vin DISTINGUISHING 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. terrestris it is the 

 yellow band on the thorax that is most likely to be 

 reduced or effaced, while in B. pratorum it is the yellow 

 band on the abdomen. In B. lielferanus 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 



