138 MB. J. bljlCkwalii on the movements of insects 



.could employ without seriously injuring those delicate parts, in 

 consequence of the tenacity that the fluid would acquire by de- 

 siccation ; whereas it is well known that their movements are not 

 in the least impeded by this circumstance. Plausible as this 

 reasoning is, it appears to be based on the erroneous supposition 

 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 connexion with the phe- 

 nomenon; in fact, the fluid merely assumes a gelatinous con- 

 sistency on exposure to the atmosphere, and is readily removed 

 from the pulvilh, when redundant, by the customary mode of 

 cleansing those organs employed by insects, which it could not 

 possibly be were it of the tenacity implied by the foregoing con- 

 jecture. 



That flies are unable to walk on polished vertical surfaces when 

 breathed upon till the aqueous vapour expelled from the lungs is 

 copiously condensed thereon is an acknowledged fact; but it does 

 not appear to be known that when thus treated they cannot even 

 retain the position they occupy, whether they make any visible 

 efl'ort to do so or not, a circumstance that seems to be quite in- 

 explicable on the hypothesis that they are supported by the agency 

 of atmospheric pressiire, but which admits of a satisfactory expla- 

 nation 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 scopulae ; but the latter, when they perceive 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 inso- 

 luble in water. 



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

 seen adhering so firmly to the upright surface of the glass of win- 

 dows that they are incapable of extricating themselves though 

 they make every exertion to accomplish that object, yet, when 

 breathed upon till the aqueous vapour exhaled is condensed about 

 them, they speedily fall from the spot to which they were pre- 

 viously attached so strongly. Now that this remarkable afiection 

 of the house-fly cannot be caused solely by a low state of atmo- 

 spheric temperature, as it has been surmised, is evident from the 

 circumstance that it often occurs in the hottest period of the year ; 

 in the months of July and August 1864, upwards of twenty in- 

 stances of this curious fact were noticed; it must be ascribed. 



