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uppermost. This is effected by the property which the 

 centre of the thorax has, of repelling water ; hence, as soon 

 as the pupa has once forced this part of the body above the 

 surface, the water retreats from it on all sides, leaving an 

 oval space in the disc which is quite dry ; and the attrac- 

 tion of the air to the dry part of the thorax is sufficient to 

 overbalance the specific gravity of the pupa. If a drop of 

 water be let fall on the dry portion, it instantly sinks to 

 the bottom, but soon returning to the surface, it remains 

 suspended as before. Just previous to the exclusion of the 

 Fly, the thorax is seen to split in the middle ; the air 

 enters and forms a brilliant stratum resembling quicksilver, 

 between the body of the insect and the pupa-case, and the 

 former, pushing forth its head and fore legs, like the Gnat, 

 rests the latter on the water, and in a few seconds extricates 

 itself wholly from the puparium.* 



* British Cyclopaedia, article Ckironomus. 



