AQUATIC INSECTS 879 



tenor end of the alimentary canal, but they also occur on other 

 parts of the body. 



Since tracheae are the established channels of air distribution in 

 the bodies of insects, and nearly all insects are hatched from the 

 egg in possession of a number of them, it is natural that tracheal 

 gills should be more commonly developed in the larvae of the 

 group. A tracheal gill differs from a blood gill chiefly in that it is 

 traversed by minute capillary branches of tracheae, and the air 

 is taken up by and distributed through the tracheae. Tracheal 

 gills are usually developed apart from and quite independently of 

 the spiracles or breathing pores. They arise from the thin interseg- 

 mental membranes of the body. They may be developed upon 

 the internal walls of the rectum, forming a large and very perfect 

 gill chamber, as in the young of dragonflies. More frequently, they 

 are developed on the outside of the body. They may be flat and 

 lamelHform, as in the three caudal gills of the damselflies and in the 

 paired dorsal abdominal giUs of Mayflies, or they may be filamen- 

 tous, simple, branched or tufted, as in most other forms. Another 

 sort of tracheal gill (the so-called "tube giU") is developed directly 

 from the prothoracic spiracles in certain diptera at the assumption 

 of the pupal stage, in the form of respiratory trumpets (mosquito 

 pupae), combs (black fly pupae), brushes (midge pupae), etc. With 

 the development of gills, insect larvae have become independent of 

 the surface. Many of them remain wholly submerged through- 

 out their entire larval life. A few of them have progressed 

 farther from shore and into deeper water. Corethra has been already 

 mentioned as a plankton organism. A few larvae of midges and 

 a few caddis worms are constant denizens of the bottom silt in our 

 deeper fresh-water lakes. This seems indeed considerable prog- 

 ress into a new and totally different environment, when one re- 

 members that they are tied by parentage to the shore. 



It is to be noted in passing that only in the Coleoptera and 

 Hemiptera has the adaptation of adults and immature stages been 

 parallel. In the other groups the adults do not live in the water. 

 The possession by a few adult insects {Pleronarcys, etc., among 

 stoneflies, and CMrotonetes , etc., among Mayflies) of rudimentary 

 giUs does not indicate, as was once thought, that this is the primitive 



