664 BREATHING-APPARATUS OF LARV^. OF MUSCOIt) FLlES, 



base of the outer slit. About the middle of its length, it bends 

 inwards slightly. " Blister-structure, " when present, generally 

 situated about the point where the inward bend takes place. The 

 outer edge of the inner slit is fairly straight; the inner edge fol- 

 lows more the contour of the inner margin of the band. It is 

 slightly convex. " Blister-structure" at approximately the maxi- 

 mum point of curvature. The " intermediate structure ' occurs 

 between the middle and inner slits, but, in many cases, it is but 

 slightly developed. The "blister-structure" is generally present 

 on the inner side of the inner slit, and absent on the other two 

 slits. It may, however, be seen on all three slits. 



Pycnosoma varipes. (Plate Ixx., figs. 3, Sa). 



Anterior Spiracles. Cap differentiated into two parts. Tu- 

 bules much shorter than in F. rufifacies. 



Posterior Spirxccles. — Band extremely thick and strong, and 

 showing the same differentiation into two parts as in P. rufifa'-ies; 

 the same remarks apply equally in this case as in the previous 

 one. The inner margin of the band projects slightly downwards 

 between the outer and middle slits. The band is broken at the 

 base, leaving the space open and incompletely surrounded. On 

 the inner side of the spiracular plate, the band shows a develop- 

 ment of chitin like a square flag —the minimum distance between 

 the plates has been taken from the edges of these flags. The slits 

 run the whole length of the plate. 'J he outer slit is fairly 

 straight and regular throughout its whole length, although the 

 outer margin conforms more or less to the contour of the inner 

 margin of the band. The "intermediate structure" is only very 

 slightly developed between the middle and inner slits, and is 

 apparently only noticeable in the later stages of the growth of the 

 maggot. The "blister-structure" on the slits is absent. 



LuciLiA sericata. (Plate Ixx., figs. 2, 2a). 



Anterior Spiracles. — Cap not differentiated into two parts. 

 Tubules short and slender. 



Posterior Spiracles. — Band fairly thin and entire, pinched in 

 between the slits as in A. augur. The space in the band is com- 

 pletely enclosed by the band, which is thickened at this spot; 



