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XXVI. On the Formation and Use of the Air-sacs and Dilated Trachece in 

 Insects. By George Newport, Esq., F.R.S., F.L.S. ^c. 8fc. 



Read December 7th, 1847. 



IT is well known to every comparative anatomist, who has paid any atten- 

 tion to the Invertebrata, that many insects in their perfect state have their 

 respiratory organs more or less dilated, in different parts of their course, into 

 vesicles, or sacs. In some tribes, as in most of the Hymenoptera, Lepidoptera 

 and Diptera, these sacs are present in almost every species, and occupy a large 

 portion of the interior of the body, more especially of the abdominal region. 

 In the most active Neuroptera the sacs are very numerous and capacious, espe- 

 cially in the Dragon-Jiies ; but they are much smaller and fewer in number in 

 the Ephemerce, the Sialidce and the Scorpion-Jlies. In the Coleoptera the sacs 

 exist only in the volant species, and are more or less numerous and capacious 

 in these in proportion to the bulkiness of the insect, and its degree of activity 

 on the wing. This difference exists not only in different genera, but in differ- 

 ent species of the same genus, according as they are winged or apterous spe- 

 cies. Thus distinct vesicles are found in the winged Carahidce, but not in the 

 apterous, in which the respiratory organs are simply tracheal. In the more 

 heavy-bodied genera the vesicles are not confined to the abdominal and tho- 

 racic regions, but are sometimes extended into other parts, as in the unwieldy 

 Stag-beetles, in which they are extremely numerous, and occupy the chief 

 portion of the interior of the mandibles. In the Lepidoptera, as in the Neu- 

 roptera, they are largest in the swiftest and most powerful species, and more 

 especially so in those in the males, which are known to be the most active on 

 the wing. On the contrary, in the majority of the Orthoptera, which are 

 merely saltatorial in their habits, the tracheae never assume the form of di- 

 stinct vesicles, excepting in a few genera, which have the power of flight. 

 They retain the arborescent form in the perfect as in the larva state, but are 



