20 NATURAL HISTORY OF AQUATIC INSECTS 



SOME CASES OF ABRIDGED LIFE-HISTORIES 



In the following pages many instances will be 

 giv r en of Insect larvae which have become completely 

 adapted to aquatic conditions. The spiracles or 

 inlets of the air-tubes are closed ; some kind of gill 

 takes their place ; fins useful only in water are 

 developed. The pupa, which succeeds to the larva, 

 is occasionally wholly aquatic too, though it is much 

 more common for the Insect when the time of 

 pupation is at hand to creep out of the water and to 

 begin to breathe gaseous air. Still there are a very 

 few cases of pupae whose respiration is adapted to a 

 completely submerged life. The pupae of Chironomus 

 and Simulium obtain their whole supply of oxygen 

 from the water. But there is as yet no one instance 

 of an imago, or adult Insect, which breathes the air 

 dissolved in water. All, when they reach their final 

 stage, become air-breathers, drawing in gaseous air by 

 open spiracles. This is true of the aquatic equally 

 with the terrestrial imago. A Dytiscus, a Hydro- 

 philus, a Water-scorpion or a Water-boatman must 

 be able occasionally to reach the air with some part 

 of its body, and will perish if prevented from doing 

 so. Some fully developed insects can, it is true, 

 remain submerged for many hours together, but all 

 require to take in gaseous air at intervals. Why, we 

 may ask, could not that adaptive power of Nature 

 which has enabled so many larvae and some few pupae 

 to subsist upon dissolved air, have furnished us with 

 one or two examples of an adult Insect capable of 

 doing the same ? 



