PI. VIII. — Insect structures: i, gill of maj^fly, Siphlurus, 

 showing the branching air-tubes ; 2 , underside of water penny 

 showing its white, fringed gills; 3, subimago mayfly (a, 

 oesophagus; h, gas-filled stomach; c, ovary; d, intestine; e, 

 Malpighian tubules) ; 4, subimago mayfly with alimentary 

 canal removed {a, muscles; b, nerve chain; c, ovary). 



Photo, by A. H. Morgan 



