1913] Biology of May-flies. 381 



specialization are examples which show the variety of ad- 

 justments within this family. As later discussions will point 

 out, they also show what diverse structures may fit an organism 

 equally well for life in the same of similar situations. 



Since the outside of an animal is the first to be influenced 

 by environment, the most important adjustments must be 

 looked for in external structures. In this study only the 

 three most important sets will be considered; those which 

 have to do with respiration, food and motion. 



SHAPE OF BODY IN THREE SUBFAMILIES OF MAY-FLIES. 



Before attempting to trace the adaption in the three sys- 

 tems just named, it is necessary to briefly describe the general 

 shape of the nymphs in the three subfamilies. 



The bodies of the Ephemerinae are elongate, more or less 

 cylindrical and tapering at either end. Those of Ephemera 

 (PI. XLIV, Fig. 8) and Hexagenia are almost perfectly cylindri- 

 cal. The heads are wedge-shaped with the mandibular tusks 

 projecting sharply in front. The bodies of Polymitarcys and 

 Potamenthus (PI. XLIV, Fig. 7) are flattened. The head of 

 the latter is short and broad with the mandibular tusks barely 

 showing beyond the labrum. A comparison of Ephemera 

 (Plate XLIV, Fig. 8), with Potamanthus, will immediately show 

 Ephemera to be the burrower. 



In the Heptageninas, the head, the body and all its append- 

 ages are depressed. In Iron and Epeorus (PI. XLII, Fig. 4),. 

 which inhabit the swiftest water, this depression is greatest,, 

 but in Heptagenia and Ecdyurus, it is also very pronounced. 



The form of the Bastinae is various. The most represen- 

 tative is the slender compressed body and rather small rounded 

 head which is characteristic of the active nymphs like Callibsetis, 

 Ameletus, (PI. XLIII, Fig. 5 and PL XLII, Fig. 3). All of these 

 nymphs have long legs for running and jumping, but in another 

 type, the body is shortened, more or less flat upon the ventral 

 side, and thickened through the metathorax. Such a form is 

 represented by the majority of the Ephemerellas. It is most 

 marked in the very short stubby bodies of Casnis and Tri- 

 corythus, which have become exclusively mud dwellers. In 

 Blasturus (PL XLII, Fig. 1) there is a tendency to a depressed 

 form. This is more pronounced in Choroterpes, which is strik- 

 ingly similar to the Heptageninae. 



