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MISC. PUBLICATION 631, U. S. DEPT. OF AGRICULTURE 



beyond the point of its separation from r 2+3 (ef. figs. 4 and 13). In 

 certain acalypterate muscoids the costa may be broken near the 

 humeral cross vein or near the apex of vein r x , or in both places. In 

 some cases a strong costal spine (cf. fig. 13) occurs near the apex of 

 vein sc. Certain modifications of the wing membrane, such as the 

 development of macrotrichia, or spurious veins or folds,' may occur; 

 those pertinent to this work are treated in the proper place in the text. 

 The wing articulates with the body on several sclerites (fig. 5) , three 

 of which are referred to in the descriptive part of this work. At the 

 base of the wing, along the costal margin, are two scalelike sclerites, 

 the epaulet (epl), which is setulose and situated next to the body, 

 and the basicostal scale (bcs), which is bare and situated next to the 



base of the costa. The subcostal 

 sclerite (sscl) , best seen from the un- 

 der side of the wing, lies at the base 

 of the subcosta and radius. Each 

 wing is connected basally with the 

 thorax by the squamae or calypteres 

 which are best developed in the 

 higher muscoids and in a few isolated 

 groups such as the Tabanidae. There 

 are two squamae associated with each 

 wing, an upper or alular squama 

 (asq) and a lower or thoracic squama 

 (tsq). 



The leg consists of five well-defined 

 parts — the coxa, trochanter, femur, 

 tibia, and tarsus. The tarsus in most 

 Diptera is five-segmented; the basal 

 segment, or basitarsus, is usually the 

 longest and is sometimes enlarged. 



The number of visible abdominal 

 segments in all the Diptera is greatly 

 reduced from the theoretically prim- 

 itive number of 12. The usual num- 

 ber in the muscoids, aside from those 

 forming the genitalia, is 4 or 5 ; these 

 are called the pregenital segments, 

 and, when the number is 4, the second 

 and third are designated as the intermediate segments. A typical 

 abdominal segment consists of 2 parts, a tergite (dorsal sclerite) 

 and a sternite (ventral sclerite). In the higher Diptera the lateral 

 plates (pleurites) that occur in many of the more primitive insects, 

 as well as in a modified form on the dipterous thorax, have disap- 

 peared; as a matter of fact, in the muscoid flies a tergite usually 

 extends over the sides of the abdomen, and its margins may meet 

 ventrally, thus partly or even completely obscuring the sternite. 

 The genital segments, or genitalia, are much used in the taxonomy of 

 certain groups of flies, but their use usually requires the spreading 

 and the careful examination of the parts. In the males the genital 

 segments form a structure, often somewhat bulky, called the hypo- 

 pygium ; it ends in a pair of posterior forceps. 



In the muscoids the arrangement of the bristles of the abdomen is of 

 some taxonomic importance, but not nearly so much as on the head 



Figuee 5. — Structures at the wing 

 base of a muscoid fly : A, Dorsal 

 surface; B, ventral surface of 

 costal region. For explanation 

 of abbreviations, see page 22. 



