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 
