Occasional Papers of the Museum of Zoology 3 



Dragonflies as adults, when they are usually most readily 

 observed, may have a very short life, and a few days differ- 

 ence in visiting- some suitable habitat may result in the failure 

 to find a single individual of a certain species which may have 

 been there in numbers a few days before or after the visit. 



Some dragonflies also vary from year to year, due to early 

 or late seasons, in the time of their appearance. If their sea- 

 son is very short, the student, if not continuously in the field, 

 looking for the species only on certain dates, may miss it 

 entirely. 



Some dragonflies, if not all, seem to occur in relatively 

 large numbers only at intervals of undetermined duration. 

 The species maintains itself continuously in a certain habitat, 

 but has "lean and fat years." This phenomenon may be 

 obscured or confused by the ebb and flow of odonate life 

 described in a later paragraph. 



Many dragonflies are very susceptible to some other con- 

 ditions which are probably meteorological. On a certain day 

 a pond, for example, may be alive with the active individuals 

 of many species. The succeeding day may be, to the observer, 

 similar to the day before, but at the same pond fewer species 

 and a smaller number of individuals may be on the wing. 



The student of habitats encounters another difficulty. 

 Dragonflies "come and go" — there are great ebbs and flows 

 over long periods. I do not refer to the occasional individual 

 strays or waifs, of which the collector finds a few during 

 many years of collecting, but rather to large and, apparently 

 at the time, successful invasions of a new habitat. Two good 

 examples come to mind. Near Bluft'ton, Indiana, is a wood- 

 land swamp of possibly three or four acres. I have had this 

 pond under observation for over twentv years. One year 

 Lihellula quadriniaculata appeared there in great numbers. 



