Movements of Insects. — BlachmlL 53 



adhering so firmly to the upright surfixce of the glass of ^vinclo^v5 that 

 they are incapable of extricating themselves, though they make every 

 exertion to accomplish that object, yet, when breathed upon, till the 

 aqueous vapor exhaled is condensed about them, they speedily foil 

 from the spot to which they were previously attached so strongly. 

 Now, that this remarkable affection of the house-fly can not be caused 

 solely by a low state of atmospheric temperature, as it has been sur- 

 mised, is evident from the circumstance that it often occurs in the 

 hottest period of the year ; in the mouths of July and August 1804, 

 upward of twenty instances of this curious foct were noticed ; it 

 must be ascribed, therefore, either to feebleness, resulting from some 

 other cause, or to an increase in the adhesiveness of the fluid emitted 

 from the papilla in the act of climbing. If it should still be insisted 

 upon, that the phenomenon is the result of atmospheric pressure, it 

 behooves the advocates of that hypothesis to explain in what manner 

 a little condensed vapor causes the liberation of insects that are unable 

 to accomplish the act by their own unaided eflbrts. That an organ 

 deemed capable of so entirely expelling the air from the space 

 between its extremity and smooth surfaces with which it is brought in 

 contact, so as to produce a vacuum, should yet be incompetent to effect 

 the exclusion of so dense a fluid as water, does certainly appear to he 

 in the highest degree improbable. 



[The adhesion of flies to the glass of windows and to other surfaces, 

 toward the end of summer and in autumn, is usually caused by the 

 growth from the interior of the body of a parasitic fungus (^SporemlO' 

 nema miiscce, Fries : Empusa muscce, Cohn.) — G. B.] 



The promptness and celerity of the movements of flies in an inverted 

 position, or with their backs downward, on highly polished surfaces, 

 and the certainty with which their hold is immediately secured when 

 they alight upon them, would seem to preclude the possibility of the 

 employment of muscular force on such occasions, adequate to the 

 expulsion of the air between their delicate climbing apparatus, and the 

 plain on which they move, to the extent required for the formation of 

 an efl[icient vacuum ; but every difficulty is at once obviated by ad- 

 mitting that a minute quantity of moderately adhesive fluid, whicli 

 acquires a gelatinous o6nsistency on exposure to the atmosphere, is 

 emitted from the organs of sustentation. Unexceptionable evidence 

 that such is the case has been obtained by observing that the extremity 

 of each papilla becomes cauterized when subjected to the action of 

 finely pulverized nitrate of silver ; and that insects, when traversing a 

 vertical surface of glass, leave upon it a visible and enduring trace of 

 their path, for the better perception of which a lens, having a high de- 

 gree of magnifying power, should be employed. 



