( cxcii ) 



area of the exoskeleton (e. g. the clypeus, the humeral tubercles, 

 the scutellum, etc.), or — and this is generally the case when 

 they appear on the abdomen — take the form of mere spots 

 or stripes, the latter often running along the margin of a 

 segment, but occasionally crossing its middle. 



Aculeates whose integument is unicolorous are generally 

 black ; but some workers of Ants, even in this country, and 

 a few sub-tropical desert-haunting forms in all groups are 

 entirely pale yellow or testaceous ; and there are also some 

 practically unicolorous Fossors, Bees, and Wasps which are 

 distinctly "metallescent," e.g. spp. of Scelijyhron, Osmia, 

 Ceratina, Halictus, etc. 



Whatever be the ground-colour of an insect, black, red-and- 

 black, yellow or metallescent, it may or may not be diversified 

 by a paler pictura albida : whether it be so or not, or to 

 what extent it be so, depends generally on the species, but 

 sometimes, as we shall see presently, upon the sex. 



Also it depends chiefly on the species, but in certain cases 

 on the sex, what the ground-colour is. There are species in 

 which one sex is black, the other red-and-black : or in which 

 both sexes are red-and-black, but one shows more red than the 

 other. Again, where both sexes are metallescent, there may 

 be a difference in the particular metallic colouring of the two 

 sexes. And to a certain extent these phenomena seem to 

 follow, as I shall try to show, certain rules. 



It is, therefore, not without exception that we can admit 

 Wallace's generalisation, quoted above, that " the two sexes of 

 the stinging Hymenoptex-a are equally well-coloured " ; though 

 I think it must be allowed that sexual difference plays a very 

 minor part in determining the general style of their coloration. 



Summing briefly what has been said, I would suggest that 

 we may roughly divide the insects under consideration into 



(1) black, (2) red-and-black, (3) pale, and (4) metallic forms, 

 and again into (1) forms with '^pictura albida," which will 

 include all the black-and-yellow (wasp-like) species, and 



(2) forms without it, including not only entirely black or 

 metallic species, but red-and-black insects like Sphecodes and 

 other similarly coloured Bees and Fossors, provided that they 

 have no white or yellow markings. And I believe that any one 



