286 w. wesch£ on the proboscis of the blow-fly, 



while on the under side are lateral rods which reach from the 

 base of the labium to the bases of the labella (Fig. 7). In 

 Leptis conspicua, Mg., which possesses the blades of the maxillae, 

 and has only lost the mandibles, two rods are present on the 

 under side exactly as in C. dorsalis (Fig. 8). In a fly nearly 

 related to L. conspicua, Chrysopilus auratus, F., the lower plate 

 (mentum) is present, exactly as in the Blow-fly, but with this 

 structural difference, that two rods have been incorporated into 

 it, these being fused in the median line and sending out forked 

 processes at the anterior end (Fig. 9). 



In a numbar of families related to these older types, a similar 

 structure of the mentum will be found. In the Stratiomyidae, 

 Bombylidae, Empidae, Dolichopodidae, Phoridae, Lonchopteridae, 

 and Platypezidae, .practically all the species examined exhibit an 

 arrangement of the parts similar to that which exists in 

 Chrysopilus. All these families agree in this particular, and all 

 differ from the Blow-fly. 



We can now see that the differences in the proboscis in these 

 two sections have been brought about by the incorporation of 

 the mandibles into the structure of the labium ; in the Blow-fly 

 they appear to be embedded on the dorsal side of the labium, 

 and are represented by the two stout lateral rods previously 

 referred to. In the older families we find species which have the 

 mandibles (1) fused together and situated in the median line on 

 the ventral side of the labium, as in Tipula or Bibio, (2) present 

 as lateral rods, as in Leptis or Chironomus, or (3) fused into the 

 mentum, as Chrysopilus or Dolichopus and the majority of the 

 Brachycera. 



The larvae of the huge family of the Muscidae, to which the 

 Blow-fly belongs, escape from the pupa-case through a circular 

 opening, whilst all the others mentioned, with the possible ex- 

 ception of the Platypezidae, escape through a T-shaped opening. 

 The character of the mentum follows the character of the pupa- 

 case, which differentiates these flies as belonging to two great 



