82 BULLETIN OF WISCONSIN NATURAL HISTORY SOCIETY. VOL. 1, NO. 2. 
pollen from one plant-species exclusively, or from a limited num- 
ber of closely related species, has been called by Loew (8j an 
ol inotropic bee, and for a bee that collects pollen on an extended 
number of species, belonging to different families the same author 
has selected the name polytropic. Robertson (9), in a paper on 
the visits of oligotropic bees refers to five oligotropic bees taken 
on four species of Salix in ^Macoupin County in Southern Illinois, 
and four of these I have observed on the flowers of Salix discolor, 
where they appear as regular and frequent visitors. Their names 
are Andrcua inariac, Rob., Andrcna illiiiociisis, Rob., Andrena 
crythrogastra, Ashm. and Parandrcna andrcnoidcs, Cr., and they 
may be seen on the flowers in both sexes, the females paying more 
attention to the staminate catkins in search of pollen and nectar, 
and the males visiting both staminate and pistillate catkins for 
nectar only. 
The willows which furnish pollen to these oligotropic bees are 
of vital importance in the economy of these bees, the latter utiliz- 
ing the pollen mixed with honey as food for their ofifspring, and 
depending strictly on the pollen produced by willows for their 
purp^oses. On the other hand, the existence of these bees is of 
great advantage to the willows, furnishing them a set of regular 
visitors, which need an ample amount of pollen, as well as of 
nectar, and which for this reason can be relied upon to visit both 
staminate and pistillate flowers, thereby rendering excellent 
services as cross-fertilizers. 
In favorable weather the Andrenidae visit the catkins of Salix 
discolor in great numbers, and they, together with members of the 
most important family of anthophilous flies — the Syrphidae — rep- 
resent a high percentage of the visitors. AMierever observations 
have been made, in this country as well as in different localities of 
Europe, the results always point to the Andrenidae and Syrphidae, 
as being the most prominent agents in effecting cross-pollination 
in the willows. Of the eighty-four insects making up the list for 
Salix discolor, thirty-one belong to these two families, amounting 
to 37 per cent, of the visitors. A comparison of these figures with 
those otbained by Robertson (10) in Southern Illinois, by Herman 
]\rueller (tt) in Germany, and by MacLeod (12) in Flanders on 
several early species of Salix, gives the following results : 
(8) E. Loew. Beobachtuneen ueber den Blumenbesuch von Insecten an Freilandpflanzen.' 
Jahresb. d Bot. Gart. Berlin. Vol. III. 1884, 
(9) C. Robertson. Flowers and Insects. XIX. Bot. Gazette, Vol. XXVIII (1899) p. 36. 
(10) C. Robertson. Flowers and Insects. XV. Bot. Gazette, Vol. XXI (1896) pp. 76-78. 
(11) H. Mueller. Die Befriichtune: der Blumen diirch Inseckten. 1873. pp. 149-150. 
Wei-ere Beobachtuneen Vol. li. 1879. p. 210. 
(12) Mac Leod. Over de bevruchting: der bloemen in het kempisch jredselte van Vlaunderen . 
Bot. Jaarboek 6, pp. 129-133. 
