354 SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 
beautiful plumed antennee. This difference in size and complexity 
probably explains the fact that the head of the fly is not developed 
within the larval head, but in the thorax. It is only at the time of 
pupation that it becomes everted, and its appendages assume the posi- 
tion which they are ultimately intended to occupy. 
At length the Chironomus wiggles out of the larval skin, and is 
transformed into a pupa. It no longer requires to feed, and the mouth 
is completely closed. It is equally unable to burrow, and usually lies 
on the surface of the mud. Two tufts of silvery respiratory filaments 
project from the fore end of the body just behind the future head, and 
these wave to and fro in the water, as the animal alternately flexes and 
extends its body. At the tail end are two flaps, fringed with stout 
bristles, which form a kind of fan. The pupa virtually consists of the 
body of the fly, inclosed within a transparent skin. The organs of the 
fly are already complete externally, and even in microscopic detail they 
very closely resemble those of the perfect animal. These parts are 
however as yet very imperfectly displayed. The wings and legs are 
folded up aiong the sides of the body, and are incapable of independent 
movement. for two or three days there is no outward change, except 
that the pupa, which originally had the blood-red color of the larva, 
gradually assumes a darker tint. The tracheal system, which was 
quite rudimentary in the larva, but is now greatly enlarged, becomes 
filled with air, secreted from the water by the help of the respiratory 
tufts, and the pupa floats at the surface. At last the skin of the back 
splits, the fly extricates its limbs and other appendages, pauses for a 
moment upon the floating pupa case, as if to dry its wings, and then 
flies away. 
This fly is a common object on our window-panes, and would be called 
a gnat by most people. It can be easily distinguished from a true gnat 
by its habit of raising the forelegs from the ground when at rest. It 
is entirely harmless, and the mouth parts can neither pierce nor suck. 
Like many other Diptera, the flies of Chironomus associate in swarms, 
which are believed in this case to consist entirely of males. The male 
fly has plumed antennie, with dilated basal joints. In the female fly the 
antenne are smaller and simpler, as well as more widely separated. 
In brisk and lively streams another Dipterous larva may often be 
found in great numbers. This is the larva of Simulium, known in the 
winged state as the sand fly. The Simulium larva is much smaller than 
that of Chironomus, and its blood is not tinged with red. The head is 
provided with a pair of ciliary organs, fan-like in shape, consisting of 
many longish filaments, and borne upon a sort of stem. The fringed 
filaments are used to sweep the food into the mouth. The larva of 
Simulium subsists entirely upon microscopic plants and animals. 
Among these are great numbers of diatoms, and the stomach is usually 
found half full of the flinty valves of these microscopic plants. The 
Simulium larva seeks its food in rapid currents of water, and a brisk 
