Linnaan Society. 383 



sylvestris, R. Br., upon the sense of taste, in reference to diminish- 

 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 paper 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 Hymenoptera, Lepidoptera 

 and Diptera. They are numerous and capacious in the Dragon-flies 

 among the Neuroptera, but are smaller and fewer in the Ephemeree, 

 the Sialida and the Scorpion-flies . In the Coleoptera they exist only 

 in the volant species ; and even in the same tribe, as in the Cara- 

 bidcE, 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 dilatation 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 author 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 of 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 which 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 



