262 CLASS vin. 



conclude from hence that the circulation of blood is unnecessary in 

 Insects, and consequently does not exist. The circulation of blood 

 has not respect to respiration alone, it is not merely for the con- 

 version of venous blood into arterial ; it is necessary that arterial 

 blood should circulate that it may serve for nutrition and secretion. 

 Many Insects live in water : but of these the greater number 

 breathe atmospheric air ; like whales amongst mammals some come 

 to the surface of the water for that purpose. But usually there are 

 special arrangements for conducting the air, so that the Insect can 

 remain under water. This is seen ex. gr. in the larva? of Dtytera, 

 which live under water; those of Culex have at the posterior 

 part of the body a lateral canal with fine hairs at the orifice ; the 

 larvse of Stratiomys have a canal at the end of the abdomen, whose 

 orifice is fringed with a circlet of plumose hairs ; the genera Nepa 

 and Ranatra have a tail composed of two filaments at whose base 

 are two air-slits 1 . These water-insects die in a few hours if the air 

 has no access to the water. Other Insects breathe in the water itself, 

 that is, they breathe the air that is diffused through the water, as 

 fishes do by their gills. Such Insects have no air-slits : the air 

 must therefore penetrate the walls of the tracheae, which to that 

 end are spread out either in filiform or capillary appendages (in the 

 larvae of Gyrinus, of Semblis, the nymphce of Chironomus) or in 

 leaf-like plates at one side of the body (Ephemera), or at the 

 extremity of the abdomen (Agrion). These parts have been termed 

 Gills*; they are not found in perfect Insects. Gills of this sort, 

 from which blind air-tubes arise, occur in the rectum of the larva 

 of Libellula, as five rows of plumose incised leaflets. From them 

 arise six longitudinal steins, of which two, larger than the rest, 



universum corpus disperguntur, sic ut singulce paries aeris particulars per pulmones et 

 sanguinis portiones per arterias recipiat." MALPIGHI Anatome plantarum, Op. om. I. 

 p. 15. 



1 Figures of Culex in SWAMMERDAM, Bibl. nat. Tab. xxxi. figs. 4, 5 ; of Stratiomys, 

 ibid. Tab. xxxix. ; of Nepa, in DUFOUR, 1. 1. The abdomen of Nepa and Ranatra 

 has besides three pair of conspicuous, but closed, air-slits, in which very large branches 

 of air-tubes terminate with blind ends. 



3 This nomenclature is only in part correct. The proper respiratory organs of 

 Insects, the air-tubes, belong to the category of lungs, whether the air penetrates by 

 external apertures (stigmata), or the tubes be filled with air from endosmotic action. 

 The air in fact is in the inside, and the stream of blood (along the tracheae) on the 

 outside, and this relation is just the reverse of that which prevails in gills. 



