Parthenogenesis in a Beetle

Abstract

I DO not know whether any instances have been recorded of parthenogenesis in the coleoptera, nor does the interest of the case I am about to relate consist in the discovery of the operation of a not uncommon mode of insect reproduction in a new field, but rather in its altogether abnormal and fortuitous character in the species of beetle concerned, viz., Gastrophysa raphani. My own observations hitherto on this species have been uniformly to the effect that unimpregnated females lay always barren eggs, and that one intercourse with the male renders fruitful all eggs subsequently laid. I bred the female in question from the egg this year, and have kept her isolated since her exclusion as an imago. She has laid, up to the present, about twenty batches of eggs consisting of about thirty-four and fifty-one alternately in the batch. Of these, some fifteen batches have been observed; and only in one of these, No. 10, to wit, consisting of thirty-four eggs, and in one of these thirty-four only were any traces of development observed. This batch was laid between the 2nd and 4th of August. On the 5th I noticed in one an appearance which is usual about this time in fertilised egg, which I have been accustomed to think about as the “embryonic scroll,” and which, on reference to Huxley's “Anatomy of In-vertebrated Animals,” pp. 444–445, I am inclined to think may be what is there called “the sternal band (Keimstreif of the German embryologists).“This scroll is invariably present in gastrophysa eggs regularly developing, and enables one to predict with certainty the position of the ventral aspect, and of the head and tail of the future larva. On the 6th this same appearance was more distinctly marked. On August 10 a further well-defined stage of development had been reached. On the 11th the ocelli were plainly visible. Next day I noted the antennae, mandibles, palpi, and legs. The segments, warts, and spiracles were also to be seen. On the 12th and some subsequent days I saw plainly somewhat feeble but unmistakable and decided movements of the legs, especially of the tarsi and ungues. At this season of the year the egg should have been hatched in about ten or twelve days. I have no longer any hope of this, and all larval movements appear to have ceased. All the other (thirty-three) eggs have undergone the usual degeneration, but this one presents a striking contrast to them, showing all the external parts perfectly formed and distinctly visible, as far as the position of the larva (which is just the reverse of the usual one, namely, with the dorsum in place of the venter next the surface of attachment) allows them to be seen. There is an un-unsual appearance of brownish coloration towards the caudal end, the nature of which I have not made out. The failure to hatch out, however, does not hinder this from being a decided case of embryonal development in an egg laid by a female of Gastrophysa raphani whose virginity is assured; and it is a solitary instance occurring among some eight or nine hundred eggs laid by the same beetle both before and after and along with it, all of which (as far as observed) were normally and uniformly barren.