384 NATURAL HISTORY OF AQUATIC INSECTS CH. 



The resting position, moreover, brings the spiracles 

 within reach of a fresh supply of air, though not in 

 this Insect actually to the surface. Dytiscus when at 

 rest has its spiracles fully exposed to the air. If 

 such Insects are disabled, they have only to cease to 

 struggle in order to gain the surface in that position 

 which is most convenient for breathing. Swimming 

 beneath the surface is accomplished with various 

 degrees of efficiency. The winged Dytiscus is per- 

 fectly adapted for submerged navigation. The even 

 contour, the glossy surface, the powerful oars, fringed 

 with bristles, working simultaneously, and feathering 

 during the return stroke, the steering action of the inter- 

 mediate legs, the arched back, and the angulated keel 

 will delight those who can appreciate a finished piece of 

 mechanism. The soldering of the broad bases of the 

 oar-like hind legs to one another, and to the back part 

 of the thorax as well, stiffens the body, and it may be 

 prevents the loss of power which would result from 

 the incessant squeezing out of water enclosed between 

 the basal joint and the body. I suspect that there is 

 a further mechanical advantage in this structure which 

 I am at present unable to explain. Polynema travels 

 about in the water by the help of its wings, like a 

 Penguin. Many long-bodied Dipterous larvae move 

 by a lashing action, striking the water sideways and 

 instantly reversingthe stroke. In Corethra and the Gnat 

 the lashing action of the larva is aided by a fin com- 

 posed of close-set bristles. The Simulium larva runs 

 about its network of silken threads in the water of a 

 rapid stream, holding on by one or both of its clusters 

 of hooks, and throwing out a new thread whenever 



