552 TERTIARY INSECTS OF NORTH AMERICA. 



blades converging anteriorly and terminating in two attingent rounded 

 lobes attached to the inner edge of the blades The anterior spiracles are 

 seen in a single specimen as a simple, rounded, dark spot just outside the 

 middle of either lateral half; the two lateral tracheal vessels may be seen 

 in nearly all the specimens, and especially at the hinder extremity, and 

 fragments of them are frequently scattered about on the stones ; they are 

 very large. The integument is generally rather dark and more or less 

 blotched, and covered profusely and almost uniformly with backward- 

 directed hairs ; these are short, tapering, and moderately stout, though 

 minute. 



Length of contracted bodies. 11.5" 1 " 1 : breadth of same, 6.25""": length 

 of bodies not contracted, 17.5 mm ; breadth of same, 5.75""" ; length of skins, 

 25 mm ; breadth of same, 7.25 inm : length of blades of mandibles, 3.25 mm ; 

 diameter of trachea;, 0.6 mm ; of anterior spiracles, 0.4 mm ; distance of latter 

 apart, 2.75 mm 



Chagrin Valley, White River, Colorado. Several specimens (W. Den- 

 ton). 



MUSCA BIBOSA. 

 P). 5, Fig. 73. 



Musca bibosa Sciukl., Bull. U. S. Geol. Geogr. Snrv. Terr., Ill, 757 (1877). 



Another species is represented by a single body and one skin and its 

 reverse, which seem to belong to the same. It is closely allied to M. asca- 

 rides, but differs from it in some essential features. When contracted the 

 body does not taper regularly from the middle of the front half to the tail, 

 but the whole hinder half is much slenderer than the front and toward the 

 tip has nearly parallel sides, so that the body is flask-shaped and about 

 twice as long as broad. A similar, though not so abrupt, change of contour 

 is seen in the skin. The structure of the mandibles and of the tracheae may 

 be seen to be the same as in the preceding species, but the integument is 

 naked, being entirely destitute of any of the hairs which roughen the skin 

 of M. ascarides. 



Length of rontracted body, 14 mni ; breadth of same in front, 7.5 mm ; 

 behind, 3.75" 1 " 1 ; length of skin (a small one), Ifi""" ; greatest breadth of 

 same, 5.25 mm ; length of mandible blade, 2.75""" ; diameter of trachea, <).7o""". 



Chagrin Valley, White River, Colorado. Two specimens (W. Denton). 



