257 
with it, and the bees were dead *. No experiments 
can more clearly shew the necessity of oxygen gas 
to the life of insects ; nor more completely demon- 
strate its almost entire conversion into carbonic acid. 
544. The ova of insects, not less than the perfect 
animals, require the combined operation of water, 
heat, and air, to carry forward their evolution. 
Many of these ova are deposited in water, and are 
evolved only during their immersion in that fluid ; 
and the influence of heat is strikingly exemplified by 
the fact, that it is only during the wanner part of the 
year that the young broods of insects appear. That 
air, also, is necessary, may be concluded from the 
experiments of Spallanzani, who placed the eggs 
both of terrestrial and aquatic insects under the ex- 
hausted receiver of an air-pump, but none of them 
were hatched, although every other condition, neces- 
sary to their development, was present f He found, 
also, that the ova of various insects were evolved in 
large vessels, even when the vessels were hermeti- 
cally sealed ; but, in proportion as the size of the 
vessels was diminished, the progress of evolution 
was retarded, and when the volume of air was re- 
duced to a few inches, no signs of evolution then 
took place J. 
545. The same observations may be extended to 
the intermediate, or larva state of insects. Spallan- 
zani ascertained that these larvae soon died when 
confined in vacua, or in close vessels of air, though 
* On Air and Fire, p. 148. 155. 
t Opuscules de Phys. t. i. p. 141. J Ibid. t. ii. p, 251. 
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