344 
POPULAK SCIENCE EEVIEW. 
object of which in the vegetable economy is not so evident : 
viz. by actually eating it. This has chiefly been observed in 
the case of flies or Diptera belonging to the class Syrphidse, 
the movements of which in summer and autumn, in hovering 
over flowers and then suddenly darting upon them, are so 
remarkable. Many entomologists doubt whether it is possible 
for flies, which have no mandibles, and whose only food- 
obtaining organ is a proboscis adapted for suction, to masticate 
so comparatively hard a substance as pollen-grains. This need 
not, however, present a difficulty to anyone who has smarted 
under the irritating attacks of flies and midges during rainy 
weather in mountainous countries. We have ourselves dis- 
sected the bodies of flies belonging to this family, and found 
their stomachs in many cases perfectly loaded with pollen- 
grains. Prof. Muller takes this view very decidedly, and gives 
some admirable drawings, from which we have taken our 
fig. 5, to show the manner in which the extreme tip of the 
proboscis is furnished with a number of cross-bars, r, by means 
of which, as he has himself observed, these insects are able 
even to tear asunder the fine threads by which the grains of 
pollen are frequently attached to one another, as in the 
evening primrose. Fig. 5, a, represents the extremity of the 
proboscis of one of the largest and commonest of these 
hoverers,” Eristalis tenax, closed ; b the same open, showing 
the cross-bars. 
It is often a matter of surprise to the cultivators of flowers 
that many species which flower luxuriantly in our gardens 
never produce fruit or seed, though all the separate organs of 
the flower appear to be perfectly developed. This is the case, 
for instance, with the large white Convolvulus grown frequently 
against the walls of houses, and with the yellow jessamine 
which flowers in the winter, and to a less extent with the 
Calycanthus or allspice-tree. The reason of this is no doubt 
generally the absence of those insects which serve as their 
fertilisers in their native country, our native species either not 
being attracted by their foreign nectar, or not possessing the 
mechanical appliances necessary to obtain it, and hence not 
visiting the flowers. 
We mentioned at the outset that though the large majority 
of flowers are cross-fertilised, yet there are exceptions to the 
rule. Darwin has described the peculiar contrivance by which 
self-fertilisation is effected in the singular Bee-orchis {Ophrys 
apifera) of our chalk hills, alone among our native orchids. 
There are not a few flowers which never or scarcely ever com- 
pletely open their petals so as to allow either the entrance of 
an insect or the escape of the pollen. An instance of this is 
furnished by the pretty little bog plant the sundew {Drosera 
