INSECTS. 



93 



Each spiracle consists of a horny ring placed, as we have seen, between the epimt 1:1 

 and episterna of a segment. The opening is provided with a pair of valves by which 

 it may be closed, and besides there is frequently a strainer of fine hairs and intei Incin- 

 meshes, the object of which is to prevent foreign particles from entering the air tubes. 



The air tubes, or tracheae, are minute branching canals, arising from the inside of 

 the spiracles, by which the air necessary for respiration is conveyed to all parts of the 

 body. They are composed of three layers, the middle 

 of which only possesses any popular interest. This is 

 composed of a filament wound in a spiral between the 

 other two, giving the trachea the spiral character so 

 often seen in microscropic preparations. In many 

 cases branches of the tracheae unite to form a con- 

 tinuous air tube along each side of the body, and 

 oftentimes large cavities or air sacs are formed in 

 various portions of the body. These, when filled 

 with air, tend to reduce the specific gravity of the 

 insect, and hence may play an important part in 

 flight. In Peripatus the tracheal tubes are irregu- 

 larly distributed over the inner surface of the body 

 cavity, the anterior and posterior portions of the ali- 

 mentary canal and the oviduct ; in most other forms 

 the branches are regularly arranged, and have a more 

 extensive distribution. 



In the aquatic larva? of many insects there are 

 no stigmata, and to replace these openings, gills are 

 introduced, usually on the abdomen. These gills, 

 the purpose of which is to extract oxygen from the 

 water, differ materially from those of crustaceans, 

 for they are penetrated by tracheae instead of blood-vessels, and these air tubes convey 

 the oxygen to other portions of the tracheal system. With the development these 

 gills are almost invariably lost, the stigmata appear, and a connection is established be- 

 tween them and the system of air tubes. 



In the spiders an additional feature appears, the existence of so-called lungs. 

 These are formed by a trachea which arises from a spiracle in the ordinary manner. 

 It then breaks up into a number of small flattened branches or plates, which are 

 arranged like the leaves of a book. These are the organs to which Professor Lankester, 

 as mentioned on a preceding page, would compare the gills of a horseshoe crab. 

 There may be one, two, or four pairs of these lungs or pulmonary organs, which are 

 only found in the abdomen. 



In some of the insects, as the lower mites, and the " spring tails " (Collembola) the 

 tracheal system has entirely disappeared, and respiration is carried on through the 

 general integument of the body. 



The act of breathing can be easily witnessed in the larger insects, especially in 

 those which, like the grasshopper or hornet, have the abdomen naked and not covered 

 with the wings or hairs. Holding one of these insects with the fingers or with a pair of 

 forceps, the abdomen will be seen to elongate and contract with great regularity. 

 Each time it elongates air is drawn in through the spiracles, while the contraction, by 

 lessening the capacity of the body, forces out the air which has been used in aerating 



FIG. 129. Trachea of an insect. 



