4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



tion ; embr} ologists say there are only three. Many entomologists, 

 though they reject the multiple segment theory, have, nevertheless, 

 thought it necessary to postulate four consecutive rings in each of the 

 three segments, conceived to have arisen from four primitive trans- 

 verse folds of a soft-bodied ancestral form, in which the sclerites of 

 the segment were originally laid down as chitinizations of the integu- 

 ment. This theor}' at first appeared to have much in its favor, but 

 the more the thoracic skeleton has been studied, especially in connec- 

 tion with the musculature, the more the four-ring theory loses support, 

 and gradually entomologists have relinquished it in favor of the idea 

 that the various sclerites are secondary divisions of primitively simple 

 plates. The last review of the evidence against these theories of 

 thoracic segmentation and annulation is given by Weber (1924), and 

 the theories should now be laid away for the historian, and respected 

 for the fact that they have been helpful. 



Recently another theory has been proposed by Feuerborn (1922), 

 which would make the insect thorax a rather complicated assortment 

 of parts derived from four segments. It appears, however, that this 

 conception has been based on a misinterpretation of the facts in the 

 metamorphosis of certain Diptera. Feuerborn's theory has been 

 opposed by Martini (1922) and by Weber (1925, '27) ; particularly 

 by Weber has the evidence put forth in its favor been thoroughly and 

 critically examined and found to be wanting in essential points. 



I. FUNDAMENTAL STRUCTURE OF AN ARTHROPOD SEGMENT 



The anatomical form of most ])arts of animals, lieing a patchwork 

 of changes or modifications that have been adopted in different stages 

 of the animal's evolution, according to its changing needs, consists 

 of a series of characters overlapping or built one upon the other into 

 a concrete whole. The work of the morphologist, therefore, is largely 

 one of analyzing compound structures, of separating them into their 

 component elements, and of determining the chronological order of 

 the evolutionary processes that have combined them. This he must 

 do both from a study of embryology, and by the use of his imagina- 

 tion, guided by a knowledge of comparative anatomy. As a conse- 

 quence, morphology is largely theoretical, and morphological theories 

 continually supplant one another as new facts throw new light on 

 the subject of animal structure. In the present pa]>er, some of the 

 older views on the structure of the insect thorax are discarded ; but 

 a base is taken from a later theory, combined with selections from 

 others, some new observations are added, and from the mixture the 



