DIPTERA. 43 
that the most essential facts in the history of those little creatures 
would soon be made known to us. What enjoyment more worthy 
of the calling they have chosen could these pious men pursue than 
that which would place before their eyes the marvellous creations 
of an Almighty Power? Even their leisure would then incline 
them to adore that Power, and would furnish them the means to 
make others do so who are occupied by too serious or too frivolous 
employments.” 
After having changed its skin three times in a fortnight or 
three weeks, the larva of the gnat throws off its covering for a 
fourth time, and is no longer in the larva state. It is changed 
both in shape and condition. Instead of being oblong, its body 
is shortened, rounded, and bent in such a way that the tail is 
applied to the under part of the head. This is the case when the 
animal is in repose; but it is able to move and swim, and then, 
by bending its body and straightening it again, propels itself 
through the water. 
In this new condition, that is to say, in the pupa state 
(Fig. 29), it does not eat. It no longer possesses digestive _ 
organs, but it 1s necessary, even more than before its 
metamorphosis, that it should breathe atmospheric air. 
Besides, the organs of respiration are greatly changed. 
During the time the insect was in the larva state, it was 
through the long tube fixed to the posterior part that it 
received or expelled the air; but in casting its skin it 
loses the tube, two appendages resembling an ass’s ears 
being for the pupa what the tube was for the larva, the 
openings of these ears being held above the surface of the 
water. From this pupa the perfect insect will emerge ; 
it is developed little by little, and the principal members 
may be distinguished under the transparent membranous skin 
which envelops it. 
When the insect is about to change from the pupa state, it lies 
on the surface of the water, straightening the hind part of its 
body, and extending itself on the surface of the water, above which 
the thorax is raised. Before it has been a moment in this position, 
its skin splits between the two breathing trumpets, the split in- 
creasing very rapidly in length and breadth. 

Fig. 29. 
Pupa of 
the Gnat. 
