92 THE EEPORT OF THE No. 36 



AQUATIC INSECTS. 



Robert Matheson, Truro, N.S. 



Water is the most abundant mineral of our earth. It covers at least % of its 

 surface, and also constitutes a large part of our continents. Fuller estimating 

 that the entire amount of underground water would form a belt 96 ft. in thick- 

 ness. W. J. McGee estimates that the first 100 ft. of ground of the United States 

 contains 17 ft. of water. Water is the most essential of all compounds. All 

 living organisms consist of a large percentagie of water. Undoubtedly life 

 originated in the water, and to-day all forms of life are more intimately associated 

 with water than with almost any other single substance. It is not necessary for 

 me to enumerate the many peculiarities of water, its color, odor, freezing and melt- 

 ing temperatures, its specific and latent heat, its point of maximum density, the 

 formation of vapor, rain, fogs, dew, frost, etc., its solvent powers, etc., etc. Yet 

 all these chemical and physical properties of water are what makes life possible on 

 our globe. Is it any wonder then that our seas, lakes, rivers, ponds, and streams 

 teem with living organisms? 



Turning our attention to the insects, there is no question that they constitute 

 the dominant animal group. Insects are more numerous in species, constituting 

 4-5 of the known forms, but also probably exceeding in actual bulk all other 

 terrestrial animals. From such considerations one would be inclined to con- 

 clude that insects would be found in great abundance in our waters, yet the very 

 opposite in the case. Our great oceans and seas are practically devoid of all insect 

 life, only one genus of water striders, Halohates, being found distant from our 

 shores. In our inland waters, insects are found near the shores, in shallow water, 

 among aquatic vegetation, only a few forms being found in the Plankton 

 (Corethra). The open water is practically devoid of all insect life. Aquatic 

 insects are practically all littoral. 



The explanation of this paucity of forms is found in the fact that all insects 

 were originally terrestrial animals. The evidences of this are so numerous and 

 obvious that I need scarcely recount them; — 



(1) The chitinous armour, impermeable to water and air. 



(2) The taking in of air through open spiracles. 



(3) No insect form breathes air dissolved in water throughout its life. 



(4) Many aquatic larvae breathe air directly. 



(5) Larvae possessing gills are widely distributed and not restricted to any 

 one group or closely associated groups. 



(6) No adult insects are true aquatics, breathing air dissolved in the water. 

 Undoubtedly like many mammals, insects have become readapted to an aquatic 



life. This readaption has probably been brought about either by the scarcity of 

 food on land or its abundance in water, or by both, and as a result of the terrible 

 competition existing among land forms. Gradually certain forms have pushed 

 their way out into the water and this adaptation to an aquatic environment has 

 arisen independently in widely divergent groups. At the present time scarcely a 

 single large order is without aquatic representatives. In many of these orders the 

 aquatic habit has risen independently several times. Miall estimates that adapta- 

 tion to aquatic situations has risen independently at least one hundred times. To 

 the student of evolution no other single class offers such a teraptin? field for the 

 study of adaptation to a common environment by many widely divergent forms. 



