RESPIRATION IN INSECTS AND SPIDERS. 



367 



air-breathing animals (except at certain seasons, when they frequent 

 the sea-shores), although they breathe by gills. 



659. In Insects and other proper air-breathing Articulata, however, 

 the character of the respiratory apparatus is very different. The tran- 

 sition from one form to the other is effected through such animals as the 

 Leech and the Earthworm, which seem able to live almost equally well 

 in air or water, and whose respiration appears to be carried on chiefly, 

 if not entirely, through the medium of the external surface alone. These 

 animals are furnished with a series of small sacs, disposed at regular 

 intervals along each side of the body, and opening by a row of pores, 

 which are termed spiracles or stigmata ; but these sacculi do not seem 

 to participate in the respiratory function, their office being rather to 

 secrete a protective mucus. But in the Myriapods, these sacculi are 

 respiratory organs, and communicate more or less freely with each other. 

 And in Insects, the spiracles, instead of forming the entrances to so 

 many distinct sacs, open into a pair of large tubes, one of which tra- 

 verses the body on either side, along its whole length. These tubes, 

 termed trachece, have many communications with each other across the 

 body ; and they branch out into innumerable prolongations, the ultimate 

 ramifications of which are distributed to every portion of the system. 

 They occasionally present dilatations of considerable size (Fig. 102, A) ; 

 especially in the thoracic region of the body, in those insects which are 

 endowed with great powers of flight. These dilatations or air-sacs appear 

 destined to serve as reservoirs of air, during the time that the insect is 

 upon the wing, its spiracles being then partially closed ; and they may 

 also be useful in diminishing the specific gravity of the body. The air- 

 tubes are prevented from having their cavity obliterated through the 

 pressure of the surrounding parts, by means of an elastic spiral fibre ; 

 which winds round them, between their outer and inner membrane, 

 from one extremity to the other (Fig. 102, B) ; and which answers the 

 purpose of the cartilaginous rings and plates, in the trachea and 

 bronchi of air-breathing Vertebrata. 



Fig. 102. 



Respiratory apparatus of insects: A, air vesicles and part of traeheal system of Scolia hortorum. B, por- 

 tion of one of the great longitudinal tracheae of Carabus auratus, with one of its spiracles. 



660. In this manner, the air that is introduced through the spiracles 

 is carried into every part of the body, and is brought into immediate 

 relation with the tissues to be aerated; so that the carbonic acid which 



