40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



tain other groups, as the Hystriciinae, a different type of them is 

 correlated with a true spinose development of the abdominal macro- 

 chaetae. The cephalic bristles are uniformly present in the super- 

 family, though sometimes weakly developed, whether the abdominal 

 ones are present or absent. The same is usually true of those of the 

 thorax and scutellum. The function of the macrochaetae and bristles 

 of the abdomen is doubtless tactile. They are capable of movement 

 in life. 



In most insects the antennae, and to a less extent the palpi, are the 

 main seat of the tactile sense. The cyclorrhaphous Diptera, however, 

 have the antennae so modified as to preclude this function. It is 

 probable that the vibrissa? are functionally tactile, and the frontal 

 and vertical bristles as well. The vibrissa? project straight out in 

 front near the ends of the ptilinal suture, and naturally serve as 

 anterior tactile organs for the protection of the lower portion of the 

 head. Likewise the frontal bristles serve as anterior and superior 

 cephalic, and the vertical bristles as superior and posterior cephalic 

 tactile organs. The fact that the vertical bristles are almost invaria- 

 bly stronger and longer than the frontal bristles strengthens this 

 view. The inner vertical bristles correspond in development to the 

 vibrissa?. 



The macrochaetae of the thorax, scutellum, and abdomen serve as 

 lateral and dorsal tactile organs, those of anal and preanal segments 

 always being the strongest of the abdomen and those of scutellum 

 the strongest of the thorax. The scutellar are doubtless the main 

 dorsal tactile organs, and the anal the main posterior ones. The 

 abdominal macrochaetae, when dense and of spinose character, pos- 

 sibly serve also as a defense against insectivorous animals, as in 

 Dcjcania, Parade jeania, Bombyliomyia, Hystricia, Hystrichodexia, 

 and others. 



The macrochaetae, especially those of the abdomen, constitute the 

 most recent form of specialization in the Myodaria, and are especially 

 characteristic of the Muscoidea. As such, and considering further 

 their probable functional character as tactile sense organs, those of 

 the abdomen at least can not be expected to furnish valid characters 

 for the separation of higher categories in these flies than species, 

 genera, and at most tribes. 



The macrochaetae of the head, thorax, and scutellum appear to be 

 of far longer standing than those of the abdomen. With the excep- 

 tion of most of the (Estridae, they are present not only in all Mus- 

 coidea, many of which lack abdominal macrochaetae, but also in prac- 

 tically all of the Myodaria except the CEstridae already named and 

 Conopidae, which two families stand well apart from the other Myo- 



