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 7s 47.5 go 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 Melissodes 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. enici; Coreopsis palmata: M. coreopsis ; Helianthus: Halic- 
toides marginatus; Asteroideae: Colletes armatus, Andrena 
nubecula, Pterandrena asteris, solidaginis, Pseudopanurgus 
