DIPTERA 779 



CH^TOTAXY 



OR THE ARRANGEMENT OF THE CHARACTERISTIC BRISTLES OF DIPTERA 



In certain families of the Diptera some of the setas with which the body is 

 clothed are stout bristles, termed macrochcEtcE. In the classification of these 

 families much use is made of the number, position, and arrangement of these 

 bri&tles. This has made necessary the establishment of a set of terms by which the 

 different bristles or sets of bristles can be designated. Such a terminology was 

 proposed by Osten-Sacken in 1881, and is still in use with additions. 



In the choice of terms Osten-Sacken and later writers have used those that in- 

 dicate the places of insertion of the bristles. But owing to the fact that the homol- 

 ogies of the sclcrites of the head and thorax had not been definitely determined at 

 the time Osten-Sacken wrote he proposed a "purely conventional terminology" 

 for the areas upon which the bristles are inserted ; and in this he has been followed 

 to the present time. The result is that some of the terms are misleading; as for 

 example, the so-called frontal bristles are not inserted on what is really the front 

 but on the vertex. But the use of these terms is so firmly established that it is not 

 probable that they will be changed. In the following account I have endeavored 

 to indicate the homologies of the parts named in those cases where the terms 

 applied to them differ from those used in accounts of other orders of insects, and 

 which are defined in Chapter II. In defining the special terms used by writers on 

 chsetotaxy I have made free use of the definitions given by Osten-Sacken ('81), 

 Hough ('98), Williston, ('08) and Walton ('09). 



THE PARTS OF THE HEAD 



The homologies of the areas of the head were determined b}'' Peterson ('16) 

 who studied and figured the heads of representatives of nearly all of the families 

 of Diptera found in our fauna, and who gives a diagram representing a hypothetical 

 type of the head-capsule of Diptera (Fig. 998). The conclusions of Peterson are 

 based on comparisons of heads of the more generalized Diptera with those of the 

 more generalized members of other orders of insects; for descriptions of the 

 latter see above, pages 37-40 and 96-97. 



The more important landmarks for determining the homologies of the areas of 

 the cephalic aspect of the head, the region in which the greatest confusion exists, 

 are the stem of the epicranial suture (Fig. 998, .y. e. s); the arms of the epicranial 

 suture (Fig. 998, a. e. s); and the positions of the invaginations of the dorsal arms 

 of the tentorium (Fig. 998, i. d.); and of the anterior arms of the tentorium 

 (Fig. 998, i. a.) 



In Figure 999 is given a diagram illustrating the terms applied by writers on 

 chastotaxy to the areas of the head. These terms are defined below. 



The antennal fossa, fovea, or groove. — Depressed areas of the fronto-clypeus in 

 which the antennas rest (Fig. 999, a.f.) 



The Bucca.^Th.a.t part of the wall of the head on each side that is ventrad of 

 the transverse impression, and ventrad of the eye, extending ventrad to the edge 

 of the mouth opening, cephalad to the vibrissal ridge and continuing caudad on 

 the gena to the caudal margin of the head (Fig. 999, b.) 



The cheeks. — This term is used differently by different systematists; by some 

 is applied to the space on each side of the head that is between the lower border of 

 the eye and the oral margin, differing from the bucca only in that it does not 

 extend over the caudal aspect of the head; by others it is applied to this space and 

 the so-called gena of writers on chaetotaxj'^ ; and by others to the so-called gena 

 alone. 



The cheek- grooves. — A more or less distinct depression on each side below the 

 eye. 



The clypeus. — See fronto-clypeus. 



The epistoma. — The oral margin and an indefinite space immediately con- 

 tiguous thereto. 



The face. — That part of the cephalic aspect of the head lying below an imag- 

 inary horizontal line passing through the base of the antennas. (Fig. 999, Fa.). 



