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Ch. XL.] 



MIGRATION OF INSECTS. 



377 



indmduals of six years' g-rowtli, and the tAvo largest were four 

 inclies long and tliree inches and a half broad,"^ 



From this example we learn the manner in which oysters 

 may be diffused over every part of the sea where the crab 



rig. 136. 



Eggs of freshwater MoUiisks. 



Fig. 1. Egg'sof Ampullaria ovata (a fluviatile a dead leaf lying under water, 



species) fixed to a small sprig whick had fallen Fig. 3. Eggs of the common Limneus (X. 



into the water. vulgaris), adhering to a dead stick under water. 



Fig. 2. Eggs of Planorhis albus, attached to 



wanders ; and if they are at length carried to a spot where 

 there is nothing but fine mnd^ the foundation of a new oyster- 

 bank may be laid on the death of the crab. In this instance 

 the oysters survived the crab many days^ and were killed at 



last only by long exposure to the air. 



GEOGEAPHICAL DISTEIBTJTIOK AIS'D MIGEATIOKS OF mSECTS. 



The entomological provinces coincide very closely with 



as already described. Few species 



animal 



have a very wide range, but there are exceptions to this 



r 



m 



butterfly {Va7iessa cardui)^ which re-appears at the Cape 

 of Good Hope and in New Holland and Japan with scarcely 



^ Mr. Brodorip observed that this have stated that the species moults 



crab, which was apparently in per- 

 fect health, conld not have cast her shell ^^^ 



for six years, whereas some naturalists of the animal. 



annually, without limiting the moulting 

 period to the early stages of the growth 



