No. 64J DIPTERA OF CONNECTICUT: MORPHOLOGY 69 



likewise enables us to be certain that a torsion has occurred in this 

 direction^ and indicates the path of displacement followed in the 

 shifting- of the seventh and eighth sternites which occurs in the 

 males of the higher Diptera. 



The fact that the spiracles are borne within, or just below, the 

 lateral borders of the tergites enables us to determine which sclerites 

 are tergites, and which are sternites, when there has been a marked 

 displacement and distortion of the parts in male Cj'clorrhapha ; and 

 the displacement of the spiracles, particularly the downward shift 

 of the dextral spiracle of tlie seventh segment (or even that of the 

 sixth segment) into the sternal region — or even on around into the 



I lower region of the insect's left side (see Fig. 12, H) , serves to indicate 

 the extent of the torsion of the parts in these Diptera. The distor- 

 tion and displacement of the accompanying tracheae is likewise of 

 value for indicating the nature of the torsion process, and furnishes 

 important evidence that a left-to-right torsion has taken place in the 

 seventh and other abdominal segments, although the condition exhibit- 

 ed by the spiracles and their tracheae is of little value for determin- 

 in,g the nature of the torsion of the parts behind the seventh segment, 

 since the latter is the last spiracle-bearing segment in male Diptera. 



The spiracular openings are features of prime importance for 

 determining the nature of the tergites in the basal region of the 

 abdomen, since the corresponding spiracles remain distinct when the 

 first two tergites which originally bore them become more or less 

 completely united to form a syntergite, or composite apparent first 

 tergite in certain Diptera. The spiracles are not infallible indices 

 of segmental fusion, however, since the spiracles of the seventh 

 abdominal segment may shift forward into the sixth abdominal 

 segment (which then bears two pairs of spiracular openings) in the 

 postabdomen of certain female Cyclorrhapha, althougli the sixth 

 abdominal segment is not composite in such cases. (See Fig. 9a,B,C 

 and D.)* 



The occasional persistence of the spiracles, when the segments 

 which originally bore them become membranous and the segmental 

 boundaries become obliterated, is a feature of some value for deter- 

 mining the segmental composition of the abdominal region in some 



' of the highl}^ specialized Diptera. Althougli the location of the 

 spiracles does not furnish us with accurate information concerning 

 the exact boundaries of the segments in such cases, it is at least 

 evident that the segmental boundaries must lie between the areas 

 occupied by the spiracles in these Diptera. 



*In males of the anthomyid Fannia canicularis, the left spiracle of the sixth abdom- 

 inal segment migrates back into the synsternite (.composed of the lateroverted sev- 

 enth and inverted eighth sternites) behind it, so that the systernite bears two sinistral 

 spiracles (those of the sixth and seventh segments), and a somewhat similar condi- 

 tion occurs in the male of the calliphorid Callipliora erythrocephala. In most male 

 tachinids the sinistral sixth spiracle lies in the lateral membrane, but in some tachi- 

 nids it migrates back into the synsternite ( composed of the seventh and eighth ster- 

 nites) behind it, which also bears its own sinistral spiracle (that of the seventh seg- 

 ment), and in such instances the presence of the left spiracle of the sixth abdominal 

 segment in the synsternite does not indicate that the sixth abdominal segment enters 

 into the composition of the synsternite. 



