BEE-EATERS AXD SWALLOWS. 
9 
up without a proportionate waste and a reparation 
of that waste, which is effected in some degree hy 
means of respiration. The auxiliary breathing which 
they have in supplement to the common action of the 
lungs no doubt prevents the exhaustion of those 
organs, and checks fatigue, by the buoyancy which 
it gives to the bird. Feathers that take much hold 
on the air would impede their progress, and be the 
very worst adapted for them, and accordingly all their 
feathers are firm aud smooth ; so that constant exertion 
is required to preserve their elevation in the air. 
The whole of the race are strictly insectivorous, and 
never destroy anything that is useful to man. How 
much they contribute to the preservation of many 
things that are valuable to him can scarcely be con- 
ceived, the number of insects that they capture being 
beyond computation. Many of the flies that are taken 
in the upper air by Swallows, are captured while wing- 
ing their way for the purpose of depositing their eggs 
in situations where the grubs would be very injurious 
to vegetable life. Against these little depredators 
man has few direct defences within the range of his 
aids ; and it is on this account that the insect-feeding 
birds, and the Swallows which obtain their food where 
no other birds can, are so very valuable, and are of 
such use in the economy of nature. 
But let us see how the Swallow is enabled to perform 
these feats, and what is the modification of form and 
power requisite for such a purpose. 
The general characters are readily perceived. They 
are all, observes a writer in the “ British Cyclopaedia,” 
very thickly formed in the anterior part of their 
