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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 attraction 

 of the air to the dry part of the thorax is sufficient to over- 

 balance 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 sus- 

 pended as before. Just previous to the exclusion of the My, 

 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 Chironomus. 



