THE HYDROUS PICEUS. 295 
not maintain itself horizontally at the surface of the water, but 
it pokes out its head above water mark, and folds its antenne, 
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THE METAMORPHOSIS OF S/ydrophilus piceus. 
the club-shaped ends of which are partly hollowed out; and thus 
carries downwards a small bubble of air, which is pressed against 
the sides of the thorax and of the abdomen, and \. hich then comes 
