INSECT ECOLOGY 387 



idae) are predaceous. Nymphs of dragon flies are predaceous, catching 

 other insects by means of their peculiar extensile lower lip, and even 

 young fishes, tadpoles and smaller nymphs of their own kind. (R. C. 

 Osburn.) Most caddis worms are plant-eating, but some are insectiv- 

 orous, as Hydropsyche, which catches its victims in the nets that it 

 spreads in swift currents. May fly nymphs are carnivorous, or feed on 

 >lants or mud. Additional examples of food habits are given on p. 165. 



4. BIOTIC CONDITIONS 



Aquatic animals and plants, like terrestrial, form a vast complex of 

 ganisms that interact upon one another in innumerable ways and are 

 cted upon by the environment; but the interactions are different to the 

 extent that the aquatic environment differs from the terrestrial. As 

 a whole, the physical and chemical conditions of existence are more 

 uniform in water than on land. Furthermore, the number of species 

 concerned is fewer; thus aquatic insects form only 3 per cent, of all 

 insects. As regards food supply, the truism that its diminution lessens 

 the numbers of the animals dependent upon it, applies of course, as with 

 terrestrial forms. Aquatic insects are, however, much less specialized 

 than terrestrial as regards food habits. Thus the plant-eating species 

 are seldom limited to one species of plant, therefore can always find 

 food, even though there are fewer species of aquatic plants than of 

 terrestrial. The great majority of aquatic insects are, however, carniv- 

 orous, and many are omnivorous, and rarely suffer from lack of food. 

 Though the predaceous habit is highly developed among aquatic 

 insects, the parasitic habit has developed almost entirely among terres- 

 trial forms; and aquatic insects, while actually in the water, are practi- 

 cally free from the attacks of parasitic insects. 



On the whole, the interrelations of aquatic forms, though incon- 

 ceivably complex, are less extensive than those of terrestrial species. 



The so-called equilibrium or " balance of nature" is maintained, 

 which is, as on land, a condition of continual fluctuation within rela- 

 tively narrow limits; with a smaller range of fluctuation in the aquatic 

 environment. 



III. ENVIRONMENTAL FACTORS IN GENERAL 



The important factors of the environment have been considered 

 individually. In nature they do not operate singly, however, and by 



