362 SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 
water, and the air is renewed without effort through the thin-walled 
bladders. 
Why should the position of the respiratory organs be changed from 
the tail end in the larva to the head end in the pupa? Chironomus, 
the gnat, Corethra, and many other aquatic insects exhibit the same 
phenomena. Evidently there must be some reason why it is more con- 
venient for the larva to take in air by the tail, and for the pupa to take 
in air by the head. Let us consider the case of the larva first. Where 
it floats from the surface, or pushes some part of its body to the sur- 
face, it is plain that the tail must come to the top and bear the respira- 
tory outlet, for the head bears the mouth and mouth organs, and must 
sweep to and fro in all directions, or even bury itself in the mud in 
quest of food. To divide the work of breathing and feeding between 
the opposite ends of the body is of obvious advantage, for the breath- 
ing can be done best at the top of the water, and the feeding at the 
bottom, or at least beneath the surface. Such considerations seem to 
have fixed the respiratory organs at the tail of the larva. Why then 
need this arrangement be reversed when the insect enters the pupal 
stage? There is now no feeding to be done, and it surely does not 
signify how the head is carried. Why should not the pupa continue 
to breathe like the larva, by its tail, instead of developing a new appa- 
ratus at the opposite end of its body, asif for change’s sake? Well, 
it does not appear that, so far as the pupa itself is concerned, any good 
reason can be given why the larval arrangement should not continue. 
But a time comes when the fly has to escape from the pupa case. The 
skin splits along the back of the thorax, and here the fly emerges, 
extricating its legs, wings, head, and abdomen from their close-fitting 
envelopes. The mouth parts must be drawn backward out of their 
larval sheaths, the legs upward, and the abdomen forward, so that 
there is only one possible place of escape, viz, by the back of the 
thorax, where all these lines of movement converge. If then the fly 
must escape by the back of the thorax, the back of the thorax must 
float uppermost during at least the latter part of the pupal stage. 
Otherwise the fly would emerge into the water instead of into the 
air. Granting that the back of the thorax must float uppermost in 
the pupal condition, it is clear that here the respiratory tubes must 
be set. 
I need hardly speak of the many insects which run and skate on the 
surface of the water in consequence of the peculiar properties of the 
surface film. They are able to do so, first, by reason of their small 
size; secondly, because of the great spread of their legs; and thirdly, 
on account of the fine hairs with which their legs are provided. The 
adhesion of the surface film is measured by the length of the line of 
contact, and accordingly the multiplication of points of contact may 
indefinitely increase the support afforded by the surface of the water. 
