the genitalia of Insects as guides in Pliylogeny. 303 



C C-^). Telescopic and with cliitiniscJ joints as in 



Ortalidc€. 



Conelusion. I have already alluded to the difficulty of 

 estimating a position in a systematic list brought about by 

 the contradictions of characters, an archaic being contra- 

 dicted by what we miglit call a late specialisation. But 

 search will usually reveal an overwhelming balance on one 

 side or the other, and it is usually the archaic character 

 that stands. A comparison of two well-known flies will 

 illustrate my point. The mouth parts and the male 

 genitalia of Callij}]iora erythroccplhala, Mg., and Scatopliaga 

 stercoraria, L., are absolutely homologous part for part, 

 and quite close together in all respects. But C. erytliro- 

 cepliaJa has a telescopic ovipositor extended by rods, while 

 in S. stercoraria it is non-telescopic, Seatopliaga is un- 

 doubtedly the older type, and the wide space between the 

 eyes of the male (Williston's dichoptic), the small calyptra, 

 and the open first posterior cell of the wing confirm the 

 older type of ovipositor. 



To take a more difficult case, the Phorid Trineura 

 atcrrima, F., has an ejaculatory apparatus in the male 

 genitalia, usually only found in the Muscidas. Against 

 this, the species has dichoptic eyes, D type of ovipositor, 

 no ptilinum, and mouth parts which have characters in the 

 labium and mentum only found in the Brachycera and 

 Nematocera, and a sense organ on the palpi practically 

 exactly similar to that found in the Nematocerous Dilo- 

 phus. The evidence is overwhelming that Trineura has 

 no place, even in the older families of the Muscida^. We 

 get collateral evidence when we find that other Phorids 

 are without the ejaculatory sac and apodeme, and that the 

 Pipunculid Chalurus spu.rius, Fin., together with a 

 styliform penis, has a similar ejaculatory apparatus. 



1. From these observations it may be assumed that 

 those characters both of the male and the female genitalia, 

 which are found in the Nematocera, when they can be 

 recognised in other suborders, are the more reliable as 

 guides in Phylogeny. 



2. That a similarity of arrangement, when very ex- 

 ceptional and aberrant from the usual type, may also be 

 relied on, 



3. It can also be assumed that though genitalia cannot 

 invariably be relied on to solve problems in Phylogeny, 

 the evidence they afford is valuable and must be carefully 



