44 THE CANADIAN ENTOMOLOGIST 



indicate that the phasiid stocks are much older than the oestrid 

 and cuterebrid stocks. 



Other facts point to the same conclusion. The specialization 

 toward partial and complete atrophy of the mouthparts in the 

 oestrid stocks, toward partial atrophy of the same in the cutere- 

 brid and megaprosopid stocks and toward antennal reduction 

 in these and kindred stocks indicates that the extreme shortening 

 and constriction of the facial plate are connected with a more or 

 less complete loss of mouth and antennal functions. Certainly 

 this is comparatively recent specialization, for the primitive stocks 

 must have had highly functional mouthparts as well as high an- 

 tennal development. 



Facial plate reduction has probably followed antennal and 

 mouth reduction. In other words it is a consequence of loss of 

 nutritive and olfactory functions in the fly, and thus marks an 

 extreme stage of parasitism and host-adaptation, particularly 

 one in which the sexes may easily find each other, in which the 

 female may easily find the host, and in which the maggots may 

 easily store a large food-supply. The two muscoid stocks which 

 are apparently of most recent evolution, the Masiceratid and 

 Hystriciid, in which the mouthparts and antennae are both still 

 highly functional and the facial plate in consequence still retains 

 its full development, have much less perfect host-relation, sex- 

 relations and food-supply conditions. They must search as- 

 siduously for their hosts; the large fecundity which is necessary 

 to their peculiar host-relations demands extensive feeding in the 

 adult female, especially as she has not an unlimited food-supply 

 during her larval life; and the necessity for feeding and host- 

 searching makes the female a wanderer, whose discovery by the 

 male calls for well-developed olfactory organs. 



The comparison of Cobboldia with other types shows con- 

 clusively that pharyngeal atrophy (atrophy of pharynx and rostrum 

 of proboscis, and not necessarily of haustellum or palpi, with more 

 or less complete closure of pharyngeal cavity) is directly cor- 

 related with the evenly receding and gently-convex profile of the 

 facial plate and peristomalia, and the consequent more or less 

 complete recession of the epistoma; further that the great shorten- 

 ing of the clypeus is primarily dependent on and thus directly 



