294 Insects and their Surroundings 



face at intervals and pierce the film with its air-tube 

 in order to get a fresh supply of oxygen. But the 

 pupa of the Sand-midge with its tubular gill-fila- 

 ments (fig. 1 60 c, d), or the larva of a Mayfly with its 

 tracheal gill-plates, can remain in the water through- 

 out its life, drawing, as do the fishes, sufficient oxygen 

 from the dissolved air. It is interesting to notice that 

 within the limits of a single and restricted Order — 

 the Dragon-flies — we find some larvae breathing by 

 means of tracheal gill-plates, and others taking 

 supplies of water into the hind-gut over whose walls 

 run branching air-tubes ; while in the final nymph 

 stage the thoracic spiracles are open, and the insect, 

 raising the front part of its body above the surface, 

 breathes through them after the manner of an imago 

 (170). These various adaptations to an aquatic life 

 within a single group 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 pre- 

 sumed that larvae with the more perfect adaptations 

 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. 



Marine Insects. — Insects have not only invaded 

 rivers and lakes, they have established themselves to 

 some extent at least, along the margin of the sea. 

 On a sunny day by the shore, myriads of Flies can 

 be seen hovering over the seaweed cast up by the 

 tide. These have been developed from grubs which 

 live and feed in the decaying weed, and are able 

 to bear immersion twice daily. Around the rock- 

 pools many Midges may be noticed. Their grubs 

 feed on growing green seaweed, and spend their 

 whole life in the salt water, breathing the dissolved 



