383 
Linncean Society. 
sylvestris, R. Br., upon the sense of taste, in reference to diminisli- 
ing the perception of saccharine flavours. 
Read also a paper “ On the Formation and Use of the Air-Sacs 
and Dilated Tracheae in Insects.” By G. Newport, Esq., F.R.S., 
F.L.S. &c. &c. 
The j)aper was commenced with the remark, that the presence of 
air-sacs in insects is known to every comparative anatomist. These 
sacs are largest and most numerous in the Hijmenoptera, Lepidoptera 
and Diptera. They are numerous and capacious in the Dragon-fiies 
among the Neuroptera, but are smaller and fewer in the Ephemera, 
the Sialidce and the Scorpion- flies . In the Coleoptera they exist only 
in the volant species ; and even in the same tribe, as in the Cara- 
bidee, they are found in the winged, but not in the apterous species. 
In all insects in which they occur they are largest and most nume- 
rous in the swiftest and most powerful individuals. They are found 
in the Orthoptera only in the migratory families ; while in those 
which are truly saltatorial insects the tracheae are enlarged in some 
parts of their course, but are not to be regarded as properly saccu- 
lated, and sacs are never found in the larva state of any species of 
insect. The sacs are formed by the dilatation of tracheae during the 
metamorphoses of the insects, which commences at the close of the 
larva state, when the insect has ceased to feed. This dijatation goes 
on for the first few days only in those species which hybernate, and 
is resumed again in the spring, but it continues uninterruptedly to 
the development of the perfect insect in those which change to that 
state in the summer. 
The autiior showed that the longitudinal tracheae of the third and 
fourth segments of the larva of winged insects give off a small branch 
at the sides of each segment, which, divided into two portions, passes 
outwards and “ is involved in a fold of the new tegument that is 
formed beneath the old skin of the larva some days before its change. 
These folds of tegument supplied each with their tracheae closely re- 
semble in appearance the external abdominal branchiae of the aqua- 
tic larvae oi Neuroptera,” and afterwards become the most important 
organs of the insect in its perfect state — the wings. The expansion 
of these organs at the change is mainly effected by their tracheae, 
which instead of becoming dilated, like those within the body, are 
elongated, and thus induce a rush of blood into these portions of the 
tegument M^hich promotes their expansion into wings. This elon- 
gation, as well as the dilatation of the tracheae within the body, is 
the result of powerful respiratory efforts of the insect. The author 
remarked, that although able to show the mode in which these 
changes are effected, it is less easy to give a satisfactory explanation of 
the real use of the vesicles. He adopts, however, a view entertained 
by John Hunter, that the vesicles are mainly to enable the insect to 
alter the specific gravity of its body at pleasure during flight, and 
thus diminish the muscular exertion required during these move- 
ments. To support this opinion, the author reviewed the different 
classes of Vertebrata, and showed that although a vesicular form of 
the respiratory organs exists 'in the whole, yet that Birds approach 
