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Show 302 SILK-MOTITS. CHAP, VIII. by cultivators, ani has not been fixed by selection. Captain Hutton, in tho paper before referred to, has argued with much force that the da~k tiger-like marks, which so frequently appear duri~g tho later moul~s m tho caterpillars of val'ious breeds, are due to Tcvcrswn; for tho catorpilla~s of several allied wild species of Bombyx arc m11rkcd .and ~olourcd m this manner. Ho separated some caterpillars with tho tJger-Wm .marks, and in tho snccccding spring (pp. 149, 298) nearly all the ca~m·pillars ~cared from them were dark-brindled and tho tints became still darker m the third generation. Tho moths r~arcd from. these catcrp~lars 71 al~o b~came darker, and rcscm bled in colouring the wild B. Ilu.ttom. 0~ this. VlCW of the tiger-like mn.rks being duo to reversion, the persistency with whiCh they nrc transmitted is intelligible. . . . . Several years ago Mrs. Whitby took great pams m brccd~ng silkworms on n, large scale, and she informed me that some of he~· caterpillars had ~ark eyebrows. This is probably the first step in rcvcr~I~n towards the tJgcrlikc marks, and I was curious to know whether so trifling a character would be inherited; at my request she separated in 1848 twenty of these caterpillars, and having kept tho moths separate, bred fr~m them. Of the many caterpillars thus reared, "every one without exceptiOn had eyebrows, some darker and more decidedly m11rked than tho others, but. all had eyebrows more or Jess plainly visible." Black catc.rpillars oc~as10nally appear amonrrst those of the common kind, but m so vanablc a manner, that ~ccording to M. Robinct the same race will one year exclusively produce white caterpillars, and the next year many black ones; nevertheless, I have been informed by M. A. Bossi of Geneva, that, if these black caterpillars arc scpn.ratcly bred from, they reproduce the sn,mo .colour; but the cocoons n,nd moths reared from them do not present any difference. The caterpillar in Europe ordinarily moults four times before passing into the cocoon stage ; but thoro arc races " a trois roues," and the Trevoltini race likewise moults only thrice. It might have been thought that so important a physiological difference would not hn,vo arise~ under dorr:csticn, tion; but M. Robinct 72 states that, on tho one hand, ordmary caterpillars occasionn,lly spin their cocoons after only three moults, and, on tho other hn,nd, " presque toutos les races a trois muos, quo nous n,vons oxpcrimontees, ont fn,it qun,tro muos a la sccondo ou a la troisicmo ann6e, co qui semble prouvor qu'il a suffi do los placer dans des conditions favorables pour leur l'Cndro uno facultc qu'cllos avn,icnt perdue sons des influences moins favorablcs." · Gocoons.-Tho cn,torpillar in changing into the cocoon loses about 50 per cent. of its weight; but the amount of loss diffcl's in different bl'ceds, and this is of impol'tanco to tho cultivate!'. The cocoon in tho diffol'ont r~ces presents characteristic differences; being lal'go or small ;-nearly sphcncal with no constriction, as in tho Race de Lor'iol, or cylindricn,l with either a deep or slight constriction in tho middle ;-with the two ends, or with ~ne end alone, more or less pointed. The silk varies in fineness and quality, .and in being nearly white, of two tints, or yellow. Generally the colour of ;1 'Transact. Ent. Soc.,' ut supra, pp. 153, 308. i~ Robinet, idem, p. 317. CIIAl'. Vllf. THEIR DIFFERENCES. 303 tho silk is not strictly inherited: but in tho chapter on Selection I shall give a curious account how, in tho course of sixty-five generations, tho number of yellow cocoons in one breed has been reduced in Franco from one hundred to thirty-five in tho thousand. According to Robinct, the white race, called Sina, by careful selection during the last seventy-five !cars, "est arrivco a un tel etat de purctc, qu'on no voit pas un seul cocon Jau~e dans des millions de cocons blancs." 73 Cocoons are sometimes formed, as IS well known, cnt~cly destitute of silk, which yet produce moths; unfortunately. Mrs. Whitby was prevented by an accident fl'om ascertaining whether this chn,ractcr would provo hereditary. .Adult stage.-I can find no account of any constant difference in the moths of the most distinct races. Mrs. Whitby assured me that there was none in the several kinds bred by her ; and I have received a similar statemcn~ .from the eminent natu~alist M. de Quatrefages. Captain Hutton also s~ys ' 1 that the moths of all kinds vary much in colour, but in nearly the same mconstant manner. Considering how much the cocoons in the several races differ, this fact is of interest, and may probably be accounted for on t~e sn,mc principle as the fluctuating variability of colour in the caterpillar, namely, that there has been no motive for selecting and perpetuating any particular variation. Tl~e males of the wild Bombycidre "fly swiftly in tho day·timo and ovcnmg, but tho females are usun,Uy very sluggish n,nd inactive."75 In ~cvcral ~oths of this family the females l1ave abortive wings, but no mst~nco IS known of tho males being incapable of flight, for in this case the spccws could hardly have been perpetuated. In the silk-moth both sexes have ~perfect, crumpled wings, and are incapable of flight; but still thoro 1s a trace of the characteristic difference in tho two sexes · for though, on comparing n, number of males and females, I could detect no differ?ncc in tho development of their wings, yet I was assured by Mrs. Whitby that the males of the moths bred by her used their wings more than the females, n,nd could flutter downwards, though never upwards. She also states that, when tho females fust cmcr()'c from the c~c?on, their. wings are less expanded than those of the male. o The degree of ImperfectiOn, however, in the wings varies much in different races and under different eircumstanccs; M. Quatrcfagcs 76 says that he has seen a number of. moths with their wings reduced to a third, fourth, or tenth ~~rt of thmr norm;.l dime~sions, ~n~ even to mere short straight stumps: il me semble qu 1l y a la un von table arret de devoloppement partiel." On the. other hand, he describes tho femn,le moths of the Andre J can brood as hav~g "lc.urs ailes largos et 6tal6es. Un soul presente quelques courb~ rcs nTeguhercs et des plis anomaux." As moths and butterflies of all ki~ds reared from wild cn,tcrpillars under confinement often have crippled wmgs, the same cause, whatever it may be, has probably acted on silk- 73 Robinct, idem, pp. 306-317. 7~ 'Trapsuct. Ent. Soc.,' ut supra, p. 317. ia Stephens' Illustmtions, ' Haustellala,' vol. ii. p. 35. See also Capt. llutton, 'Transact. Ent. Soc.' idem p. 152.. ' 76 ' Etudes sur les Maladies du Y er h. Soie,' 1859, pp. 304, 209. |