394 PHYSIOLOGY. 



231. 



Respiration by means of gills is found only in such insects as live 

 wholly in the water. The situation, form, and differences of these 

 organs have been given above ( 126) in sufficient detail : we will 

 merely add here somewhat upon the mechanism of this mode of 

 respiration. By their deficiency of external apertures the gills are 

 chiefly distinguished from the other organs of respiration. The reception 

 of atmospheric air within the tracheae is thereby naturally rendered 

 more difficult, for its imbibition through the tunic of the gills must 

 proceed more slowly than its mechanical reception through numerous 

 apertures. The gills, consequently, form large broad leaves or long 

 bunches of hair, around which circulates the medium containing the 

 oxygen. A second condition of the reception of this gas by means of 

 gills is the constant motion of these organs, by means of which motion, 

 fresh particles of water, saturated with this gas, are brought into 

 contact with the gills. This motion of the branchiae varies accord- 

 ing to their situation and form. 



Lamellate gills, situated at the sides of the abdomen, move like the 

 fins of fishes from front backwards, so that throughout the whole series 

 of these branchial leaves a constant undulating motion is perceived. 

 The first lamellae bend forwards, whilst the posterior ones strike back- 

 wards, and while the former strike backwards, the latter are bending 

 forwards. Thus the motion of all the gills is not contemporaneous, 

 but both progressive and alternating. By this means these larvae do 

 not swim in thrusts, but regularly, as by means of a portion of the 

 leaves of their gills they are constantly propelled the while another 

 portion reposes, and by this portion they are kept in motion when the 

 preceding is again inactive. By this continued motion of the branchiae, 

 the larva is constantly changing place, and thereby an incessant influx 

 of fresh air is promoted. 



But if the lamellate or hair-shaped gills are placed at the anal 

 extremity of the body, motion is produced by the serpentining of the 

 abdomen, just in the same way as worms without swimming leaves move 

 in water. Thus the larvae of the Agrions swim and breathe at the 

 same time. And, lastly, if the gills lie in the colon itself, as in the 

 larvaa of Msclma and Libellula, by the opening of the anus and the 

 distension of the colon, water is received in the. cavity of this organ, 



