72 COXXECTICUT GEOL. AND NAT. HIST. SURVEY [Bull. 



of tlu- sevonth spiracles in the pretrenital plate (e.g.. that labelled 

 T.s- and 8.s in Fig. 13, D) does indicate that the seventh segment par- 

 ticii^ates in the formation of the composite pregenital plate, since a 

 stiidv of a series of transitional stages clearly indicates that the 

 seventh segment enters into the composition of the pregenital plate: 

 and it is also possible that the marginal area immediately dorsad of 

 the seventh spiracles is formed by the reduced seventh tergite, al- 

 though this is not certain. Furthermore, it should be noted that the 

 ])i'esence of the seventh spiracles in the pregenital plate of male Cy- 

 clorrliapha such as those shown in Fig. 13, D, or Fig. 14, B (where 

 the ])regenital plate is labelled 7s and 86-) makes it imjjossible to 

 interpret the entire pregenital plate as the eighth segment alone, as 

 is done by some investigators, since tliere are no spiracles in the eighth 

 abdominal segment of any male Diptera whatsoever, and the presence 

 of the seventh spiracles in this case indicates that the seventh seg- 

 ment (as well as the eighth) enters into tlie composition of the 

 })regcnital plate of these Diptera. 



The doAvnward shifting of the dextral spiracles of the sixth and 

 seventh abdominal segments into the ventral region of these segments, 

 and ou around into the insect's left side, in such male Diptera as 

 those shown in Fig. 12. D and H, enables one to visualize the tremen- 

 dous amount of distortion and displacement which has occurred in 

 tho neighboring sclerites in these insects, and in such instances as 

 these, the positions assumed by the spiracles reveal the extent of 

 the torsion process which has occurred in these insects. On the other 

 hand, the final position of the spiracles may conceal rather than reveal 

 tlie antecedent distortion and displacement of the parts, as is the case 

 in the males of such higher Diptera as 7'richopoda pennipes, in which 

 the torsion process has apparently continued until a more or less 

 comi)lete circumversion has taken place, and a final re-alignment of 

 the sclerites and spiracles results in a deceptively symmetrical ar- 

 langement of the parts outwardly, although a loopmg up of the 

 ejaculatory duct over the top of the hindgiit reveals the extent of the 

 preceding torsion internally. 



While instances such as these indicate the danger of depending 

 too nuich on the evidence of tlie spiracles alone, in attempting to in- 

 terpret the homologies of the sclerites about them, this does not 

 detract from the value of the evidence of the spiracles when tested 

 by that from other sources as well, and for this purpose the evidence 

 furnished by a study of a series of intermediate forms is of the ut- 

 most value for interpreting the sclerites of the terminal abdominal 

 segments of the higher Diptera in general, and of male Cyclorrhapha 

 ni particular. 



The Segmentation of the Ahdomen. A study of the embryology 

 of the orthopteroid insects, which are the nearest living represent'a- 

 tives of the ancestors of the Holometabola, indicates that the abdom- 

 inal region is primitively composed of eleven true segments (of which 

 the eleventh bears a pair of cerci), followed by a non-segmental, anus- 



