POLLINATION OF THE BRITISH PRIMULAS. 131 
admitting that Thrips are capable of effecting pollination, the amount of such 
pollination must be exceedingly small, owing to their minute size and the 
fact that they are not known to be capable of a flight of sufficient length to 
enable them to pass from plant to plant *. 
As to the few small pollen-eating bees forming Group 3: they are all 
quite incapable of effecting the pollination of the flowers in the regular and 
orthodox manner. They are all robbers merely, and their frequent visits to 
the flowers are made solely for the purpose of stealing pollen. Their visits 
are, therefore, prejudicial, rather than beneficial, to the plant. 
Nevertheless, it seems certain that these bees do pollinate the flowers 
effectually, to a certain extent, but those of the long-styled form only, and even 
those only in an irregular and unorthodox manner. ‘Thus, the pollen they 
gather must come entirely from flowers of the short-styled form (in which 
the anthers are exposed at the mouth of the corolla-tube) ; for, in flowers of 
the long-styled form, the anthers are placed too far down the tube to 
be accessible to them. If, however, one of these bees, having visited a 
short-styled flower and become dusted with its pollen, happens to visit, even 
momentarily, a long-styled flower (in which the stigma is exposed in the 
mouth of the corolla-tube), there is a probability that it may leave some 
pollen upon (and thus effectually pollinate it) ; but this cannot take place in 
the case of short-styled flowers (in which the stigma is situated far down the 
tube and is quite inaccessible to the bee). 
Of the last-named of these bees, Andrena gwynana, Müller, after stating 
that it gathers pollen from short-styled flowers only, adds t that— 
It visits the long-styled form also, but flies away immediately—not, however, 
without performing cross-fertilization in the momentary visit. I have never seen a 
pollen-collecting humble-bee alight on a long-styled flower. It seems to recognize 
them at a distance and to avoid them f. 
Prof. Weiss, in the course of his observations in Shropshire §, noticed that, 
from short-styled flowers, this bee 
gathered the pollen greedily and, in getting at it, poked its head deep into the mouth 
of the corolla. It naturally [he adds] stayed longer in the short-styled flowers, but 
its movements in the long-styled forms [? flowers] were sufficient to pollinate the 
protruding stigma. 
From this, he concludes that these bees are “very active agents of cross- 
pollination of the Primrose ”—a conclusion I am quite unable to accept. . 
* See Bell, ^ Primrose and Darwinism,’ pp. 35, 36. 
+ ‘Fertilization of Flowers,’ p. 385 (see ante, p. 118). 
+ If this is really so, these bees, in their visits to the flowers of Primula, are “ constant,” to 
a certain extent, to one furm, just as most insects, when visiting other flowers, are constant, 
more or less, to one species. 
§ New Phytol. ii. p. 103 (see ante, p. 112). 
