THE FLIES THAT CAUSE MYIASIS IX MAX 19 



or several accessory vibrissae near the bases of the true ones. The 

 parafacial often bears one or more bristles. There is usually one, some- 

 times two. pairs of ocellars {orb) on the ocellar triangle; the vertex 

 may have an inner (ivr) and an outer (ovr) pair. The bristles most 



commonly occurring on the frontal vitta are a pair of cruciate frontals 

 (cfr) directed inward and crossing each other. The bristles of the 

 parafrontals are highly important. They consist of the frontals (ft I . 

 a row of inwardly directed bristles next to the margin of the frontalia : 

 the proclimate fronto-orbitals (pfro >. usually two but sometime- more 

 or less, and directed forward; and the reclinate fronto-orbitals ( rfro ). 

 directed backward. 



The dipterous antenna is variable in form and show- many gradu- 

 ations from the long many-segmented filiform or moniliform antenna 

 of some nematocera to the highly specialized three-segmented antenna 

 of the muscoid. In the three-segmented type the third segment bears 

 on its dorsal surface and usually near its base a bristlelike arista I ar) 

 which, upon careful examination, may be seen to be composed of two 

 or three segments, an indication of its composite structure as a remnant 

 of the missing terminal segments of the antenna. The form of the 

 arista is of considerable taxonomic value: it may be bare, pubescent, or 

 pectinate or plumose part way or to the apex. In the many-segmented 

 antenna the segments beyond the second are often fu>ed. forming an 

 apparently annulated third segment: in some cases, the true third seg- 

 ment is much larger than those that follow, the latter located at the 

 apex of the third to form a style. 



The proboscis consists of three parts — the rostrum (rs)\ the haus- 

 tellum ( hs ) . and the labella (lb). The rostrum bears a pair of sensory 

 palpi (pip ). Flies that take no food in the adult stage may have the 

 proboscis reduced to rudiments ( cf . figs. 52 and 61) ; in the others it 

 may be modified in accordance with the food habits. Bloodsucking 

 species usually have a long haustellum and small or rudimentary 

 labella. or the mouth parts are in other ways modified for piercing and 

 for sucking blood; other species that are flower feeders might have an 

 elongated haustellum with small labella. 



The mesothorax. or intermediate thoracic segment, is greatly devel- 

 oped at the expense of the other two segments. The sclerites of the 

 prothorax which are most clearly evident are the unpaired presternum, 

 which lies between the anterior coxae and may be bare, hairy, or bristly, 

 and the paired propleura (ppl), which are usually bare except on the 

 lower part, but which may be hairy. The mesothorax (fig. 2) is a 

 rather complicated structure. The areas used in the descriptive mat- 

 ter that follows can best be located by means of the illustrations. On 

 the dorsal side, above the wing bases, are the paired humeri ( km ). the 

 mesonotum {m,snt) which is divided into two parts by the mesonotal 

 suture ( 8Ut), the paired notopleura ( n [>1 ) and postalar calli ( pac ) . the 

 scutellum (scut), and the postscutelluni (pscut). The pleural areas 

 on each side below the wing base are the mesopleuron (mspl), the 

 sternopleuron (stpl), the pteropleuron (/>//;/). and the hypopleuron 

 (hpl). The metathorax. like the prothorax. is greatly reduced, but a 

 pair of rudimentary wings, the halteres (hoJf). arise one from each 

 metapleuron (mtpl). The prothorax lacks spiracles; there is. how- 

 ever, a mesothoracic pair (mss) located between the propleura and 

 the mesopleura. and a metathoracic pair (mts) between the hypopleura 

 and the metapleura. 



