Notes on some Colour-Mutations of Certain Lepidopterous Species, with Remarks on the Preference of Like for Like as Affecting the Breeding of such Mutations.

By A. V. Chappell
Communicated By L. W. McCaskill

[Read before the Canterbury Institute, September 4, 1935; received by the Editor, March 9, 1935; issued separately, October, 1935.]

An interesting example of the preference of like for like was afforded by the Arctiid species, Tyria jacobaeae, which has been introduced into New Zealand by the Cawthron Institute in an endeavour to control Ragwort (Senecio jacobaeae) in the less accessible regions of the country.

In the course of handling emergents from many thousands of pupae, two very distinct and radical colour-mutations of this species were encountered. The normal form is black and crimson. But several specimens obtained were of black and yellow colouration, while in one other the crimson was entirely replaced by dull brown. The author attempted cross-breedings between these mutations and the normal forms, but without success. In each case it was very difficult to obtain a cross-mating, whereas normal-to-normal matings presented no difficulties whatsoever.

The procedure employed was to isolate a single specimen of a mutation with several normal specimens of the opposite sex, care being taken that all individuals were sexually mature. In no instance was copulation observed, but it must have occurred in one case, as a female yellow mutation laid a number of eggs which later proved to be fertile. The resultant larvae, however, failed to survive, all becoming victims, in spite of precautions taken, of a polyhedral disease which attacked them at an unusually early age. (In the Nelson region of New Zealand Tyria jacobaeae is particularly susceptible to the ravages of a polyhedral disease, a disease from which most native Lepidoptera suffer but little.) The only other mutative female to yield eggs was the single dull brown specimen. In this case the eggs were very few and all infertile. Although normal forms were so reluctant to pair with the unusual mutations, they would, on removal to a cage containing normal representatives of the opposite sex, mate within a short time. A more frequently encountered and less radical colour-variety of Tyria jacobaeae has the crimson replaced by pink. But normal forms will mate readily enough with specimens of this variety.

This preference of like for like as exhibited within the Arctiid species Tyria jacobaeae and the Pterophorid species Alucita monospilalis raises several interesting points.

It will be readily appreciated that such preference must exercise a strong control on the continuity or perpetuation of such colour-mutations or sports as should happen to arise within a species. Should the number of colour-mutations be large, there is a fair chance of such mutations being able to mate with like forms, thus increasing the possibility of the appearance of certain mutations in succeeding

certain species of other genera inhabiting the same localities. Moreover, in the usual habitat of E. hemipteraria the colouration of this mutation is, if anything, even more effective than the normal colouration in rendering the moth inconspicuous. So this mutation can scarcely be deleterious to the existence of the species. Yet, judging by the extreme rarity of this particular mutation, most of such individuals as do occur would seem to die without mating and thereby having the opportunity of transmitting their characteristics to future generations.

Another New Zealand species of the same genus and occurring in the same localities as E. hemipteraria is also of interest. E. alectoraria is a fairly variable species, though the variation is usually confined to gradations of the normal colouring; but occasionally there appear forms having extra markings of conspicuous and unusual colourings. Again the colouration of these occasional mutations is at least equally effective for purposes of concealment as is the normal colouration; so again the mutations have every chance of surviving long enough to secure a mate. But the infrequency of the appearance of these more widely divergent mutations yet again argues that most of those which do appear die unmated. Of course, there is the rather remote possibility that a few of these rare mutations might secure like mates, or, even more remotely, normal mates. But even after consideration of the variable results of the Alucita monospilalis and A. monospilalis var. lycosema breedings, one must, I think, come to the conclusion that if such matings occurred in the cases of Tyria jacobaeae and the two species of Epirrhanthis the radical mutations should appear more commonly than they do.

Whatever the sex of the radical mutation the difficulty of securing a normal mate seems to be the same for each; so the natural passivity of the female has little bearing on the matter. Such passivity might account for the inability of a female mutation to secure a normal mate, but the same explanation would not be applicable in the case of the active male mutation. Hence we are forced to the conclusion that the colour-preference is of equal importance to both sexes. It must, however, be clearly understood that this preference of like for like applies most noticeably to radical mutations. In the case of mutations which are not widely divergent from the normal form the preference is not so stringent in operation.

It would be interesting to know to just what extent this preference of like for like is existent in species having sexual dimorphism or polymorphism; further, whether there is in the instance of “mimetic” species of the same range and appearing at the same season as the “models” always a structural or other barrier to cross-breeding. In the instance of “mimetic” species and “model” a colour-preference would not, of course, constitute a barrier to cross-breeding if such breeding were structurally possible. And so, in the absence of a barrier such as a scent-preference there would be a distinct possibility of hybridization in at least some instances. There is no reason why such hybridization should not occur if coition were possible, and if “mimetic” species and “model” were closely related, no reason why such hybrids should not be fertile inter se.