DIPTERA 853 



Subsection I— THE ACALYPTRAT^* 



The Acalyptrate Muscids 



In the families included in this subsection of the Myodaria the 

 alulae or calypteres are always small or rudimentary; the subcostal 

 vein is often indistinct or vestigial, but well preserved in some forms; 

 vein Ri is shortened and is often very short; the thorax is without a 

 complete transverse suture; the posterior callus is usually absent; 

 and the abdominal spiracles, with some exceptions, are in the con- 

 junctiva?. The flies are usually small or very small, they are never 

 large. 



The subsection Acalyptratae includes many families, twenty- 

 three of which are represented in our fauna. Some of these families 

 include well-known species that have attracted attention on account 

 of their economic importance or for other reasons; but most of the 

 families have been studied comparatively little in this country. 



Family CONOPID^ 

 The Thick-headed Flies 



With the members of this family the head is large, being broader 

 than the thorax. The body is more or less elongate ; it may be naked 

 or thinly clothed with fine hair, but it is rarely bristly. 



The ocelli may be either present or absent. The antennse are 

 prominent and project forward; they are three-jointed; and the third 



Fig. 1 107. — Wing of Physocephala affinis. 



segment bears either a dorsal arista or a terminal style. The radial 

 sector is only two-branched (Fig. 1107); veins R4+5 and Mi -1-2 end 

 near together or coalesce at their tips. The medial cross-vein is 

 present. Vein M3 coalesces with vein Cui for nearly its entire length. 

 Veins Cuj and 2d A coalesce at their tips, and sometimes for nearly 

 the entire length of vein Cu2. 



*Acalyptratse: a (d); without; calypter (koKvitti^p), a sheath. 



