THE FLIES THAT CAUSE MYIASIS IN MAX 23 



or thorax. The bristles on or near the posterior margin of the seg- 

 ments are the marginal abdominals; those in a median position in re- 

 lation to the anteroposterior axis are the discal abdominals. Both 

 discals and marginals may occur either in isolated pairs or in rows; 

 there may be more than one pair or row. serially arranged, of discals 

 to a given segment. 



LARVA 



The typical museoid larva (fig. 6. A), such as that exemplified by 

 Sarcophaga. Musca, and GaRipJwra, is a cylindrical, headless, legless 

 maggot, usually broader on the posterior than on the anterior half, 

 strongly tapering anteriorly, and with a more or less truncate posterior 

 face. The two prominent posterior spiracles impress one unfamiliar 

 with maggot anatomy as being eyes, and for this reason earlier writers 

 have sometimes confused the anterior end of the larva with the 

 posterior. 



There are 12 apparent body segments, including the cephalic ( cs \ 

 and the anal segments (Sab), both of which, though apparently sim- 

 ple, are of a compound origin. The first or cephalic segment is short 

 and conical : ventrally, it is armed with a pair of strong mouth hooks 

 (mh). The first thoracic (prothoracic) segment (If ho) bears an 

 anterior spiracle (asp) on each side; this structure is typically tuber- 

 cular, its apex terminating in a varying number of processes each of 

 which ends in a spiracular opening. The anterior spiracle is nonfunc- 

 tional in the first instar. although it can sometimes be seen under the 

 integument. The cuticle of the thoracic and abdominal segments may 

 be bare ; usually, however, it bears rows or bands, either complete or 

 incomplete, or areas, of spines along the anterior ( spa ) or po>terior 

 i spp) margin, or both, of certain segments. The last abdominal seg- 

 ment ( Sab ) is typically more or less truncated posteriorly : usually 

 its posterior face is flush or convex, though sometimes there is a prom- 

 inent slit or depression in which the posterior spiracles are located (fig. 

 6. C). Above and below this depression, or. if it is absent, the area 

 which it would occupy, are a number of tubercles, typically 3 above 

 and 3 below on each side. To each side of the anus is a prominent anal 

 protuberance ( ap \ . 



The structure of the posterior spiracle (fig. r>. D) is of great im- 

 portance in larval taxonomy. The spiracle, or spiracular plate, usu- 

 ally has an outer strongly >clerotized area, or peritreme {per), which 

 may completely encircle the spiracle or may open ventrally. In a 

 ventral position there often occurs a rounded structure, the button 

 (bt) ; this may lie in the opening of the peritreme. or may be sur- 

 rounded by that structure, or may be completely within its inner 

 margin. The respiratory openings are the respiratory slits I si), with 

 three in the third-stage larva: these may be straight, curved, or sinu- 

 ous; occasionally one or more of the slits may be fragmented into two 

 or more parts. Sometimes there are breaks in the membrane between 

 the slits. The number of slits is usually considered indicative of the 

 larval stage, one for the first, two for the second, and three for the 

 third. In drawings it is customary to show both spiracles or to give 

 some indication as to their distance apart and relative position in re- 

 spect to each other, since these relations are often of taxonomic value. 



The internal skeleton of the head region (cephalopharyngeal skele- 



