42 THE CANADIAN ENTOMOLOGIST 



with it; these together form a basis for the Group-unit ExoristicE. 



The eleven family types indicated in the diagram farther on 

 show pronounced adult characteristics which cause them to stand 

 forth prominently, as in bas-relief, from the mass of the Mus- 

 coidea. These characteristics are reinforced by valuable char- 

 acters drawn from the eggs, early stages and reproductive system. 

 But the limits of the groups which these eleven family types re- 

 present are often greatly obscured in the external anatomy of 

 the adult, and it is the function of the egg,^ early-stage and re- 

 productive characters to clear up this obscurity in all cases. This 

 is the first time in the history of muscoid taxonomy that we have 

 had the means of definitely segregating these various families and 

 accurately determining their limits, and they may well be main- 

 tained now on the sum-totals of their respective characters. But 

 in synoptic treatment, as will appear later, these family groups 

 are unwieldy and do not aid us as such, though their divisions 

 may be employed as leading directly to the group-limits. 



The characters of the facial plate apparently continue to hold 

 better in the main for the indication of family types than do those 

 of any other single external adult structure. They become sub- 

 ordinated to the characters of the female reproductive system 

 and early stages occasionally, as in the case of the Sarcophagidae 

 and Dexiidae, but this is in accordance with the well-known law 

 of contraction of values, and the consequent fluctuation of charact- 

 acters, which cannot hold for all groups. They are reinforced 

 by various other external adult characters in the several groups. 

 Where they fail from lack of differentiation to mark off other- 

 wise prominent groups, the characters of the first and second 

 categories are always available. The following scheme of deriva- 

 tions illustrates well, reading the group from left to right, the 

 successive retrograde modifications of the facial plate that 

 appear to have taken place in these flies. 



(1) Phasid stem (Facial plate remains wide and elongate). 



(2) Muscid (3) Exoristid (4) Hystriciid (5) Masiceratid stem 

 (Facial plate shortens but remains wide). 



(6) Phasiopterygid (7) Dexiid stem (Facial plate shortens fur- 

 ther and becomes constricted below). 



(8) Sarcophagid stem (Facial plate in the typical stock very 



