MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
535 
It passes along the antenna nearest its under surface, and in the V anessa urticce, Steph., 
gives off laterally thirty-four pairs of minute branches, one pair to each segment. 
In the twelve segments which constitute the club or apex of the antenna, the 
trachea becomes very minute, and in the last four segments of the club it is divided 
into numerous ramifications [Plate XXXVI. fig. 6. b.~\. About an hour after the 
change to the pupa state the tracheae from the first spiracle, which ramified over the 
oesophagus, are enlarged to about double their original size, and instead of continuing 
of a pencillated structure, are almost of equal diameter throughout, and are beginning 
to be detached from the oesophagus, which is becoming narrower. At seven hours 
these changes have all been carried further, and the air-cells in the abdomen are much 
larger. At twelve hours, besides the gradual enlargement of the tubes, the chief 
thing observable is the diagonal direction of the tracheae from the seventh spiracles, 
which supply the pyloric extremity of the stomach, which proves that the stomach is 
gradually becoming shorter previous to the detachment of these tracheae, which sub- 
sequently takes place. Eighteen hours after the change all the longitudinal and 
thoracic tracheae, with those of the head, are still further enlarged, and the tracheae 
from the third pair of spiracles, which are given to the cardiac extremity of the 
stomach, are partly detached from that organ, and are more dilated than any of 
the others. The tracheae from the ninth spiracles, which are given to the colon, 
are beginning also to assume a vesicular form. The stomach is still supplied with 
tracheae from six spiracles. At twenty -four hours the changes are still advancing. 
At thirty-six hours the tracheal branches distributed to the different ganglia are en- 
larged ; and at forty-eight hours the development of these parts is so far advanced 
that nearly all the tracheae in the body have become a little dilated, and this dilata- 
tion continues until the insect has become perfect. The only difference between the 
development of this insect and the Sphinx, or those which undergo their metamor- 
phoses in the earth and remain in the pupa state during the winter, is in the rapidity 
of the changes ; and even this difference exists only in those diurnal insects which 
are developed in the beginning and middle part of the summer, since in those indi- 
viduals which are produced late in the season, and consequently remain in pupa 
through the winter, all the circumstances are precisely similar. The real use of the 
pulmonary sacs, which are found in all volant insects, appears to be, as supposed by 
John Hunter, to enable the individual to alter its specific gravity at pleasure by en- 
larging its bulk, and thus render it better able to sustain itself on the wing with but 
little muscular effort. That this is the real use of the sacs may further be inferred 
from their non-existence in the larva or infant condition of the insect, and from their 
almost entire absence in all insects which are destined to live entirely on the ground. 
This opinion is further supported by the fact that they are most developed, relatively 
to the size of the individual, in those insects which sustain the longest and most 
powerful flight, as Hymenoptera, Lepidoptera, some of the winged Coleoptera, the 
Hemiptera and Lucani, while in none of these insects in the larva state is there any- 
