68 SUBSTITUTION OF A LEG FOR A WING IN A MOTH. 



kindly given me some particulars regarding it. He says "In this, 

 as is usual, the supplementary wing is between the others, and 

 partakes of the characters both of the fore and hind-wings ; this 

 and the hind-wing are about equal in size and are rather smaller 

 than the hind-wing on the other side." 



Mr. Mason also informs me that he possesses several examples 

 of the duplication of an organ in beetles, amongst which is a 

 specimen of Chrysomela mentliastri, with three perfect tarsi on one 

 leg. This I should place in the same class of monstrosities as the 

 cases I have mentioned of the duplication of a wing. Professor 

 Westvvood says, in his paper, that a considerable number of 

 instances have occurred in insects of an extra number of legs, or 

 antennae, or of the joints of those organs, but that the duplication 

 of a wing is very rare. 



I will now proceed to consider, with the help of the evidence 

 before me, in what respects this specimen of Z. filipendulw differs 

 from the other cases I have mentioned. It is a well known fact 

 that supplementary limbs are not uncommon amongst vertebrates, 

 and Professor Windle says that they are of the same nature as the 

 adjacent limbs, except where portions of two distinct individuals 

 are present, giving, as an instance of the latter, an arm projecting 

 from a tumour near the thigh. It may not be strictly correct to 

 assume that the same laws hold good as regards monstrosities in 

 invertebrates, but still I think that we may look on the two as 

 somewhat analogous, and, according to the above law, we may 

 regard this fifth wing in my specimen of Z. filipendulw as corres- 

 ponding to the arm projecting from the tumour near the thigh in 

 the vertebrate. Eggs containing two germs, or double-yolked eggs, 

 are not at all unusual amongst fowls, and I have found them more 

 than once in the case of the common earthworm, and though I am 

 not aware that they have been recorded amongst insects, still I see 

 no reason why they should not occur. There is little doubt that 

 a double monster cannot be produced except from one of these 

 <&,'*, though, as a rule, two distinct and separate individuals 

 w..\ild be formed, as has been the case with the double worm's 



