1908] Habits of Insects as a Factor in Classification 73 



ADAPTATION TO AQUATIC LIFE. 



The insects which Hve in the water show very distinct hnes of 

 divergence from the terrestrial habit and we can trace in a great 

 many different groups the connection with the land inhabiting 

 forms with very great certainty. 



There is at present probably no question as to the general 

 principle of the derivation of aquatic forms from those inhabiting 

 the land, but the relations of the different groups and the particu- 

 lar lines of adaptation are open to further study. One feature 

 that is perhaps sometimes overlooked is that the groups in which 

 the aquatic habit is most perfectly established are those which 

 have been for the longest time fitted for such existence, and there- 

 fore show less perfectly the connection with land inhabiting 

 forms. The groups of dragon flies and May flies may both be 

 looked upon as having established the aquatic habit at a very 

 remote period, and as showing at present a very perfect adjust- 

 ment in the larval stage for this mode of life. In both of these 

 groups the larvas are capable of aquatic respiration and have 

 been provided with tracheal gills, adapted on different lines, so 

 that they need not come to the surface in order to gain air. The 

 development of these tracheal gills, however, must have been a 

 matter of long evolution, but the fact that they are associated 

 with a complete but modified tracheal system is evidence of their 

 origin, subsec^uent to that of trachea. 



In the mosc|uito, on the other hand, we have forms in which 

 such a perfect aquatic respiration is not developed, the larvae, 

 in nearly all species, being obliged to make frequent trips to the 

 surface of the water in order to acquire fresh supplies of air. And 

 here, too, there is a distinct interchange of the contents of the 

 tracheal tubes permitted by the open spiracles. A more spec- 

 ialized condition is shown in the buffalo gnats where there is a de- 

 velopment of gill filaments capable of absorbing oxygen from the 

 water and permitting strictly aquatic respiration without any 

 recourse to the atmosphere. It has lately been shown, however, 

 that tracheal tubes are present but become much reduced in 

 these gill filaments so that there is every reason to assume that the 

 aquatic respiration is of comparatively recent origin. 



In the aquatic Hemiptera we have a series of families which 

 show successive stages of adaptation to aquatic life, and we may 

 trace by easy stages the passage from land or shore living forms to 

 those which are most completely specialized for aquatic life. 



