October, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



239 



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j nLy..,iUi of Arc 



Apparent Motion of VI. and VII. in 1904-5. 



A similar confusion formerly pre\ailed in Saturn's 

 system, and was remedied by dropping the numbers 

 and substituting names. 



The four Galilean satellites were long ago named lo, 

 Europa, Ganymede, and Callisto. These names, how- 

 ever, are now seldom used. It would seem to be an ap- 

 propriate time t,o revive them in lieu of the more 

 prosaic numerals, and to give proper names to the 

 three new satellites. 



The tenth satellite of .Saturn, recently detected on the 

 Harvard plates, is interesting from its period being 

 21 days, almost the same as that of Hyperion. This 

 presents another case of linked satellites, but in this 

 case, unlike that of \T. and VH., the planes of motion 

 are probably nearly identical, so that very close ap- 

 proaches are possible; it is rather curious that soon 

 after the discovery of Hyperion its minuteness 

 suggested that it might be one of a ring of satellites 

 analogous to the zone of asteroids, an idea which R. A. 

 Proctor endorsed in his imaginative essay, " A Voyage 

 to the Ringed Planet." This anticipation seems 

 worthy to rank with Swift's and X'oltaire's suggestions 

 of two Martian moons as a remarkable astronomical 

 prophecy. 



It is satisfactory to learn from a recent Harvard 

 circular that Phoebe has again been photographed 

 during the present apparition of Saturn, the positions 

 agreeing so closely with those predicted from the ele- 

 ments given last year as to remove the smallest doubt 

 as to the substantial accuracy of the adopted orbit. 



Dr. F. E. Ross has been engaged on the study of the 

 orbit and perturbations of Phoebe, and I understand that 

 his researches have already been published in the 

 Harvard Annals, but they do not seem to have arrived 

 in England as )'et. When they arrive they will be 

 studied with great interest, as likely to throw much 

 light on the perplexing problems which these distant 

 satellites present to us. 



Mimicry among Insects. 



By Percy Collins. 

 It has been said tliat the strongest testimony to the 

 value of warning coloration is afforded by the like- 

 ness which harmless insects sometimes bear to dan- 

 gerous or noxious ones. Such instances are generally 

 referred to as " mimicry," although the title is also 

 (though somewhat unwisely) employed in descriptions 

 of deceptive appearances which should really be spoken 

 of as protective resemblance. True mimicry, accord- 

 ing to the accepted scientific meaning of the term, 

 consists in the external likeness of a poorly-protected 

 creature to a well-protected one, whereby the former 

 is enabled to share in the immunity from attack en- 

 joyed by the latter. 



Not infrequently, the young student finds some 

 difficulty in comprehending fully the theory of mimicry, 

 when first the subject is presented to his mind. In 

 such cases a direct appeal to nature is usually more 

 fruitful than abstract explanations. Let us, therefore, 

 take an actual instance of mimicrv among British 

 insects. 



The poplar clearwing (Scsia apiformis), in its general 

 appearance, is exceedingly unlike a tvoical moth. Its 

 wings are transparent, tinged with yellow ; its thorax 

 is brown, with a square patch of bright yellow on each 

 side in front ; its abdomen is yellow with a brown belt 

 near the base, and another near the middle ; while its 

 legs are deep orange. It has, moreover, a general 

 aspect of trimness and alertness very unusual among 

 the Lcpidoptera. 



But although the poplar clearwing is unlike a moth, 

 it is very much like a hornet. Indeed, it is doubtful 

 whether a person unversed in the study of entomology 

 could distinguish between the two insects merely by 

 ocular examination. Yet a hornet and a moth belong 

 respectively to totally distinct orders of insects ; what, 

 therefore, can be the meaning of the close external like- 



