230 ON THE MOVEMENTS OF INSECTS 
they were previously attached so strongly. Now that 
this remarkable affection of the House-fly cannot be 
caused solely by a low state of atmospheric tempera- 
ture, 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 instances of this 
curious fact 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 secretion emitted from the papille in 
the act of climbing. If it should still be insisted 
upon that the phenomenon is the result of atmo- 
spheric pressure, it behoves the advocates of that 
hypothesis to explain in what manner a little con- 
densed vapour causes the liberation of insects that 
are unable to accomplish the act by their own 
unaided efforts. That an organ deemed to be 
capable of so entirely expelling the air from the 
space between its extremity and smooth surfaces 
with which it is brought in contact as to pro- 
duce a vacuum should yet be incompetent to 
effect the exclusion of so dense a fluid as water, 
does certainly appear to be in the highest degree 
improbable *. 
* The adhesion of flies to the glass of windows and to other 
surfaces, towards the end of summer and in autumn, is usually 
caused by the growth from the interior of the body of a parasitic 
fungus (Sporendonema musce, Fries ; Empusa musce, Cohn). 
