MOVEMENTS OF INSECTS ON SURFACES. 227 
should be pursued contemporaneously, and, as far as 
opportunities will admit, in combination. 
Having clearly ascertained by repeated inspections 
of the pulvilli 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 papille with 
which they are provided separately contribute to 
produce 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 demon- 
strate, 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 un- 
impaired, but also to establish the important fact that 
individuals occasionally remain 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 papille. The sole sug- 
gestion hitherto advanced, which has even the appear- 
ance of at all affecting the validity of the conclusion 
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 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 
a2 
