534 
MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
off its old skin as a larva, there is a cessation of its efforts for a few seconds. It then 
makes a few slow but very powerful respirations, during which, as in every forced 
inspiration, the abdominal segments are much distended, after which the longitudinal 
layers of muscles of these segments become very much contracted, and the segments 
themselves shortened. While this is taking place the circulatory fluid contained in 
the vessels of the abdomen is propelled forwards, and the wings, which at the mo- 
ment of slipping off the old skin are scarcely so large as hemp-seeds, are distended 
at their base, and at each inspiration of the insect are gradually enlarged by the pro- 
pulsion of circulatory fluid into them, and are carried down over the lateral and under 
surface of the trunk, and the ventral surface of the first two segments of the abdomen. 
This is exactly what takes place in the Sphinx ligustri, as well as in Vanessa urticoe. 
From the fact of all the tracheae being enlarged immediately after the insect has 
changed to the pupa state, it seems not improbable that this enlargement is occa- 
sioned by the closing of the spiracles, and the expansion of the air within the tracheae 
during the powerful respiratory efforts of the insect in effecting its transformation, — 
that it results from the recession of the circulatory fluid from the vessels of the ab- 
domen into the partially developed wings taking off pressure at the instant from the 
tracheal tubes, which then become distended by the natural elasticity of the air con- 
tained within them. 
Professor Carus attributes the development of the air-bags and dilatation of the 
tracheae entirely to the closing of the spiracles and the expansion of the contained 
air, which he thinks is increased in quantity * during the development of the insect. 
But it seems more probable that the formation of air-bags is occasioned simply by a 
continuance of the same cause, the elasticity of the contained air, which produces 
the enlargement of the tracheae in the first instance, and that this enlargement or 
dilatation keeps pace with the gradually decreasing size of the digestive organs, since 
the spiracles are not permanently closed during the pupa state, but are in constant 
action, except during the period of complete hybernation. It is at the actual moment 
of transformation that the anterior pair of large vesicles begins to be formed, and the 
tracheae in the next segment are a little more enlarged. The antennae, which just 
before the change were coiled up within the sides of the head, are now extended along 
the sides and abdomen, and the tracheal vessels within them may be readily ex- 
amined. If the antenna be separated from the pupa while soft and transparent, it will 
be seen that the trachea within it extends from its base to its apex [Plate XXXVI. 
fig. 5. a. &.]. It is a continuation of a large trachea that comes from the first spi- 
racle, and crossing the segment above the oesophagus and dorsal vessel, sends off its 
cruciform branches immediately behind the brain, at the back part of the head. 
There are four branches given off at that point, as before noticed ; the two external 
ones are those which supply the antennae. The main tracheal vessel of the antenna 
at this period of the insect is very small, but afterwards becomes much enlarged. 
* Introduction to Comparative Anatomy, translated by Gore, 1827, vol. ii. p. 167. 
