52 3Iovements of Liseds. — Blackivall. 



adhesive power is sufficient to sustain tlieni in the position they 

 occupy ; but, low as it undoubtedly is, it greatly exceeds that of 

 atmospheric air, and it is evident that the efficiency of the adhesive 

 agency to support them on a polished, vertical surface in vanio, thus con- 

 ceded, must be ample to enable them to move on the glass of our windows 

 in perfect security, under ordinary circumstances, without the adventi- 

 tious aid of atmospheric pressure ; the question of specific gravity, 

 therefore, may be safely eliminated as being of no moment in any 

 attempt to solve this interesting physiological problem. 



The argument so much relied upon by opponents, is, that if flies 

 retained their position on polished, vertical surfaces by means of an 

 adhesive fluid emitted from the hair-like papillae on the inferior surface 

 of their pulvilli, they would, after remaining long in any situation, 

 be unable to quit it by any muscular effort they could employ, 

 "without seriously injuring those delicate parts, in consequence of the 

 tenacity that the fluid would acquire by dessication ; -whereas, it is well 

 known that their movements are not in the least impeded by this cir- 

 cumstance. Plausible as this reasoning is, it appears to be based on 

 the erroneous supjwsition that the properties of the fluid resemble 

 those of animal glue, or vegetable gum, an assumption which is at 

 variance with all the particulars that have been ascertained in connec- 

 tion with the phenomenon ; in fact, the fluid merely assumes a gelatin- 

 ous consistency on exposure to the atmosphere, and is readily removed 

 from the pulvilli, when redundant, by the customary mode of cleans- 

 ing those organs employed by insects, which it could not possibly be 

 were it of the tenacity implied by the foregoing conjecture. 



That flies are unal^le to walk on polished, vertical surfaces when 

 breathed upon, till the aqueous vapor expelled from the lungs is copi- 

 ousl}^ condensed thereon is an acknowledged fact ; but it does not ap- 

 pear to be known that when thus treated they can not even retain the 

 position they occupy, whether they make any visible effort to do so or 

 not, a circumstance that seems to be quite inexplicable on the hypo- 

 thesis that they are sujDported by the agency of atmospheric pressure, 

 but which admits of a satisfactory explanation on the principle of a 

 solvent fluid acting upon a gelatinous and moderately adhesive animal 

 secretion ; and these remarks apply to numerous species of insects, and 

 also to spiders provided with scopuke ; but the latter, when they per- 

 ceive their footing to be insecure, frequently attach themselves to the 

 spot by emitting from their spinners a little of the viscid material of 

 which their silken lines are formed, that possesses the property of being 

 insoluble in water. 



In spring, summer, and autumn, house flies may frequently be seen 



