VIII MAY-FLIES 297 



open into the mouth or throat as in other animals. 

 After long examination, I believe that I have dis- 

 covered the openings on the under side of the thorax, 

 nearly in the same place as I afterwards found them 

 in Grasshoppers, where, however, they are easily seen. 

 Since the Palingenia-larva lives in water and mud, it 

 is natural that the openings should be narrow and hard 

 to discover.^ From these observations it is clear why 

 the Palingenia larvae, when the water of the river rises, 

 creep upwards and betake themselves to new tubes, 

 in order that they may get the air which they require 

 to breathe. For the same reason they follow the 

 water as it sinks, lest they should be surrounded with 

 air, and dried up. When the air-tubes are examined 

 in an Insect which has been dead some days, so that 

 the viscera have turned black, they appear like pearls, 

 or bright silver on a dark ground. The firmness of 

 their texture prevents them from rapid decay, and 

 they preserve their shape for a considerable time. 

 To make sure whether air is really contained in these 

 vessels or not, it is only necessary to compress them 

 with the point of a needle under water, when bubbles 



^ Swammerdam is mistaken in supposing that there are such 

 openings in the larva of Pahngenia or other EphemericUc. At 

 times of moult indeed the future spiracles are momentarily- 

 opened to permit the removal of the old lining of the air-tubes, 

 which comes off with the cast skin. Lubbock {Linn. Trans.., 

 \o\. XXV., p. 480) says that this does not happen with the larva 

 of Chloeon dimidiatum, except at its last moult. Only in the 

 imago (winged fly) and the sub-imago (a stage peculiar to 

 l'",phemerida;, in which the imago is shrouded in a thin skin, 

 formed within the proper pupal skin) do the thoracic and 

 abdominal spiracles serve for the admission of air. 



