Vol. XXV] ENTOMOLOGICAL NEWS. 69 



positae and say: "It is not so surprising that some of these 

 are exclusive when we consider that at their maximum the 

 Compositae form 34 per cent, of the indigenous flowers." 



At Carlinville the phenological positions of the indigenous 

 Compositae and their oligotropes are : 



Apr.-May Jn.-July Aug.-Sept. Oct. 



Compositae 10.8 57.6 86.9 42.3 



Oligotropes 7.5 47.5 90 50 



It may be that some of these originated under the maximum 

 of the Compositae. But it is a little too much to assume that 

 they originally had a short flight, turned to the Compositae 

 and happened to fall into a nice phenological correlation. My 

 view is that they have a short flight and form their maximum 

 under that of the Compositae because they are oligotropes. 

 They are the most abundant at the time when competition 

 would be the least. Whenever competition becomes the most 

 severe at this point, it will be an advantage to change food 

 habits, or fly earlier or later. 



That the pressure of competition has already reached a 

 severe stage is indicated by Melissodes and at least some 

 genera of Panurgidae. In my opinion Mclissodcs is 

 typically a genus of Compositae oligotropes and the polytropic 

 species, as well as those which are oligotropes of other flowers, 

 were originally developed from oligotropes of Compositae. 

 The same may be true of the Panurgidae, but I am doubtful 

 about it except in some of the genera. 



But it is misleading to speak as if Compositae oligotropes 

 were all competitors. Some do not occur at the same time as 

 others ; probably the flight of no two exactly coincides. More- 

 over they are not all oligotropes of the same flowers. They 

 fall into the following fourteen more or less non-competitive 

 sets. Where two tribes are mentioned most of the visits are 

 to the first : 



Krigia: Pterandrena krigiana; Boltonia: Perditella bol- 

 toniae; Vernonia'. Melissodes vernoniana, vernoniae ; Cnicus: 

 M. cnici; Coreopsis palinata: M. coreopsis; Hclianthns: Halic- 

 toides marginatus ; Asteroidcae : Colletes armatus, Andrena 

 nubecula, Pterandrena asteris, solidaginis, Pseudopanurgus 



