No. 64] DIPTERA OF CONNECTICUT: MORPHOLOGY Tl 



into the anterior region of their respective segments. In most of the 

 Orthorrhapha and the athericeran and acalypterate Cyclorrhapha, 

 the spiracles are found in the membrane between the tergites and 

 sternites, while in most of the calypterate Cyclorrhapha they occur 

 in the ventro-lateral region of the tergites which become enlarged in 

 these Diptera, although there are exceptions to this general rule. 



In many cases the occurrence of the spiracles is of considerable 

 i]nportancG for determining the segmental composition of the sclerites 

 around them, but a failure to realize that in some instances the sj)i- 

 racles may shift without reference to the adjacent sclerites has resulted 

 in so much confusion in the attempts to identify the neighboring 

 sclerites by the presence of the spiracles, that, at the risk of seeming 

 to be unduly repetitious, attention should be called to this fact again, 

 in discussing the occurrence of the spiracles in the abdominal region. 



The occurrence of two pairs of spiracles in the basal abdominal 

 tergite clearly indicates that the first two spiracle-bearing tergites 

 have united to form the apparent first tergite, since a study of a 

 series of intermediate stages (which is the most reliable means of 

 interpreting the parts correctly) clearly proves that a union of the 

 first two spiracle-bearing tergites has taken place. On the other 

 hand, the occurrence of two pairs of spiracles in the sixth abdominal 

 tergite of such female Diptera as the one shown in Fig. 9a, D, does 

 not indicate that this tergite is formed by the union of two spiracle- 

 bearing tergites (the sixth and seventh), as some investigators main- 

 tain, since a studj^ of the transitional forms shown in Fig. 9a clearly 

 indicates that the spiracles of the seventh abdominal segment have 

 merely migrated forward to join those already present in the sixth 

 abdominal tergite. 



Since the spiracles are typically borne in or near the ventro- 

 lateral region of their corresponding tergites, their occurrence should 

 be of some value for determining which sclerites are tergites, and 

 which are sternites, when the sclerites become distorted and displaced 

 in the torsion process which occurs in the males of the higher Diptera ; 

 but here again, extreme caution must be used in attempting to identify 

 the sclerites by their relation to the spiracles, since in some cases the 

 spiracles appear to shift without reference to the sclerites with which 

 they are normally associated. Thus, for example, in males of the 

 anthomyid Hylemyia antiqua (shown in Fig. 13, D) the sinistral 

 spiracle of the sixth segment lies in the membrane in front of the 

 pregenital plate (labelled 7s and 85 in Fig. 13, D), while the sinistral 

 spiracle of the seventh segment is borne in the anterior region of 

 the pregenital plate. In males of the dexiid Theleira nigripes, on the 

 other hand, both of these spiracles (belonging to the sixth and seventh 

 segments) lie in the membrane in front of the pregenital plate, while 

 in certain tachinids such as Gylindromyia hinotata (or in the antho- 

 myid Fannia canicularis) both of these spiracles are borne in the 

 pregenital plate itself. 



Although the spiracles of the sixth and seventh abdominal seg- 

 ments apparently shift about in such instances as these, the occurrence 



