276 NATURAL HISTORY OF AQUATIC INSECTS CH. 



but this is not the case. The larva creeps by means 

 of its six legs, sometimes aiding its progress by an 

 undulating, side-to-side movement of the abdomen. 

 In a confined space it may often be seen to move 

 its abdomen in vertical undulations, which cause a 

 swirl in the water, and no doubt promote respiration. 



Several aquatic larvae, not at all closely related to 

 Sialis, have similar tracheal gills. Gyrinus and 

 Orectocheilus are examples taken from the Coleoptera. 

 One peculiarity of the gills of Sialis, viz. their division 

 into joints, is shared by a Dipterous larva, Dicranota. 

 It is worth notice that aquatic Insects, living side by 

 side under identical conditions, should show so great 

 a variety of respiratory organs. Some have tracheal 

 gills, not all of one kind ; others breathe by spiracles ; 

 others have no special respiratory organs at all. 



Though the Sialis larva lives in water, and can 

 spend months without coming to the surface, as I 

 have found by observation of captives, it can survive 

 a rather prolonged exposure to air. Full-grown 

 larvae, taken out of the water before the time of 

 transformation has fully come, and placed on damp 

 earth, walk about till they find a convenient crevice, 

 and then bury themselves. They live and retain the 

 power of . active movement for days when thus se- 

 cluded. This faculty of enduring exposure to dry 

 conditions is useful, as will shortly be seen, on two 

 occasions, which inevitably occur during the lifetime 

 of the Insect — viz., immediately after hatching, and 

 before pupation. "^ 



In May or June, when the larva is full-fed, it quits 

 the water and seeks a place where it can bury itself 



