360 SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 
coronet is extended it forms a basin open to the air and impervious to 
water, by reason of the fineness of the meshes between the component 
filaments. Were the larva provided with a basin of the same propor- 
tions formed out of continuous membrane, it might float and breathe 
perfectly well, but the old difficulty would come back, viz, that of 
freeing itself neatly and quickly when some sudden emergency required 
the animal to leave the surface. As it is, the plumed filaments collapse 
and their points approach; the side branches are folded in, and the 
basin is in a moment reduced to a pear-shaped body, filled with a glob- 
we of air, and reaching the surface of the water only by its pointed 
extremity. Down goes the Stratiomys larva at the first hint of danger, 
swimming through the water with swaying and looping movements, 
somewhat like those of Chironomus. When the danger is past, it 
ceases to struggle and floats again to the surface. The pointed tip of 
its tail fringe pierces the surface film, the filaments separate once more, 
and the floating basin is restored. 
The larva of Stratiomys is extremely elongate. The length of its 
body has evidently some relation to the mode of life of the larva, but 
none at all to that of the fly which is formed within it. The pupa is so 
much smaller than the larva as to occupy only the fore part of the space 
within the larval skin.* The interval becomes filled with air, and 
during the pupal stage the animal floats at the surface within the 
empty larval skin. 
Stratiomys, both in its larval and pupal states, floats at the surface 
of the water. The larva can descend into the water when attacked, 
but the pupa is too buoyant, and too much encumbered by its outer 
case, to execute any such maneuver. Provision has accordingly to be 
made for the protection of the helpless pupa against its many enemies. 
It is probable that hungry insects and birds mistake the shapeless lar- 
val skin, floating passively at the surface, for a dead object. The con- 
siderable space between the outer envelope, or larval skin, and the 
body of the pupa may keep off others, for the first bite of a Dytiscus 
or dragon-fly larva would be disappointing. Still further security is 
gained by the texture of the larval skin itself. The cuticle consists of 
two layers. The inner is comparatively soft and laminated, while the 
outer layer is impregnated with calcareous salts, and extremely hard. 
The needful flexibility is obtained by the sub division of the hard outer 
layer. Seen from the surface,it is broken up into a multitude of hexag- 
onal fields, each of which forms the base of a conical projection, reach- 
ing far into the softer layer beneath. The conical shape of these calea- 
reous nails allows a certain amount of bending of the cuticle, while 
the whole exposed surface is protected by an armor, in which even the 
uralists have actually described the latter as a parasite (Westwood’s ‘‘ Mod. Classi- 
fication of Insects,” vol. 0, p. 532). 
