poe Oo LINGUISHING THE SPECIES. 147 
face of the armature much depend on the way that 
the light falls on it, and are of no particular import- 
ance. 
Variation in coat colour in a species consists 
chiefly in an increase or reduction of black, but in 
a few species the light colours may, to a slight 
extent, encroach upon or displace one another. 
On the Continent dark and light varieties, often 
separated geographically, of almost every species, are 
to be found; but in this country, except in the cases 
of Bombus ruderatus and Psithyrus campestris, which 
are often black all over as the result of dimorphism, 
the range of variation in blackness is, as a rule, 
small, and some species scarcely show any at all. 
Examination of series of specimens of the different 
species shows that the darkening generally extends by 
black areas encroaching upon the yellow, red, or white 
ones, but there is often more or less mingling of the 
black hairs with the lighter ones, the latter, at the same 
time, losing their brightness : in some few cases, however, 
the pale hairs gradually deepen without any admixture 
of black hairs. It depends upon the species to what 
extent and in what parts of the body any of these pro- 
cesses takes place. In the queens of B. ¢errestris it is the 
yellow band on the thorax that is most likely to be 
reduced or effaced, while in 5. pratorum it is the yellow 
band on the abdomen. In B&B. helferanus the thorax 
darkens by a mingling of black hairs and the abdomen 
by a deepening of yellow and brown. Yellow bands 
usually divide in the middle, or grow narrow, as they 
darken ; in many species they do both. 
In the descriptions of the species that are to 
follow, it has been found convenient to speak of 
