76 The Microscope. 



theory which was long held about them was this 

 that they are suckers ; that the air presses on them, 

 and that no air can get under them, as a vacuum 

 is formed beneath by the raising of their central 

 parts. 



There appears much to be said in support of this 

 theory, and the sucker is an instrument by no means 

 unknown in nature. The feet of a lizard called the 

 Gecko are supplied with a contrivance of the kind, 

 and this is also the case with some of the water- 

 beetles; I have seen one of them hold on quite 

 tightly to the edge of a basin by means of the suckers 

 on its feet. It would appear, however, that the fly 

 cannot f c support its position 3} in this way. An acute 

 observer of nature, Mr. Blackwall, at Manchester, 

 noticed that flies under the glass of an exhausted air- 

 pump were still able to adhere to it, and, in fact, 

 could not be detached without the employment of a 

 small degree of force. This observation led to many 

 experiments being tried; and it was found that the 

 adhering power exists, not exactly in the flat append- 

 ages, or pulvilli, but in the minute hairs which sur- 

 round them, and, being tubular and terminating in 

 minute disks, partly act as suckers, and emit a fluid 

 which makes the adhesion perfect.* 



Fig. 4 represents a foot of the boat-fly, so called 

 from its rowing itself in the water with a pair of feet 

 much resembling oars. This is one of its hind feet, 

 and it will be observed that, besides being very flat, 



* See Mr. Hepworth's communications to the " Quarterly 

 Journal of Microscopic Science," Yols. II. and III. 



