STONE-FLIES. 73 



thus secured from fishy enemies, protrudes only its 

 head and two rudimentary fore-legs out of its case, 

 and so it can readily roam over the bottom of the 

 water, seeking for its food. The case, it must be ob- 

 served, has almost invariably a seeming irregularity 

 about it, to the curious observer. If made of small 

 stones, it will generally be perceived that a small 

 piece of lighter material wood, leaf, or bit of rush 

 is attached to it. Again, when constructed of lighter 

 substances such as pieces of wood, leaves, or aquatic 

 plants a stone or two will be found adhering to the 

 structure. Though at first sight this seems a rather 

 incongruous sort of architecture, where so much inge- 

 nuity is displayed, it can nevertheless be satisfactorily 

 explained. The larva being of the same specific gra- 

 vity as the water in which it lives, it follows, as a 

 matter of course, that the case must be as nearly as 

 possible of a similar weight. For if the case be 

 heavier, the larva could not drag so weighty a house 

 with it when roaming and feeding at the bottom of 

 the water ; whilst, on the other hand, if the case 

 should be lighter, it would raise the larva from the 

 ground, to be carried away by the current. Thus our 

 little hydrostatic engineer, if it finds its case too light, 

 ballasts it with a stone or two, but if too heavy, in- 

 stead of discomposing the case by throwing off bal- 

 last, the insect merely attaches a bit of wood or other 

 light material, to give it the buoyancy required. In 



