PARTHENOGENETIC PURE LINE OF APHIS AVEN/E FAB. QI 



THE EFFECTS OF CONTINUED PARTHENOGENETIC REPRODUCTION 

 ON THE VIRILITY AND METABOLISM OF A STRAIN. 



Whether or not long-continued asexual reproduction will affect 

 the virility of a strain of animals has long been a debated ques- 

 tion. The time was when zoologists in general held that con- 

 jugation or some form of amphimixis was absolutely necessary 

 in order to prevent the eventual dying out of a strain of animals 

 that had long been reproducing asexually. The more recent 

 work of Woodruff, 1 however, has shown us that in the case of 

 Paramcscium, at least, conjugation is not necessary in order 

 to rejuvenate a race. The question that next suggests itself is; 

 can insects reproduce indefinitely by means of parthenogenesis? 

 Probably the prevailing opinion among entomologists has been 

 that parthenogenesis could not continue indefinitely in any species 

 of the insects. Certain it is that in all those well-studied species 

 where parthenogenesis is known to occur males have been found 

 to exist. In fact, we find that frequently the parthenogenetic 

 offspring are all males, and are produced thus according to def- 

 inite law. Further, in the case of some insects, as aphids, males 

 are produced according to the season. On the other hand, in 

 the case of some insects that have not been sufficiently studied, 

 males are unknown. In some of the species of Thysanoptera 

 females have been found by hundreds, yet no males are known. 

 In the case of our common pear-slug saw-fly, Eriocampoides 

 limacina Retzius, I have counted hundreds, and reared scores of 

 individuals without ever seeing a male, yet males are supposed 

 to exist. Males must be extremely rare in the case of our 

 common oyster-shell scale, Lepidosaphes ulmi Linn. I have 

 reared scores and examined thousands of individuals without 

 ever finding a male; however, the male has been described. 



Since taking up this work with Aphis. avence Fab., I have 

 observed this species to pass the entire winter on the Pacific 

 coast in the agamic form, and, as a matter of fact, have never 

 observed the sexual form in that region of the country, although 

 it probably exists there to a limited extent. We have several 

 experiments on record of aphids having been reared partheno- 



1 Woodruff, L. L. For a review of the work of Woodruff see, Middleton, A. R., 

 1913, "Work on Genetic Problems in Protozoa at Yale," Amer. Nat., Vol. XLVIL, 

 pp. 434-439. 



