HUMBLE-BEES AND OTHER MATTERS.
Two humble-bees, Bombus thoracicus and B. violaceus,
are found on the pampas; the first, with a primrose
yellow thorax, and the extremity of the abdomen bright
rufous, slightly resembles the English B. terrestris;
the rarer species, which is a trifle smaller than the
first, is of a uniform intense black, the body having
the appearance of velvet, the wings being of a deep
violaceous blue.
A census of the humble-bees in any garden or field
always shows that the yellow bees outnumber the black
in the proportion of about seven to one; and I have
also found their nests for many years in the same proportion;
about seven nests of the yellow to one nest of the
black species. In habits they are almost identical,
and when two species so closely allied are found inhabiting
the same locality, it is only reasonable to infer
that one possesses some advantage over the other, and
that the least favoured species will eventually disappear.
In this case, where one so greatly outnumbers the
other, it might be thought that the rarer species
is dying out, or that, on the contrary, it is a new-comer
destined to supplant the older more numerous species.
Yet, during the twenty years I have observed them,
there has occurred no change in their relative positions;
though both have greatly increased in numbers during
that time, owing to the spread of cultivation.
And yet it would scarcely be too much to expect some
marked change in a period so long as that, even through
the slow-working agency of natural selection; for it
is not as if there had been an exact balance of power
between them. In the same period of time I have
seen several species, once common, almost or quite
disappear, while others, very low down as to numbers,
have been exalted to the first rank. In insect
life especially, these changes have been numerous,
rapid, and widespread.
In the district where, as a boy, I chased and caught
tinamous, and also chased ostriches, but failed to
catch them, the continued presence of our two humble-bees,
sucking the same flowers and making their nests in
the same situations, has remained a puzzle to my mind.
The site of the nest is usually a slight depression
in the soil in the shelter of a cardoon bush.
The bees deepen the hollow by burrowing in the earth;
and when the spring foliage sheltering it withers up,
they construct a dome-shaped covering of small sticks,
thorns, and leaves bitten into extremely minute pieces.
They sometimes take possession of a small hole or
cavity in the ground, and save themselves the labour
of excavation.
