided according to function 

 into two parts. These two parts were termed by Chabrier, the 

 collier and tronc alifere; by Kirby, the manitruncus and ali- 

 truncus ; and by Verhoeff, the proterothorax and deuterothorax. 

 This idea is no longer accepted. 



Audouin, '24, upon whose investigations the modern con- 

 ception of the thorax is based, points out that it is composed of 

 three simple segments and was the first to work out a terminol- 

 ogy for the thoracic sclerites, which has been used by all sub- 

 sequent entomologists. Beginning with the anterior thoracic 

 segment, he named them the pro-, meso-, and metathorax. 



