Jan , 1893. AGGRESSIVE MIMICRY AMONG FLIES. 55 
But it is rather in size, and especially in mode of flight, that 
V. bombylans resembles the Anthophore. In both cases the insects 
have a dashing impetuous habit, interrupted by periods of hovering. 
During such rapid movements exact markings fail to catch the eye, 
and the general impression produced by the two is by no means 
dissimilar. 
It is further maintained by some observers that var. mystacea 
resembles B. muscorum. This bee has tawny yellow hairs on the 
thorax, while on the abdomen the hairs are yellow, more or less 
mixed with black, or even entirely black. Whether mimicry can be 
said to exist in this case is a matter of opinion, but it should be borne 
in mind that both B. muscorum and the mystacea variety of V. bombylans 
vary in appearance and colour-effect, according to the direction from 
which they are observed. A freshly-emerged mystacea, if viewed from 
in front, and from a low elevation, would probably be considered by 
very many persons to resemble B. muscorum, while, if viewed either 
from directly above or from behind, the resemblance would probably 
be denied. Moreover, a faded specimen of muscovum loses much of 
the tawny colouring and appears of a yellowish grey. 
Both varieties of V. bombylans have been observed to emerge from 
nests of the three species of Bombus which are chiefly attacked, viz., 
B. muscorum, B. lapidarius, and B. hortorum. ‘The first-named of these 
is apparently the most infested. In this connection the habits and 
demeanour of the bees are of interest. 8. muscovuwm constructs a nest 
on or near the surface of the ground, and is of a comparatively mild 
and pacific disposition. The other two construct their nests at some 
depth from the surface, and are, especially dapidavius, fiercely irritable. 
Thus the nest of mwscovuwm is clearly more accessible to Volucella on 
account both of the weakness of its strategic position and the faint 
heart of the defenders. 
Be this as it may, it is exceedingly difficult to comprehend how 
the colour of Volucella can in any way impose upon any of the bees, 
for so soon as the fly has entered the approach to the nest it must be 
in more or less total darkness, and from that moment the value of 
mere colour and markings is reduced to nil. Any Bombus which 
encountered a Volucella in the darkness could not by any possibility 
discriminate by the sense of sight between friend or foe. It seems 
far more probable that the hairiness of V. bombylans (in both varieties) 
is the feature which should be spoken of as an example of “‘ aggressive 
mimicry.” It is the sense of touch which alone can enable the Bombus 
to detect V. bombylans in the semi-darkness, and surely the hairy body 
is less likely to arouse suspicion when touched than would a smooth 
body, for the latter could in no case enter the nest of Bombus as 
belonging to one of the natural members of the colony. 
This contention derives much support from the fact that the 
other three species of Volucella mentioned above have hairless bodies 
and are parasitic upon hairless Hymenoptera. Our knowledge at 
