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TEXT-BOOK OF ZOOLOGY 



with a protecting case, spun together 

 out of small pebbles, plant stalks, 

 mollusc shells, etc. Many of them 

 carry this case, from which only the 

 hard head and strongly chitinous thorax 

 protrude, about with them as a snail 

 carries its house. They are therefore 



CAD] 



FLIES AND THEIR METAMORPHOSIS. 



In Centre : Empty cases ; composed of : vegetable bodies disposed a. , longitudinally, b. , trans- 

 versely ; c., small pebbles ; d., shells of molluscs (slightly less than natural size). In and 

 above the Water: Larvae of different species in cases composed of different substances ; 

 L., two larvae removed from their cases ; Ph., pupa cases (those on the left hanging to the 

 root of a tree) ; P., pupa which, having crept out of its case, is about to cast its skin and 

 develop into the perfect insect. Above the Water: Three fully developed insects of 

 different species. 



slowly moving creatures, which live on decaying refuse matters. They 

 breathe by tracheal gills, visible externally in the form of fine threads. 



