EMBRYONIC DEVELOPMENT 



215 



insects. In some kinds the changes are very small from stage to stage, 

 as in the bugs (Fig. 188). These sucking insects shed their skins periodi- 

 cally as they grow, and at each change they are a little more like the adult. 

 At the very beginning, however, they are easily recognized as bugs. 

 Such a series of changes is scarcely metamorphosis at all but is usually 



Fig. 187. — Metamorphosis of frog. 1-4, growth of gills; 5-6, covering of gills by 

 operculum, degeneration of sucker; 7—10, growth of legs (9 shows greatly elongated intes- 

 tine) ; 11, fore legs pushed through operculum; 12-14, degeneration of tail. {Rearranged 

 from Newman, "Outlines of General Zoology,^' by perm.ission of The Macmillan Company.) 



called incomplete metamorphosis. Contrasted with this gradual trans- 

 formation is the very marked one which flies, butterflies, bees, and beetles 

 experience. In the flies (Fig. 189) the larva is a legless wormlike animal 

 called a maggot. This changes, in a very brief operation, into a quiescent 

 nonfeeding form, the pupa. After a definite (usually short) time there 

 bursts from the pupa shell the adult insect. The development of the 



