MR. NEWPORT ON THE TEMPERATURE OF INSECTS. 
325 
insects ; these always have the largest respiratory organs, and breathe the greatest 
quantity of air. In the terrestrial insects the amount of heat is greatest in those which 
have the largest respiratory organs, and breathe the greatest quantity of air, whatever 
be the condition of their nervous system. In the larva state the respiratory organs 
are smaller than in the perfect insect, compared with the size of the body, and the 
larva, we have seen, has the lowest temperature. But in these comparisons we must 
observe that the activity of respiration is equal in the individuals which are compared. 
Thus although the respiratory organs are larger in the pupa than in the larva, the phy- 
siological condition of the insect is lower, its respiration is inactive. These facts, it 
will be seen, are all in strict accordance with each other, and point to the chemical 
changes in the air during respiration as the immediate source of animal heat. But it 
may be matter of inquiry how it is that the heat evolved within the body of the insect, 
during respiration, becomes evident so rapidly. This, it may be urged, tends to show 
that it results from the influence of the nervous system. But when we remember 
that in insects the circulatory vessels are in close and most extensive communication 
with the respiratory organs over the whole body of the individual, and that, unlike 
the vessels in those vertebrated warm-blooded animals which have extensive respira- 
tion, they are neither strictly venous nor arterial, but probably intermediate between 
the two, may it not arise from only a very small amount of heat evolved at each re- 
spiration becoming latent, while nearly the whole becomes free, and is liberated as 
quickly as produced, and that this is the occasion of the temperature of the insect 
being so quickly raised during its respiration, and so rapidly diminished as the acts 
of respiration become less frequent? That, in other words, in insects the capacity of 
the blood for caloric is but very little increased during respiration? With these facts 
in consideration, and looking at the analogical condition of insects, and with Pro- 
fessor Grant* and Mr. OwEN'f'-, comparing the vast extent of their respiratory organs, 
distributed over the whole body, with a like extensive respiration in birds, and finding 
that, like birds, insects have also a greater activity of respiration, and a higher tem- 
perature of body than any other class in the division of animals unto which they re- 
spectively belong, we can hardly withhold our assent to the opinions which have long 
been advocated by many of our best physiologists, that animal heat is the direct result 
of the chemical changes which take place in the air respired. But it may be urged 
that activity of respiration is coincident with increased rapidity of circulation, and 
hence that the latter may, perhaps, precede the former, and be in reality the source 
of heat. Unto this it may be replied that the larva in its earlier state has a more 
rapid circulation, but develops less heat than in its latter. In many of the observa- 
tions on tbe Tables it is shown that the pulse may be rapid with a low amount of 
heat. It is shown in the larva, when arousing, that the pulse is not increased until 
* Lectures on Comparative Anatomy. — Lancet, 1833-34. 
f Cyclopaedia of Anatomy and Physiology, vol. i. p. 341. 
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