July 1, 1892.] 



KNOWI^EDGE. 



127 



Fig. 1. — Left hind tibii 

 of (a) Bomhiis, (b) P.iithii 

 nts. c. Corbicula. 



Bomhi are broad and flat or slightly concave, and 

 shining; no hairs grow on this shining surface, though all 

 the rest of the body is densely 

 covered, but a closely set row of 

 long, stiff, curved hairs runs along 

 each side of it. sloping backwards 

 and slightly outwards, but not 

 enough to prevent them from 

 making a sort of trough-like enclo- 

 sure, of which the polished surface 

 of the tibia forms the bottom : 

 this is the corbicula, or receptacle, 

 in which the pollen is packed, 

 after having been collected by 

 means of the dense brushes of 

 hairs which cover the body. The 

 tibia of the males is sometimes 

 nearly bare and polished, but 

 its surface is somewhat convex, 

 and the brushes of hairs at the 

 sides spread out almost on a 

 level with its surface, or are 

 indiscriminately arranged, so that no enclosure is made. 

 Now the Pxithyri have no such arrangement as above 

 detailed ; the hind tibiae of the female are not broadened, 

 their outer surface is decidedly convex, not smooth and 

 polished, but covered scantily with hairs, and no erect 

 rows of bristles fringe their side margins, and those of the 

 male are quite similar ; these peculiarities alone are 

 sufficient to distinguish these parasitic bees from their 

 industrious hosts. The Pstithyri, also, are not so densely 

 covered with hairs as the Bombi : certain parts of the upper 

 surface of the abdomen especially have the hairs short and 

 thinly scattered, so that the bare shining skin can easily 

 be seen through them, whereas the Bumhi would have 

 these parts covered completely with dense bands of fur, and 

 scarcely any trace of the skin beneath would be visible. 



Now, as these Psithyri in their larval condition are 

 nourished at the expense of the working Bombi, and are 

 therefore a constant drain on the resources of the nest, it 

 is difficult to believe that the parasites would, if recognized, 

 be tolerated in the nest unless they contributed in some 

 way to the well-being of the establishment ; and if they 

 were at all conspicuously different in appearance from the 

 true owners, it would be a risky matter to enter the nest. 

 more particularly as they are connected almost entirely 

 with those Bomhi that are underground builders, and 

 these are the most fiery-tempered of the genus. As they 

 do not, so far as is known, contribute in any way to the 

 advantage of the colony they are associated with, it is 

 easy to conjecture a reason for the remarkably close imita- 

 tion they exhibit of the appearance of their hosts, a disguise 

 30 complete that they are able to pass in and out of the nests 

 entirely without molestation, a disguise, too, which appears 

 to extend to the larva?, for it is difficult to believe that the 

 workers would consciously waste their time and efforts 

 upon foundlings which in no way serve to strengthen or 

 otherwise aid their own race. There is a curious point 

 in connection with this imitative dress of the parasites that 

 is worth notice. It is obvious that it is only the females 

 and workers that any invaders would have to fear, for the 

 males have no sting, and have therefore no means of 

 attack except their jaws ; the greatest safety would seem 

 therefore to be obtainable by the parasites becoming un- 

 distinguishable from the dangerous groups of their hosts, 

 while resemblance to the harmless males would not be so 

 important. Hence we find that not only do the females of 

 the Psithyri ape the colours of their female and worker 

 hosts, but the males do so as well, thus becoming quite ' 



different from the same sex in the Bombi. Take, for 

 example, the common red-tailed humble bee {B. lapiddrius). 

 The females and workers of this insect are generally black 

 all over except the last three segments of the abdomen, 

 which are bright red, highly suggestive of their fiery 

 temper. But the male has, in addition to its red taU, a 

 yellow-haired face, and a broad yellow band in the front 

 of the thorax. Now the male of the Psitlii/nif: which 

 accompanies this species (P. nipestris) has no yellow, but, 

 like the female, is usually coloured only black and red. 

 with at most a few greyish patches at the sides. The 

 resemblance between the two females, however, is not 

 quite so close as a description of their bodies alone would 

 lead us to imagine, for the wings of the guest are much 

 darker than those of the host. The resemblance between 

 the other species of Psithi/yi and their hosts is perhaps not 

 quite so close as in the case indicated above, but still suffi- 

 ciently so to cause them to be confounded on a casual 

 glance ; examination of the hind legs would at once show 

 their distinctness. In size they usually compare with the 

 male and female Bombi, and are therefore larger than the 

 workers. There may be another reason for the imitation 

 of the female and worker Bomhi, rather than of the males, 

 which may be found in the fact that the males much less 

 frequently visit the nest after they have once left it, and 

 therefore the parasites would become more noticeable if 

 coloured like these occasional visitors, than when i-esem- 

 bling such constant residents as the workers. 



The nests of the red-tailed bee {B. lapidnrius), are fre- 

 quented by another parasite whose relations to its hosts 

 are much more serious. This is a two-winged (dipterous) 

 fly, called Volucella bombijtans. It is something like a 

 large bluebottle in shape, and about the size of the worker 

 bees, which it imitates in colour, being black-bodied and 

 red-tailed. Entering the nests it lays its eggs there, and 

 the maggots issuing from these devour the larvas of the 

 bees, sometimes destroying almost the whole brood. It 

 seems marvellous that these depredations are not checked 

 by the bees themselves, which apparently have the remedy 

 in their own hands, since they might easily sting the larvse 

 to death, even if the parents had escaped notice. One 

 almost feels disposed to conclude that they fail to dis- 

 criminate between the fly maggots and their own larvae, 

 though there is a considerable difference between the two. 

 A similar apparent obtuseness of perception is exhibited 

 with regard to the flies themselves ; for the differences 

 between the two-winged and light-bodied fly, and the four- 

 winged and much more substantial bee are sufficiently 

 obvious, notwithstanding the disguise ; and it is scarcely 

 credible that if they did recognise the flies, and divine in 

 the least degree the object of their visit, they would allow 

 them to enter the nest unmolested. On the other hand, 

 if they really do not distinguish between the flies and 

 their own comrades, the necessity for the imitative dross 

 seems to be rather less obvious. It is generally main- 

 tained that bees recognise their comrades belonging to the 

 same nest, but several observers have suggested that this 

 is by smell rather than by form or colour, in which case 

 strange flies would seem to be placed at a disadvantage, 

 since they evidently would not possess the smell peculiar 

 to the nest they might wish to enter. Sir John Lubbock's 

 well-known experiments serve to show that the colour 

 sense is well developed in bees, but this does not imply 

 an equal power of perceiving details of form, and hence 

 it may possibly be sufficient for the parasites to show a 

 black and red body in order that they may pass muster. 

 The risk run by these flies, however, on the supposition 

 that the bees really do object to the destruction of their 

 offspring, and would fight in their defence if they knew 



