36 On the Uses of the Dorsal Vessel. [Juty, 
and its diameter is at least a millimetre (0°03937 inch) ; sometimes 
it iseven more considerable. It is from this common trunk that the 
bundles of transverse tracheze always divided into pairs proceed ; 
the ramifications of which are generally unequal. ‘lhe number of 
these bundles of trachez is always twice that of the stigmata, as two 
always proceed from each stigma. 
The insects which respire air immediately, and which have only 
arterial trachee, are those in which the respiratory system is sim- 
plest. ‘The species in which this disposition exists require to enjoy 
the influence of air as speedily as possible. Hence it is distributed 
almost as soon as it is received. 
The pulmonary trachez of the scarites gigas originate above the 
cerebriform ganglion by a transversal branch, from which proceed 
ramifications to the upper lip, the antennz,-and the eyes. ‘This 
branch is prolonged in the head by two principal trunks, which 
extend in the corcelet, and then in the rest of the body. ‘These 
trunks haying reached the corcelet, form on each side of the dorsal 
vessel a kind of semicircle, giving out numerous ramifications to 
the dorsal vessel and the surrounding muscles. The pulmonary 
trunks, when they reach ‘the breast, approach the dorsal vessel 
more and more, forming on each side erismes, semicircles, from 
the centre of which proceed the branches that form a communica- 
tion between the pulmonary and arterial trachea, ‘The common 
pulmonary trunks continue in the same manner in the abdomen, 
where they form afterwards rings in semicircles, from which proceed 
the principal branches, which form a communication between them 
and the arterial tracheee. As to the branches that come from the 
internal side, they all go to the dorsal vessel and the muscles that 
surround it. In this place the pulmonary trunks never acquire a 
large diameter. 
The trunks of the arterial traches rise below the cerebrum by 
two principal branches, which distribute themselves over the man- 
dibles, and the different parts of the meuth. These branches have 
a very considerable diameter, and a reddish colour. When they 
come to the corcelet, they unite, and form only one trunk. After 
this they send a large branch to the first pair of legs; while from 
their interior side they send branches to the trunks of the pulmonary 
trachez, and to the intestinal tube. The same thing takes place in 
the thorax. These trachez: diminish somewhat in size in the abdo- 
men, and keeping always at the side of the body, the external 
branches go to the stigmata, while the internal surround the intes- 
tinal tube and the organs of generation with a fine network of tra- 
chee. ‘The common trunks form from ring to ring semicircles, 
always’ furnishing the branches of which we have spoken. We 
observe that from each semicircle formed by the arterial trachee 
there issue two long cylindrical trachee, which ramify to infinity 
on the intestinal tube and the organs of generation. There are few 
species in which these trachea are more distinct or extensive. In 
general the abdominal tracheg are of a silver-white ; those of the 
