The South Australian Naturalitt. 



Despite a lethargic habit and usually slow and sluggish 

 movements, the larvae of the Lihellulidae, apparently without 

 effort, dart quickly into concealment when disturbed in cleai- 

 water. liCt us try a little experiment. Grasp a larva by the 

 thorax and hold it beneath the water, so that the termination of 

 the abdomen is quite near the surface. Presently, as the insect 

 struggle? to escape, a jet of water will be forcibly ejected from its 

 vent into the air. Now the larva is equipped with internal 

 tracheal gills, which extract "dissolved" air from water, which 

 is alternately drawn into and exhaled from the abdomen. The 

 aperture may be completely closed by means of terminal appen- 

 dages (fig. la). When the expulsion is produced by a quick 

 contraction of the abdomen, the contained water is violently 

 expelled; the surrounding medium offers a resistance to this 

 forceful stream, which thus acts as a motive power. The larva 

 then ''swims" forwai-d as a result of the sudden muscular energy 

 developed. 



In the demoiselles {Zygoptera) the larva is provided with 

 external, caudal respiratory appendages, which also act as 

 rudders and assist in swimming (fig. 3). Such larvae are, as a 

 rule, not more than an inch in length, and swim with a wriggling 

 motion; their colour is a brown or green, admirably matching 

 the vegetation amongst whi(di they live. The false gills consist 

 of three broad leaves permeated by tracheae — beautiful objects 

 under the microscope (fig. 3b). The finely-V) ranching air-tubes 

 filter the oxygen from the water through the pervious membrane 

 of the gills, and likewise discharge the carbonic acid gas. 



The larvae of the May-flies (Ephemeridae) have external 

 gills; these fringed organs, which are situated along the sides 

 of the abdomen, also assist in swimming. As in the adult, there 

 are three long hairs at the hinder exti'emity of the body. 



The familiar Caddis-worms are larvae of small insectR 

 belonging to the same group as the dragon-flies. The "worm" 

 protects its soft body with a case formed of small objects — tiny 

 bits of wood, shells, pebbles, etc.— glued together, or inserts itself 

 into a hollow twig (fig. 4). In our streams the last-named is 

 perhaps the most usual procedure, the hinder end of the tube 

 thus inhabited being closed by a silky cover made by the insect. 

 The respiratory tufts are arranged on the abdomen, the extremity 

 of which is provided with two hooks, with which the larva clings 

 Jlrmlv to the inside of its retreat. 



