177] HEAD OF DIPTERA — PETERSOX 



INTRODUCTION 



The head and mouth-parts of Diptera offer a rich field for research. 

 A number of excellent studies have been made bj' several investigators 

 and the}' deserve careful consideration. A review of practically all the 

 literature shows that a majority of the workers have examined only one 

 or a few species. Meinert (1881) and Hansen (1883), however, studied 

 a number of forms, bvit they were mostly specialized species; wliile an 

 important study by Kellogg (1899) deals only with the families of the 

 Nematocera. Becher (1882) is the only investigator who has studied a 

 large series of generalized and specialized species. I have made a special 

 effort to secure as many generalized and specialized species as possible, 

 since it is highly desirable and essential in homologizing structures to 

 have at hand a wide range of species. 



Extensive studies have not heretofore been made, so far as I know, 

 on the head-capsule; consequently the important relationship which ex- 

 ists between the mouth-parts and the head-capsule in generalized insects 

 has not been traced in Diptera. This relationship is just as significant 

 in ascertaining the correct interpretation of the mouth-parts of Diptera 

 as it is in other orders. Its importance is illustrated by a study of the 

 head and mouth-parts of the Thysanoptera (Peterson, 1915). 



A review of the literature, Dimmock (1881) or Hansen (1883), 

 discloses the many and varied interpretations that have been given to 

 the mouth-parts of Diptera. To arrive at a correct interpretation of the 

 fixed and movable parts of the head, the head-capsule and mouth-parts 

 of all the species studied, irrespective of the established systematic 

 position of the species, have been carefully compared with the' head and 

 mouth-parts of generalized insects. On the basis of this comparison, 

 generalized, hypothetical types have been constructed for each fixed 

 and movable part. Each hypothetical type is made up by an accumu- 

 lation of all the generalized characters found among the Diptera, and 

 should show an intermediate stage between generalized insects and Dip- 

 tera. The use of such a hypothetical type is a great aid not only in 

 showing how the dipterous type has been developed, but also in deter- 

 mining the homology of the parts. 



The scope of this investigation makes it necessary to limit the dis- 

 <;ussions to the general subject of homology; consequently many details 



