1898.] NATURAL SCIENCES OF PHILADELPHIA. 171 



bristles subject, of course, to great variation but easily recognizable. 

 When the limbs of a Muscid are in their natural position it will at 

 once be seen that so far as the fore and hind legs are concerned, 

 one surface is turned toward the median line of the body and one 

 away from it ; these are the mesal and lateral surfaces respectively. 

 The lateral surfaces are provided with stouter and more numerous 

 bristles as would be expected if, as is supposed, the function of the 

 bristles is a protective one. The terms flexor and extensor need no 

 explanation. The anterior femur in cross section has the shape I, 

 1, and its bristles (at least the prominent ones) are in three rows, 

 each of which extends from base to apex. Two of these rows are 

 near the extensor border, the one nearest that border being made 

 up of larger bristles than the other ; the third row is very near the 

 flexor border ; all three are on the lateral surface. The hind femur 

 is shaped very like the fore femur, and has on its lateral surface an 

 extensor row and a flexor row of bristles, corresponding to the row 

 nearest the extensor border and the flexor row of the lateral surface 

 of the fore femur; the mesal surface has also a flexor row, but its 

 members are smaller and less numerous than those of the flexor row 

 of the lateral surface. The middle femur is much less flattened 

 than the others, and in its natural position its surfaces are anterior 

 and posterior flexor and extensor ; it has an anterior flexor and 

 posterior flexor row of bristles and also an anterior median and a 

 posterior median row. As a rule the median rows are incomplete, 

 l. e., do not extend all the way from base to apex. The anterior 

 commonly ends half way from base to apex, the most apical mem- 

 bers of the row being the largest, the posterior often begins about 

 half way from base to apex, and extends thence to the apex, its 

 apical members being also the largest, and usually the apical three 

 or four form a transverse group of considerable prominence. The 

 tibia? are, in cross section, almost triangular with a very well marked 

 extensor border and with flexor, mesal and lateral (middle tibia 

 with flexor, anterior and posterior) surfaces. The same difference 

 in size and number of bristles that was noted in the case of the 

 mesal and lateral surfaces of the fore and hiud femora is noticeable 

 in the corresponding tibiae, but is not 30 pronounced. Each of 

 these surfaces has, ordinarily, a flexor and extensor row of bristles. 

 The middle tibia has also flexor and extensor rows of bristles on its 

 anterior and posterior surfaces, and it has, in some genera, a prom- 

 inent bristle on the flexor surface which does not occur (as far as 



