476 PROCEEDINGS OF THE NATIONAL MUSEUM vol. no 



The epistoma is unsclerotized ; the pleurostoma is lightly sclerotized ; 

 the hypostoma is short and lightly or very lightly sclerotized; the 

 hypostomal spur and stipital sclerite are fused, the stipital sclerite 

 not extending laterally beyond its point of meeting with the hypo- 

 stomal spur; the labial sclerite is somewhat square in shape and the 

 ventral part is relatively broad; each maxillary and labial palp is 

 large and each bears two sensilla, one of which is relatively large; 

 the silk press is lightly sclerotized; the labral sclerite is absent; the 

 mandibles are relatively small and are triangular in shape; they are 

 very lightly sclerotized and are without teeth; the antenna is disc- 

 shaped ; in Alegastylus the closing apparatus of the spuacle is slightly 

 smaller than the atrium and adjoins the atrium; in Plectiscus the 

 closing apparatus of the spiracle is longer than the depth of the 

 atrium and is separated from the atrium by a length of trachea equal 

 to the length of the closing apparatus ; the skin bears small spines and 

 small projections. 



Larval Key 



1. Hypostomal spur and stipital sclerite as well sclerotized as labial sclerite. 



Megastylus 

 Hypostomal spur and stipital sclerite not as well sclerotized as labial sclerite. 



Plectiscus 



Subfamily Diplazoninae 



Figure 47 



Members of this subfamily are endoparasitic in Syi'phidae. They 

 oviposit into the egg or young larva of the host and emerge from the 

 puparium. The subfamily is an isolated group. 



The following were examined: Diplazon laetatorius (Fabricius) 

 (fig. 47a), D. orhitalis (Cresson), D. scutellaris (Cresson), Zootrephus 

 rufiventrus (Gravenhorst) (fig. 47b) (the head parts other than the 

 labium were damaged in this specimen), Syrphoctonus agilis (Cresson), 

 and aS. maculifrons (Cresson) (fig. 47c). 



The epistoma is unsclerotized; the pleurostoma is lightly sclerotized; 

 a small part of the hypostoma is very lightly sclerotized as is a small 

 part of the hypostomal spur; the stipital sclerite is absent; the labial 

 sclerite is well sclerotized and of characteristic shape with a ventral 

 spine-like projection; the maxillary and labial palps are present, but 

 setae appear to be absent from the maxillae and labium; there is 

 a faintly sclerotized area at the dorsal end of the labial sclerite which 

 may represent the silk press; the labral sclerite is absent; groups of 

 sensilla are present on raised areas of the labrum; the mandibles are 

 usually very lightly sclerotized although the blade is lightly sclerotized 

 in some species; the antenna was not seen in any cast skin and is 



