CRUSTACEAN METAMORPHOSES 



By R. E. SNODGRASS 



Collaborator of the Smithsonian Institution and of the 

 US. Department of Agriculture 



INTRODUCTION 



The review of crustacean metamorphoses given in this paper con- 

 tains little that will be new to carcinologists, except perhaps a few 

 accompanying unorthodox ideas. The paper is written for students 

 in general zoology and is recommended reading for entomologists, who 

 commonly think of metamorphosis as a phenomenon pertaining par- 

 ticularly to insects. It is true that the metamorphoses of insects and 

 of crustaceans have no relation to each other, and have little in com- 

 mon, but a preliminary discussion of both will help in arriving at a 

 general understanding of the nature of metamorphosis as it occurs in 

 the arthropods. 



The first treatise on metamorphosis was written by Ovid in about 

 the year A.D. 7, but the metamorphoses that Ovid described were 

 mostly the transformations of members of the human species into 

 animals, trees, or rocks, willed by the ancient gods or goddesses in 

 revenge against some mortal that had offended them. The meta- 

 morphoses imposed on animals by nature are not punishments, ex- 

 treme as they may be in some cases, but are beneficent changes of form 

 to better accommodate the individuals of a species temporarily to a 

 more advantageous way of living. The young butterfly, for example, 

 transformed in the egg into a wormlike caterpillar, is not an elegant 

 creature as are its parents, but from a practical standpoint the cater- 

 pillar is perfectly adapted to its chief function, which is that of feeding. 



The metamorphoses of Crustacea differ essentially from those of 

 insects in that they pertain to a much earlier stage of development. 

 The young insect hatches from the egg usually with the definitive 

 number of body segments. The insects are thus epimorphic ; but if 

 the young insect has taken on a metamorphosed form in its embryonic 

 development, it appears on hatching as a creature quite different from 

 its parents. Yet a caterpillar, for example, is actually a winged juvenile 

 stage of the butterfly corresponding with the so-called nymphal stage 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 131, NO. 10 



