4 University of Michigar 



They were easily the dominant dragonfly and any student of 

 habitats would have pronounced them one of the essential 

 factors in the balance of this woodland pond. But Libellula 

 quadrimaciilata has appeared at this pond only once. Twice 

 during my observation of this pond Enallagmas have appeared. 

 One of these years was 1920, when Enallagmas probably 

 exceeded in numbers all other dragonflies on the wing at the 

 pond during the same season. Flying a few inches above the 

 water, resting on grass stems, willow tips, and spatter-dock 

 leaves, on the broad surfaces of which the females were ovi- 

 positing, they dominated the entire pond. But they were not 

 there in 1919 or in 1921. The changes which take place from 

 year to year in any odonate society such, for example, as an 

 abandoned gravel pit, may be obscured or subject to misin- 

 terpretation due to these ebbs and flows of odonate life. 



Still another factor is involved which makes general con- 

 clusions as to the character of the habitat of any species of 

 dragonfly dangerous : the great adaptability shown by certain 

 species to thrive in very different habitats. Such differences 

 are usually, if not always, associated with differences in geo- 

 graphical location. For example, Libellula incesta frequents 

 glacial lakes in northern Indiana, especially about growths of 

 Scirpus, spatter-dock and water-lilies. It never occurs about 

 small, muddy, sun-exposed buttonwood ponds ; but in Ten- 

 nessee it does occur about just such unattractive ponds, where 

 no self-respecting Indiana incesia would fly. A species may 

 be a lake-dweller in part of its range, a pond-dweller else- 

 where, and a stream-dweller at a third location. 



Another difficulty in describing habitats arises from sea- 

 sonable differences or evolutionary changes in the habitats 

 which may escape the casual observer. For example, a suc- 

 cession of muddy pools at one season may be a deep, swift- 



