56 Psyche [June 



carince, simple, branched or plumose hairs, etc. This ornamenta- 

 tion arises usually between the spiracles, and for descriptive pur- 

 poses, I have numbered these inter-spiracular spaces, 1, 2, 3, and 4, 

 beginning with the one mediad of the dorsal spiracle. See figure 1. 



(16) The nature of the delicate chitinous bars crossing the slit-like 

 spiracles (MacGregor) ; a point undoubtedly of very great impor- 

 tance, but in many species too difficult of determination to be prac- 

 tically useful. 



(17) The nature of the chitinous ring surrounding the stigmal plate; 

 whether heavy or light, complete or incomplete; the last condition 

 usually associated with the absence of the button or circular plate 

 (MacGregor) . 



The most casual student could not but note the great diversity in 

 position of the slit-like spiracles with reference to each other and to 

 the median line of the body. Banks uses in his synopsis of the 

 groups {loc. cit., p. 15) such characters as "slits arranged radially," 

 or "sub-parallel to each other" or "rather transverse to body." 

 MacGregor mentions "the orientation of the stigmata mth refer- 

 ence to the . . . longitudinal axis of the larva" and the 

 position of the slits with respect to each other and to the median 

 sagittal plane of the body. 



But, so far as I am aware, no attempt has hitherto been made to 

 use for specific description 



(18) The accurately-measured angle of divergence between the 

 several spiracles, and 



(19) Their divergence from the median line. 



However, from my observations in the Syrphidse and myiasis- 

 causing species, I am convinced that much more can profitably 

 be made of these points; and in the case of very closely related 

 species, I believe this may at times prove to be the only reliable 

 means of separation. These characters are, of course, available 

 only in those cases where the spiracles are straight, or nearly so, 

 so that their main longitudinal axis is readily determinable. 



I was much surprised, when I conceived the notion of using this 

 method of description, that I was unable to find anywhere on the 

 market the apparatus by which such angular measurements could 

 readily be made under the microscope. I finally designed a 

 micro-protractor,^ for use in the ocular of the microscope, by means 



* Made by The Bausch & Lomb Optical Co., Rochester, N. Y., who guaranteed its accuracy 

 to within half a degree. 



