THE DRAGONFLY LARVA 



By R. E. SNODGRASS 



Collaborator of the Smithsonian Institution and of the 

 U. S. Department of Agriculture 



INTRODUCTION 



Seeing may be convincing evidence of the actuality of a fact, but it 

 does not necessarily impart an understanding of the thing observed. 

 We may, for example, w^ith our own eyes witness the splitting of the 

 skin over the back and head of some young insect, such as the aquatic 

 larva of a dragonfly, and observe the emergence of the winged adult 

 fully equipped for life in the air. The change is so great as to be al- 

 most unbelievable if it were not a commonplace event, and yet with all 

 that entomologists now know about insects, this phenomenon of meta- 

 morphosis still baffles the understanding. 



The young dragonfly (fig. i) is as perfectly constructed for the life 

 it leads in the water as are its parents for life in the air. The length 

 of the larval life from hatching to maturity may be one, two, three, or 

 even five years, according to the species, and as the larva grows it may 

 shed its outer skin a dozen or more times. At each moult except the 

 last it simply repeats its own structure on a slightly larger scale with 

 a few developmental changes ; at the last moult it transforms into a 

 creature entirely different from itself, but which is a replica of one 

 of its parents. Throughout its life, therefore, the dragonfly larva car- 

 ries some latent power that finally gives rise from the larval tissues to 

 the parental form that produced the egg from which it was hatched. 



The structural changes that take place at the transformation of the 

 larva to the adult, according to Whedon (1929), do not appreciably 

 affect the nerve chain, the heart, the Malpighian tubules, or the gonads, 

 all of which develop progressively from embryo to the imago, except 

 that there probably takes place some histological changes in the nerve 

 ganglia, together with an atrophy of larval nerve branches and the 

 formation of new ones in adjustment to the reorganization of the 

 alimentary canal and the muscular system. Metamorphosis, however, 

 brings about great changes in the integument, the musculature of the 

 head and abdomen, the alimentary canal, the fat body, and the tracheal 

 system. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 123, NO. 2 



