ON DBT, POLISHED, VEETICAL SUEPAOES. 137 



"Without the slightest intention to undervalue the importance 

 of microscopic researches into the organization of the parts in 

 question, I may be permitted to remark that the careful observa- 

 tion of phenomena and judiciously selected and skilfully con- 

 ducted exjjeriments afford equal if not superior advantages with 

 regard to the determination of the function they perform ; and 

 that the two methods of investigation should be pursued contem- 

 poraneously, and, as far as opportunities will admit, in combi- 

 nation. 



Having clearly ascertained by repeated inspections of the pul- 

 villi of flies under the microscope, both in a state of action and 

 repose, that a vacuum cannot possibly be formed between them 

 and smooth surfaces to which they are applied, unless the pa- 

 pillae with which they are provided separately contribute to pro- 

 duce such an effect, it was immediately perceived that a decisive 

 test of the truth or fallacy of this conjecture might be obtained 

 by means of the air-pump, and the result of its application was 

 to demonstrate, not only that flies can traverse the upright sides 

 and the interior surface of the dome of an exhausted receiver 

 while their physical energy is unimpaired, but also to establish 

 the important fact that individuals occasionally remaia fixed to 

 the sides of the glass after they have entirely lost the power of 

 locomotion, a circumstance which admits of only one explanation, 

 namely, that an adhesive fluid is emitted from the extremity of 

 their papillae. The sole suggestion hitherto advanced, which has 

 even the appearance of at all affecting the validity of the con- 

 clusion thus arrived at, is that the specific gravity of flies is so 

 low that a very slight degree of adhesive power is sufficient to 

 sustain them in the position they occupy ; but, low as it un- 

 doubtedly 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 vacuo, thus conceded, must be 

 ample to enable them to move on the glass of our windows in 

 perfect security, under ordinary circumstances, without the adven- 

 titious aid of atmospheric pressure ; the question of specific gra- 

 vity, 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 in- 

 ferior surface of their pul villi, they would, after remaining long in 

 one situation, be unable to quit it by any muscular effort they 



