MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
533 
the earth but suspends itself in the open air, the change begins to take place while 
the insect is spinning its thread for this purpose. When it has remained a few hours 
at rest, preparatory to undergoing its transformation, the tracheae become enlarged ; 
and at about the period when the change to the pupa state takes place the insect 
appears to make several powerful respiratory efforts, accompanied with much mus- 
cular exertion, and these are continued at intervals until its old skin becomes fissured, 
and is gradually thrown off. The tracheal vessels of the fifth and sixth segments, at 
this period of changing to the pupa state, begin to assume the vesicular form, and 
become more and more dilated during the first few days after the change. In the 
Sphinx, and those insects which pass the winter in the pupa state, there appears to 
be an interval, or cessation in the development of the tracheae, as well as of all the 
other structures, during hybernation ; but when the changes have again taken place 
in the spring the development continues until the respiratory organs occupy a very 
large proportion of the body of the insect ; so that when the insect has arrived at 
the perfect state, the longitudinal tracheae in the thorax are exceedingly large canals, 
leading to, and communicating with, the roots of the wings in the thorax and the 
air-bags in the abdomen. There are four of these air-bags on each side of the ab- 
domen in the Sphinx and other Lepidoptera. The largest are close to the posterior 
part of the trunk or thorax, the others gradually decrease in size as they approach 
the anus. In the male Humble-bee, Bombus terrestris, Steph., the anterior vesicles 
are exceedingly large, and form, with those which follow them, a series of very freely 
communicating respiratory cavities, while a nearly similarly free communication exists 
between the two sides of the body in the transverse tracheae, which are dilated into 
a series of funnels which communicate with each other across the body by their 
apices [Plate XXXVI. fig. 2. g.]. 
In those insects which undergo their changes in the open air without entering the 
earth, and which pass but a few days in that condition, there is no interval or period 
of suspension of development. In the common Nettle Butterfly, Vanessa urticce, 
which during the summer undergoes its changes in at most fourteen days, and very 
often, if the season be favourable, in eight or nine, the changes in the respiratory 
organs have distinctly begun to take place about two hours after the insect has sus- 
pended itself for transformation. Meckel has observed that the air-sacs are found 
in the insect soon after it has entered the pupa state ; but I have found that the dila- 
tation of the tracheae, which are developed into these sacs, commences very much 
earlier. On examining the insect about half an hour before it changes to a pupa, I 
have always found the whole of the tracheae a little distended, particularly those in 
the under surface of the thorax, from the first two pairs of spiracles, but their distri- 
bution to the stomach and intestines has continued as regular as in the active larva. 
It is at the actual moment of transformation that all the changes take place most 
rapidly. The efforts which the insect makes at that time appear very much to affect 
the condition of the respiratory organs. When the insect has fissured and thrown 
