430 TEXT-BOOK OF ENTOMOLOGY 



THE KESPIRATOKY SYSTEM 



While land vertebrates breathe by inhaling the air through the 

 mouth into the lungs, insects respire by internal air-tubes (trachece), 

 which ramify throughout every part of the body and its appendages. 

 The air enters these tubes through a few openings, called spiracles 

 or stigmata, arranged segmentally in the sides of the body. These 

 tracheae are everywhere bathed by the blood, and thus the latter is 

 constantly aerated or kept fresh ; the blood not, as in vertebrates 

 or as in molluscs, seeking the lungs or gills, or any specialized 

 respiratory portion of the body where the oxygen combines with the 

 haemoglobin, but the respiratory tubes, so to speak, themselves seek 

 out the blood and the blood-tissue in every part of the insect body, 

 penetrating to the tips of the antennae and of the legs, entering the 

 most delicate tissues, even perhaps passing through the walls of 

 epithelial cells. As Lang remarks, the want of an arterial vascular 

 system is compensated for as well as conditioned by the extremely 

 profuse branching of the tracheae. 



The aquatic larvae of certain dragon-flies (Agrionidae), may-flies, 

 case-worms, etc., respire by means of tracheal gills or branchiae, 



which are either filamental or leaf-like ap- 

 pendages containing tracheae. Somewhat simi- 

 lar structures appended to the thorax of pupal 



^^^^^^^^^__ aquatic Diptera, as in the mosquito and its 

 FK;.3S9.Kat- tailed larva allies, enable them to breathe while stationed 



of Eristalis. 



a little beneath the surface of the water. 



Other larvae, as the rat-tail larva of Eristalis, etc., lying at the 

 bottom of shallow pools or in ditches, etc., can breathe by raising 

 slightly above the surface a long appendage with two spiracles at the 

 end, through which the air enters the tracheal system. (See p. 461.) 



Although Aristotle, as well as the natural philosophers of the 

 Middle Ages, supposed that insects did not breathe, one can easily 

 see that they do by holding a grasshopper or dragon-fly in one's 

 hand and observing the rhythmical rise and fall of the upper 

 and lower walls of the abdomen, during which the air enters and 

 passes out of the air-openings or spiracles on each side of the 

 body. 



It is plain that insects consume very little air, since caterpillars 

 may be confined in very small, almost air-tight tin boxes, and con- 

 tinue to eat and undergo their transformations without suffering 

 from the confinement. According to H. Muller an insect placed in 



