132 RESPIRATION CHAP. 



thus forming a remarkable contrast to the amalgamated tracheal 

 system of the other Orders of Insects, where, even when the 

 tracheal system is much reduced in extent (as in Coccidae), it is 

 nevertheless completely unified. Gryllotalpa is, however, said 

 by Dohrn 1 to be exceptional in this respect ; the tracheae con- 

 nected with each spiracle remaining unconnected. 



Water Insects have usually peculiarities in their respiratory 

 systems, though these are not so great as might a priori have 

 been anticipated. Some breathe by coming to the surface and 

 taking in a supply of air in various manners, but some appar- 

 ently obtain from the water itself the air necessary for their 

 physiological processes. Aquatic Insects are frequently provided 

 with gills, which may be either wing-like expansions of the 

 integument containing some tracheae (Ephemeridae larvae), or 

 bunches of tubes, or single tubes (Trichoptera larvae). Such 

 Insects may either possess stigmata in addition to the gills, or 

 be destitute of them. In other cases air is obtained by taking- 

 water into the posterior part of the alimentary canal (many 

 dragon-flies), which part is then provided with special tracheae. 

 Some water-larvae appear to possess neither stigmata nor gills 

 (certain Perlidae and Diptera), and it is supposed that these 

 obtain air through the integument ; in such Insects tracheal 

 twigs may frequently be seen on the interior of the skin. In 

 the imago state it is the rule that Water Insects breathe by 

 means of stigmata, and that they carry about with them a supply 

 of air sufficient for a longer or shorter period. A great many 

 Insects that live in water in their earlier stages and breathe 

 there by peculiar means, in their perfect imago state live in the 

 air and breathe in the usual manner. There are, in both ter- 

 restrial and aquatic Insects, a few cases of exsertile sacs without 

 tracheae, but filled with blood (Pelobius larva, Machilis, etc.) ; and 

 such organs are supposed to be of a respiratory nature, though 

 there does not appear to be any positive evidence to that effect. 



Blood and Blood-Circulation. 



Owing to the great complexity of the tracheal system, and to 

 its general diffusion in the body, the blood and its circulation are 

 very different in Insects from what they are in Vertebrates, so 

 1 Zeitschr. wiss. Zool. xxvi. 1876, p. 137. 



