152 THE entomologist's record. 



the'other hand, a paucity of females would supply every female with 

 a distended spermatheca, and a maximum of the male element would 

 be supplied to each egg. If, in a race tending to an excess of 

 either males or females, variation occurred, as undoubtedly it would 

 sooner or later in some degree, making each sex more potent to pro- 

 duce the opposite sex than its own, it would benefit the race by 

 remedying the disparity of the sexes. In the course of the enormous 

 number of generations through Avhich our present insects have 

 descended, it is probable that some sucIj process as this has been gone 

 through by many species, more or less frequently, and that apart from 

 special selection in an opposite direction, there exists in not a few 

 insects a prepotency of each sex to produce the opposite one. 



In the Luffias we have Luffia fi'irhanltdla that is parthenogenetic in 

 the manner of certain Cynipids, i.e., producing females continuously 

 without any males. In L. fcirhaidtclla, however, there is, perhaps, a 

 closer resemblance to the Aphides, though, to prevent misapprehen- 

 sion, I must repeat that in ApJu\ the parthenogenetic female is 

 further modified as to be not, perhaps, strictly a female at all. There is 

 nothing of this sort in I.Kjfia, nor, so far as we know, is there in 

 Luffia any trace of alternation of generations, nevertheless, many 

 entomologists, perhaps from tlie analogy of AjJiis, believe that it is 

 very possible, or even probable, that, in these continuously partheno- 

 genetic forms, as in Cynipids and Tenthredinids, at some long 

 intervals, under some unknown stimulus, a generation may occur with 

 normal males and females. The evidence on which such a belief rests 

 can hardly be said to exist. The resemblance of the case of Aplii-f to 

 that of L. fcirhaiiltiila is strong in two particulars, which are impor- 

 tant ones in so far that they seem to be related to the origin of the 

 parthenogenetic condition. One of these particulars is that both are 

 apterous, the other that both have a restricted habitat. In both cases 

 we may imagine that the restricted habitat led to the loss of wings, 

 and in the second, that the combination of apterousness and a 

 restricted habitat facilitated, if they did not actually determine, the 

 appearance of the parthenogenetic hubit. It was a disadvantage 

 to the Aphis to leave its succulent leaf or twig whilst it was succulent, 

 and therefore it became apterous. [Aiffia ferchaidtdla equally had a 

 lichen-covered tree or rock, which it was well to remain upon. In this 

 respect we must not, however, regard L. JcrcJiaultdla alone, but 

 rather the whole of the Micro-Psjchids, or, rather, the Solenobiids, 

 Taleporiids, and Lufitiids, all of which are lichen-feeders. There are 

 parthenogenetic forms in the Solenobiids also, and the operative 

 causes are doubtless the same in them as in L. feirltanltdla. Why 

 apterousness should in some cases proceed to parthenogenesis in these 

 families, and sometimes not, is one of those questions impossible to 

 solve ; it amounts to this — why is a difficulty overcome sometimes in 

 one way and sometimes in anothei' '? I think we may, perhaps, guess 

 so far as to say that the benefit of cross-fertilisation in L. lapiddla is 

 about as great on the one hand as that derived from parthenogenesis 

 in L. ferclicudtdla is on the other. What is the latter ? It is 

 difficult to say, but if apterousness were an advantage, in allowing 

 successive broods to remain in their restricted habitat, an advantage, 

 first, in not taking them from it to be lost, and, secondly, in not 

 requiring the waste of developing useless or injurious appendages, 



