386 NATURAL HISTORY OF AQUATIC INSECTS CH. 



oar-like legs. Donacia and perhaps the pupa of 

 Paraponyx tap the air-chambers of submerged plants. 

 The larvae of Ptychoptera and Eristalis have respira- 

 tory tails, which can be protracted or withdrawn, but 

 each has a mechanism peculiar to itself. The long 

 respiratory filament of the Ptychoptera pupa is alto- 

 gether different from both. Nepa and Ranatra 

 breathe by a pair of valves, which cohere to form a 

 tube, and conduct air to the spiracles. Other aquatic 

 Insects breathe only dissolved air, and this in many 

 different ways ; by the outer skin, or by bunches of 

 thin-walled tubes, occupied in some cases with 

 tracheae, in others with blood-spaces, or by flat plates 

 which can be waved to and fro, or by intestinal folds. 

 Tube-dwelling larvae have their own difficulties and 



o 



remedies. Chironomus larvae, Caddis-worms, and 

 some aquatic caterpillars which inhabit sheaths made 

 out of leaves, maintain a flow of water through their 

 tubes by incessant undulation of the body. This 

 operation is aided by peculiar structures, such as 

 fringes of bristles, retractile processes which regulate 

 the distance of the body from the wall of the tube, 

 and so forth. The Chironomus larva employs 

 haemoglobin as a means of storing up oxygen. The 

 spiracles of almost any Insect, whether aquatic or 

 terrestrial, abound in singular features, nearly all un- 

 studied at the present time. The spiracles of Dytiscus 

 are among the wonders of nature ; it would take a 

 lifetime to master their details. 



Attack and Defence call forth yet more contri- 

 vances. Protective resemblance, concealment within 

 burrows, by webs, by portable cases, by fixed cases, 



