INSECTS IN THE WATER 315 



During the latter stages of their development, however, 

 the thoracic spiracles of all dragon-fly nymphs are open, 

 and the insect obtains at least some of its oxygen by 

 raising the front part of its body above the surface of 

 the water. " These various adaptations to an aquatic life 

 in a single group " (writes Professor Carpenter) " indicate 

 clearly that the habit of living in water is not primitive 

 among insects, but that it has become acquired by different 

 races at different times in the course of development. It 

 may be presumed that larvse with the more perfect adapta- 

 tions for breathing when submerged — leaf-like or thread- 

 like gills — are older inhabitants of the water than those 

 which have to rise periodically to the surface to take in a 

 supply of air." Gill structures always disappear with the 

 last moult, except in the case of stone-flies (Plecoptera), 

 some of which retain their gills in the adult state. To 

 what extent the organs retain their original function is 

 not known ; but the fact that they persist is very remark- 

 able, and suggests that the aquatic habit of stone-flies 

 may be of very ancient origin, going back to a 

 time when the atmosphere was much more heavily 

 charged with moisture than is the case to-day. 

 Indeed, the fossil remains of gill-bearing adult insects, 

 possibly ancestors of existing stone-flies, have been 

 found in rocks belonging to the immensely remote 

 carboniferous period. 



Passing reference has already been made to the larva? 

 of certain midges (Chironomidte) which are known popu- 

 larly as "blood-worms." They are of unique interest, 

 because their blood and our own contains the same kind 

 of colouring matter. This substance, called hemoglobin, 

 has the power of forming what is termed a " loose 

 combination" with oxygen, so that the latter can be 

 surrendered to the tissues of the body without chemical 



