152 INTRODUCTION 
The power of folding up the lower mouth-parts into the cavity of the head for 
biting, and of unfolding and protruding these parts for sucking, exists in Digging 
Wasps (Fossores) much as in Prosopis. Although these insects are sometimes found 
on flowers, most of them store up other insects in their holes in the earth or 
in walls for the use of their larvae. Prosopis, therefore, is not specially adapted 
to flower-food. 
Hermann Miiller (‘ Fertilisation,’ p. 49) goes on to say that Sphecodes, Halictus, 
and Andrena (genera related to Sphecodes, but much more specialized) are consider- 
ably higher in the scale than Prosopis, in regard to adaptation to flower-food. In 
all three the tongue is still moderately short (Fig. 65, 4, 77; Fig. 60, 1), but is able to 
Fic. 65. Sphecodes (after Herm. Miller). (2) Head of Sphecodes, with mandibles opened, but lower 
mouth-parts folded and hidden under the labrum; seen from in front and below. (3) The same, after 
removal of the mandibles and of the labrum, with unfolded and protruded lower mouth-parts. (4) End of 
labium more highly magnified, seen from above. References as in Fig. 64. 
insert itself into somewhat deeper nectar-receptacles, not so much on account of its 
own length as by the greater elongation of the mentum and cardines. The ligula 
(unlike that of Prosopis) is here pointed, more or less hairy, and marked by fine 
transverse lines at its tip (Fig. 65, 4). In some species of Andrena and Halictus it 
has assumed a much narrower and more slender form, for it is here less concerned 
with nest-building, as the bees in question smooth the walls of their brood-chambers 
(which are mostly underground) with only a very little slime. 
In these four genera, increased protrusibility of the ligula is therefore attained 
by a lengthening of the mentum and cardines. Owing to the length of the head, 
beneath which these parts must be withdrawn when the mandibles are used, this 
extension is naturally limited. Hence the only further adaptation enabling more 
deeply seated nectar to be reached is by elongation and increased development of 
the ligula itself, with the extension of the connecting piece between the mentum 
and cardines. 
We therefore find, among the higher forms of both scopulipides and dasygastres 
(‘ Fertilisation,’ pp. 56 et seq.), that the ligula, which in bees little adapted to flower- 
food is much shorter than the mentum and retractile into it, may be many times 
the length of the mentum. At the same time, the transverse striations (absent in 
