162 



KNOWLEDGE. 



May, 1913. 



It gets its name Psithyrus, that is, " whispering," 

 from this. If, then, it cannot quite deceive us, how 

 can it deceive the owner of the nest in which it 

 wishes to lay its eggs ? And in the darkness of the 

 nest where it probably first meets the owner, a 

 resemblance depending largely on colour would be 

 of little avail ! Moreover, are we not told that bees 

 recognise each other and strangers rather by smell 

 than by sight ? 



Recent studies of the parasite and its host, how- 

 ever, compel us to change our point of view some- 

 what. For it does not appear to be the object of 

 Psithyrus to slip unnoticed into the Bombus 

 nest, and lay its eggs unobserved. According to the 

 account given by Mr. Sladen in his recently 

 published work, " The Humble Bee," it invades the 

 nest rather to fight with and kill the lawful queen. 

 The parasites, in fact, rely more on their " exceed- 

 ingly thick and strong skin, covering them like a 

 coat of mail and protecting them from the stings of 

 the Bombi." Moreover, being idle, they are further 

 protected by having no soft wax-yielding membrane 

 between the dorsal segments, as have the humble- 

 bees. The sting is also stouter, and more curved. 

 It does not appear, then, that the resemblance can 

 be of any advantage to the parasite, or that it can 

 deceive the humble-bee. Psithyrus, in fact, neither 

 requires nor makes use of its remarkable 

 resemblance. 



Let us suppose, however, for a moment that the 

 resemblance were perfect, and that the Bombus 

 (jueen took the invader for another queen of her 

 own kind. What would happen ? We turn to Mr. 

 Sladen's book, "The Humble-bee," for an answer. 

 Some of the later-appearing individual queens of 

 the Red-tailed Humble-bee do not take the trouble 

 to start nests of their own, but enter those of others 

 to lay their eggs. At first the stranger is ignored, 

 but soon jealousy arises, and there is a mortal duel. 

 One of the queens is killed, generally, says Mr. 

 Sladen, the intruder. Thus the advantage to the 

 parasite of being like the host is more than doubtful. 



Again, the Humble-bee shows by its different 

 behaviour that it recognises the parasite. " The 

 Bombus queen," writes Mr. Sladen, " on first meet- 

 ing the Psithyrus in her nest, shows a certain 

 amount of agitation and may advance to attack her, 

 but her courage failing she draws back." Evidently 

 the resemblance does not deceive her. And if it be 

 true that bees usually recognise each other by scent, 

 and that they, can in this way detect one of their 

 own species from a different nest, we need not be 

 surprised at this, however perfect the outward 

 resemblance. The first alarm of the Bombus queen 

 at the presence of Psithyrus is followed by a " kind 

 of despondency," and her interest and pleasure in 

 her brood seem less. In the course of time there is 

 a fight, and the parasite generally wins, sometimes 

 killing several workers which have come to the 

 assistance of their mother, as well as the Bombus 

 queen. 



In the face of all these facts what becomes of the 

 theory that the mimicry of Psithyrus is protective, 

 and has been acquired as a useful character by 

 natural selection ? The resemblance does not 

 deceive the host, and it would apparently be no 

 advantage to Psithyrus if it did. 



The resemblance, as Mr. Sladen tells us, is not 

 merely superficial, as in the case of bee-like flies. 

 Psithyrus is almost a Bombus in general structure. 

 And this suggests a close relationship, as if the 

 Psithyrus had been a Bombus which took to 

 parasitism — as individuals of certain species do 

 to-day. Thus, individual queens of the Red-tailed 

 Humble-bee, will, as we have seen, enter the nests 

 of others to lay their eggs. And, if this is so, then 

 it is the points of difference that have to be evolved, 

 and not the points of resemblance. 



Having disposed of the Bombus queen, Psithyrus 

 reigns in her stead. She lays her eggs, utilising the 

 wax she finds in the nest, since she cannot make any 

 of her own. Her young are tended and fed by the 

 Bombus workers. Apparently, as a sort of protest 

 against the usurper, the latter begin to lay eggs, 

 though these would only produce males. The 

 usurping queen, in her turn, eats them ! It is 

 obviously her interest to do so, for thus the workers 

 will devote all their time to the rearing of the young 

 parasites. 



And a very curious problem is suggested by the; 

 fact that the young of the Psithyrus queen are reared 

 by the Bombus workers. For there are no workers 

 among the parasites as there are among the Bombi. 

 And the difference between these workers or neuters 

 and the queens among the Humble-bees is believed 

 to be the result of diet. The workers must settle 

 which are to be queens and which neuter. If a 

 given grub is to become a queen, it must from a very 

 early age be fed with a richer diet. Yet the Psithyrus 

 female grubs all turn out queens ! Are they all fed 

 on the royal diet, or are they all fed on the more 

 meagre diet which some superiority of constitu- 

 tion enables them to turn to better account than 

 would the grubs of Bombus? In either case it 

 is curious that the Bombus workers should not 

 make a difference, feeding some for queens and some 

 for workers, as they would in the case of their own 

 kind. And it would obviously be to the interests of 

 the Red-tailed Humble-bee as a species if the 

 Psithyrus grubs were all fed on so meagre a diet 

 that they all turned out neuters. On the other 

 hand, the feeding of all on royal diet might be a 

 last effort at self-preservation — a vaulting ambition 

 which overleaps itself and falls, on the part of 

 Bombus. For might not one of the grubs fed 

 royally turn out a Bombus queen ? But the only 

 result would be the full complement of Psithyrus 

 queens. If, however, they thought to destroy the 

 parasite race by rearing neuters instead of queens 

 then the superior constitution of Psithyrus, thriving 

 on the more meagre worker diet, developed the 

 queenly character in spite of their efforts. 



