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THE BOOK OF NATURE STUDY 



The number of aquatic insects is so large that some process 

 of selection must necessarily be exercised. We shall consider 

 here first a few which should be studied for the sake of observing 

 the salient facts in regard to the metamorphosis of insects, and 

 secondly a few permanently aquatic forms. Before beginning 

 it may be well to point out that, since insects are terrestrial 

 animals, furnished with a beautiful system of air-tubes (tracheal 

 tubes), by means of which they breathe, all forms found in water, 



without exception, have had 

 terrestrial ancestors, and show 

 secondary modifications which 

 fit them for the aquatic life. 



As examples of aquatic larvae 

 we shall consider caddis-worms, 

 dragon-fly larvae, and gnat larvae. 

 These are only a few examples 

 of the numerous insect larvae 

 which pass their early life in 

 water, but they are easy to 

 rear and illustrate the essen- 

 tial points. We shall begin 

 with the caddis-worms, as being 

 forms easy to obtain and very 

 interesting to watch. 



There is no difficulty in find- 

 ing caddis larvae ; any pond or 

 stream in summer will show a 

 number of curious cases, an inch 

 or more in length, sometimes attached to stones, but oftenest 

 free, constructed of pieces of aquatic plants, of sand particles, 

 of shells, or of little stones. Take some of these home in the 

 collecting bottle, and place them in a flat dish with plenty of 

 water weed, taking care not to overcrowd the vessel. When the 

 contained larvae have recovered from the shock of transportation 

 they will be observed to protrude from their cases the head of 

 an insect, followed by the anterior part of a body bearing the 

 three pairs of legs characteristic of insects. The larvae move 

 about rapidly, climbing over the water-plants with much dex- 



FiG. 50. Caddis-worms, showing character- 

 istic types of cases. 



