42 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [Bltll. 



Hendel (1928), Curran (1!)34), and others. Tillvard (1919) ha< 

 greatly modified Comstock's interpretation of the dipterous venation, 

 and further modifications proposed by Alexander (1927-1929), Hendel 

 (1928), Vignon (1982), and others "have almost completely changed 

 Comstock's original interpretation of the venation in the Diptera. 



The embryological deA^elopment of the Diptera (like that of all 

 other insects as well) indicates very clearly that there are only three 

 segments in the dipterous thorax (i. e., the pro-, meso- and metatli«>- 

 rax) ; and until it can be proven that the thorax is composed of more 

 than three segments in the embryonic stages, it is futile to speculate 

 concerning the supposed occurrence of additional segments in the 

 thorax of adult Diptera and other insects, as is done by Feuerborn 

 (1925) and others. A comparison of the thoracic sclerites of the 

 Diptera with those of a series of more primitive forms leading back 

 through the Mecoptera, and nemopterid Neuroptera. to the Sialidae. 

 and through these to the Isoptera. Plecoptera. and other orthopteroid 

 insects, very readily reveals the purely secondary character of the 

 various grooves, etc., which have been interpreted as marking off an 

 intercalary segment in the Psychodidae, and other Diptera, by Feuer- 

 born. 



The prothorax and metathorax are extremely reduced in the 

 Diptera, while the mesothorax is enormously enlarged to accommodate 

 the powerful muscles of flight operating the mesothoracic wings, which 

 are the only functional ones, the metathoracic wings having been 

 modified to form the halteres (which are still Aving-like in certain 

 mutants of Droso'phila). The displacement and distortion of the 

 parts (due to the unequal development of the thoracic segments), and 

 the formation of new sutures, and the re-combination of certain scle- 

 rites, make it veiy difficult to interpret the sclerites in some instances; 

 but the use of certain more or less fixed features may be of value for 

 determining the location, and extent, of some of the regions of the 

 thorax. 



1. Landmarks of the thoracic region 



The position of the spiracles (which arise in the anterior region 

 of the mesothorax and metathorax in the embryonic stages) is a^ 

 feature of some value for determining the posterior extent of the I 

 pronotum, 2)P^^i which extends posteriorly to the mesothoracic spiracle, 

 sp, (located just behind the sclerite labelled pjyn in Fig. 6, A. C. E, 

 etc.). and for determining the anterior limits of the metapleuron. 

 which extends forward to the general region of the metathoracic spi- 

 racle, sp^ (situated anteroventrad of the base of the halter, h). 



The region of the attachment of the wing (Avhich indicates the 

 line of division between the terguui and pleuron), and the region of 

 the attachment of the leg, are features of importance for indicating 

 the course of the pleural suture, ps of Fig. 6, A, C. etc., which ex- 

 tends between the episternum and epimeron, from the dorsal articu- 

 lation of the coxa, cx^ to the pleural fulcrum of the wing (above the 

 sclerite labelled sf), no matter how much it may be distorted by the 



