171 



identifying those of many of the lower famlHes the absence or pres- 

 ence of the lateral abdominal spiracles is of considerable importance. 

 It is not to be expected that a rule will be discovered as to the absence 

 or presence of these spiracles which will be of invariable applicability 

 to all the families — as at present constituted — in the Nematocera, be- 

 cause we find, from data already in hand, that there appear to be 

 several exceptional genera within the group, and also because perip- 

 neustic larvae occur in much higher families. Taken in conjunction 

 with the structure of the head, however, it is quite possible that the 

 absence or presence of the lateral abdominal spiracles may prove to 

 point to the tracheal system as of primary importance in classification. 

 It must be borne in mind that many families as at present constituted 

 include genera whose larvae have widely different habits, both ter- 

 restrial and aquatic forms occurring at times in closely allied genera 

 and, rarely, within the same genera. In this connection it seems perti- 

 nent to indicate that our system of arrangement is based primarily 

 upon the characters possessed by the imagines, but investigators may, 

 without compunction, remedy errors in the system which, by an ex- 

 amination of the immature stages, are shown to exist. 



One of the principal characters cited by Brauer for the separation 

 of the Orthorrhapha from the Cyclorrhapha is the nature of the split- 

 ting of the larval skin when the pupa or adult emerges. In the former 

 the skin splits on the dorsum in the shape of a T or + ; whereas in 

 the latter — in which the skin hardens in the last stage to form an en- 

 closing puparium — the splitting is usually transversely across the en- 

 tire dorsum and venter, so that the anterior portion of the puparium 

 comes off cap-like ; or there may be also lateral splits that cause the 

 upper and lower halves of the anterior portion to separate. The lines 

 of dehiscence in Cyclorrhapha puparia are not accidental, but are 

 clearly provided for by weak parts in the membrane. The emergence 

 of the imago in Orthorrhapha is assisted by a slight swelling in the 

 thoracic region, and by movements of the developing wings and hard- 

 ening legs; in Cyclorrhapha the operation is facilitated by the dilation, 

 with air, of a bladder-like sac immediately over the antennae, which, 

 when swollen, forces apart the anterior extremity of the puparium 

 along the lines indicated by the thinner membrane. After emergence 

 the ptilinum, as the bladder-like structure is called, is retracted within 

 an aperture above the antennae and forms a pouch-like cavity. The 

 existence or absence of this peculiar structure in the imagines con- 

 stitutes the primary character for the separation of the two subor- 

 ders — Orthorrhapha being without and Cyclorrhapha with a ptilinum, 

 its presence being indicated by what is called the frontal triangle, im- 



