46 COXXECTICUT GEOL. AND XAT. HIST. SXiRVEY [Rull. 



and I), a pre-coxal bricl,ge, /><", extends in front of the coxa, from the 

 , l)asisterniim, hs, to the episternuni, es. The furcasternum, or "ster- 



I nellum", is represented by the region labelled fs in the hippoboscid 



shown in Fig. 6, E^, but the fourth sternal sclerite, or spinasternum. 

 is not represented b}^ a distinct area in typical Diptera. A descrip- 

 tion of the sternal sclerites of the prothoracic region of such Diptera 

 as HipfoboHCd, M ijdas^ Tabanufi. Anisopus, Johamiscnomyia and Lhn- 

 nophila., is given in a paper hy Crampton (1926), in which these 

 sclerites are compared throughout the orders of insects ; and, as is 

 pointed out in the paper in question, the character of the sternal 

 sclerites furnishes some very conclusive evidence that the Diptera 

 were descended from ancestors extremely closel}' related to the Mecop- 

 tera. 



4. The mesothorax 



The enlargement of the mesothorax is accompanied by a length- 

 ening and uparching of the mesonotum, which atfect the upper region 

 of the pleuron as well, resulting in the displacement, and distortion, 

 of the dorsal plates of the pleuron in the higher Diptera. As the 

 mesothorax develops at the expense of the metathorax. the reduced 

 nietanotum shrinks away from the posterior region of the mesonotum 

 and exposes the postscutellum, which becoines a prominent feature 

 of the dorsal region, as it does in the neuropterous family Nemopter- 

 idae, in which the metathorax and hind wing are reduced in a fashion 

 somewhat suggestive of the precursors of the Diptera (see Fig. 10, 

 Plate 21 of paper by Crampton, 1931), The sternal region is not so 

 much affected by the distorting process as the other parts are, al- 

 though it is considerably affected by the modifications of the middle 

 coxae, since the sternum is folded together between them, wdien the 

 coxae become approximated in the mid-ventral region of the body, 

 and the lateral wrings of the sternum extend along t^he mesal surfaces 

 of the coxae to furnish the ventral pivotal points of the coxae in cer- 

 tain Diptera. 



The Mesonotum. The notum, or tergum, of any segment includes 

 the entire dorsal region of the segment (from the ridge of one phrag- 

 ma to the ridge of the following one), and the designation mesonotum 

 should therefore indicate the entire dorsal region of the mesothorax 

 (from the anterior margin of the prescutum to the posterior marginj 

 of the postscutellum), rather than the small portion of the true meso-^ 

 notum to which the designation is applied in the Diptera. As is; 

 the case in the wing-bearing segments of other insects, the mesonotum v 

 of the Diptera contains an anterior wdng-bearing plate, or eunotum . 

 ("alinotum" of Snodgrass, 1935), and a posterior plate behind the 

 wdng, called the postnotum /or postscutellum. psl of Fig. 6, A. The 

 anterior, wing-bearing plate becomes marked off into three principal 

 areas (Fig. 6, C) called the prescutum, psc, scutum, sc, and scutellum, . 

 si, while the postscutellum or postnotum, psh forms a fourth sclerite 

 of the dorsal series, although it has a different origin from that of the 

 first three subdivisions of the wing-bearing plate. 



