CH. XV.] SPECIES OF DIPTEROUS INSECTS. 219 



them a strong hold upon objects having irregular or 

 yielding surfaces ; and he imagined that there is 

 upon glass a kind of smoky substance, penetrable 

 by the points of their bristles. The same opinion 

 is also given by Shaw in his " Nature Displayed," 

 and, more recently, Mr. Blackwall has considered 

 that the motions of the fly are to be accounted for 

 upon mechanical principles alone ; thus, upon in- 

 specting the structure of the parts of the suckers 

 (regarding which great want of accordance exists 

 in the descriptions of authors), " it was immediately 

 perceived that the function ascribed to them by Dr. 

 Derham and Sir E. Home is quite incompatible with 

 their organization. Minute hairs, very closely set 

 and directed downwards, so completely cover the 

 inferior surface of the expanded membranes, im- 

 properly denominated suckers, with which the ter- 

 minal joint of the foot of flies is provided, that it 

 cannot possibly be brought into contact with the 

 object on which those insects move, by any mus- 

 cular force they are capable of exerting ; the pro- 

 duction of a vacuum between each membrane and 

 the plane of position is therefore clearly impracti- 

 cable, unless the numerous hairs on the underside 

 of these organs individually perform the office of 

 suckers ; and there does not appear to be any thing 

 in their mechanism which in the slightest degree 

 countenances such an hypothesis. When highly 

 magnified, their extremities, it is true, are seen to 

 be somewhat enlarged, but when they are viewed 

 in action or in repose, they never assume a figure 

 at all adapted to the formation of a vacuum." 

 Moreover, on enclosing a housefly in the receiver 

 of an airpump, " it was demonstrated to the entire 

 satisfaction of several intelligent gentlemen present, 

 that the housefly, while it retains its vital powers 

 unimpaired, can not only traverse the upright sides, 

 but even the interior of the dome of an exhausted 

 receiver, and that the cause of its relaxing its hold, 



