RESPIRATION OF INSECTS AND FISHES. 



157 



most degree, there being great numbers of tracheal tubes pervading all 

 the soft parts. These occasionally present dilatations, acting as reser- 

 voirs the foreshadowing of the respiratory cavities of the higher tribes. 

 Of such, Fig. 72, representing the air-sacs or tracheal dilatations of the 



Air-sacs of insects. 



Spiracle of insect 



scolia liortorum, is an illustration. The tracheal tubes Respiration of 

 communicate with the external air through openings which insects. 

 maybe obstructed by a valvular arrangement, as represented in Fig. 73. 

 The photograph from which this figure was taken shows such a spiracle 

 magnified 75 diameters. These organs may be seen arranged in rows 

 on each side of the body ; thus, in the common caterpillar, there are ten 

 pairs. The mode of guarding the orifice varies in different cases, some- 

 times tufts of hair being resorted to, and sometimes, as in the figure, 

 valves. 



The true lung is first recognized in the swimming bladder of fishes as 

 a simple sac. In the carp, the tendency to a multi-chambered construc- 

 tion already appears under the form of two such bladders, #, #, communi- 



eating with each oth- 

 er through a narrow 

 tube. These are con- 

 nected with the oesoph- 

 agus, 0, by means of 

 the pipe c d, the fish 

 being thus enabled to 

 remove at pleasure a part of the air contained in the sacs by muscular 

 compression. Though this mechanism is, as we have said, a rudiment- 

 ary lung, it does not properly subserve the duty of such an organ, but is 

 .employed for producing variations in the specific gravity of the animal 

 by compression or rarefaction of the included air. In these Respiration of 

 tribes the gills are the mechanism for aeration, which is ac- fishes - 



Air-sac of fish. 



