98 CONNECTICUT GEOL. AND NAT, HIST. SURVEY [Bllll. 



along the anterior margin of the inverted eighth sternite 8s in Fig. 12, 

 E, suggests that a narrow vestigial seventh tergite, bearing the seventh 

 spiracle, may have fused along the anterior margin of the inverted 

 synsternite. when the seventh sternite united with the eighth sternite 

 to form the synsternite bearing the labels 7-s^ and 85 in Fig. 13, D, 

 although the seventh tergite may simply have disappeared in the 

 latter instance. 



In the pyrgotid shown in Fig. 12,1, the eighth sternite 8s shows 

 onl}^ a slight tendency to become inverted, and still occupies a ventral 

 position in line with the slightly displaced seventh and sixth ster- 

 nites 7s and 6s, showing that it is a sternite, not a tergite. The eighth 

 sternite is hugely developed and its tergite is lost, thus clearly indicat- 

 ing that the similar large sclerite labelled 8s in the syrphid shown in 

 Fig. 12, F, is the eighth sternite and not the eighth tergite. 



The members of the key group Sj^rphidae, occupying a position 

 at the base of the lines of descent of the lower Cyclorrhapha, furnish 

 a fairly complete intra-syrphid series or series of forms extending 

 from the less modified to the rather highly modified types of Syr- 

 phidae and furnish the key for interpreting the modifications of the 

 sclerites occurring in the higher Cyclorrhapha. In such syrphids as 

 Heliophllus chaligosa^ the huge eighth sternite has not shifted as far 

 out of the sternal region as it has in the pyrgotid shown in Fig. 12, 1, 

 and the eighth tergite becomes vestigial in Ileliophilus^ thus clearly 

 indicating that the large laterally displaced sclerite labelled 8s in the 

 syrphid Sericomyia, shown in Fig. 12, F, is a sternite, and not a ter- 

 gite— and the torsion would have to be from right to left (instead 

 of being from left to right, as the looping up of the ejaculatory duct 

 ej in the syrphid shown in Fig. 14, H, indicates) to cause a tergite 

 to turn over into the insect's left side, and occupy the position as- 

 sumed by the sclerite labelled 8s in Fig. 12, F, of SeHcomyia.. The 

 seventh tei-gite 7t is hardly displaced in Sericomyia (Fig. 12, F) and 

 its spiracle occupies the normal position in relation to the tergite (as 

 do the spiracles of the preceding segments) but the seventh sternite 

 7s of Sericomyia is slightly lateroverted, or shifted up into the in- 

 sect's left side, thus tending to follow the migrating eighth sternite 8s, 

 as it does more pronouncedly in other Cyclorrhapha? 



It is important to note that the dextral spiracles are shifted 

 down into the sternal region in the syrphid Mesograpta, shown in 

 Fig. 12, D, thus indicating that there has been a profound left to 

 right torsion of the parts, resulting in the shifting of tlie seventh 

 sternite 7s up near the large lateroverted eighth sternite 8s in this 

 syrphid, as is also the case in the syrphid shown in Fig. 14, H; and 

 tlie sixth sternite 6s is also very asymmetrical in these syrphids sug- 

 gesting that a partial suppression may precede the torsion process. 

 Ihe partial "circumversion", or "strophe", of the ninth tergite con- 

 tinues around as we pass from Fig. 12, F, to Fig. 12, D. and Fig. 14, 

 H ; and in the highly modified syrphid Paragtis, shown in Fig. 13, A, 

 the '^circumversion" has progressed almost as far as it has in the cae- 

 lopid shown in Fig. 12, H. 



