I'AKTHENOGEXESIS. 27 



possesses the faculty of laying unfertilised eggs which 

 invariably yield only males. There is reason to believe 

 that they do this regularly should they be prevented, 

 from any cause whatever, from having access to the 

 males ; and the eggs are laid immediately after the 

 females have left the cocoons. And when these 

 unfertilised females are examined after oviposition, no 

 traces of spermatozoa can be discovered in the ovaries, 

 while they are easily observed in those which have 

 been fertilised. It is worthy of remark that this 

 peculiarity of rtbesii was noticed by Robert Thorn 

 as early as the year 1820 (in the ' Memoirs of the 

 Caledonian Horticultural Society/ iv, pt. 2). He 

 seems to have had an idea " that there is a connection 

 between $ and ? caterpillars ; for I have frequently 

 observed them twisted together for some time after 

 they had ceased eating, and a little before they cast 

 their skins to go into the pupa state." 



My own experiments with Jf. ribesii are completely in 

 accord with those of the writers just mentioned; while 

 with N. miliaris,* N. glutinosce, N. curtispina, and N. 

 palliatus, I have likewise been successful in getting 

 unimpregnated females to oviposit, the result being 

 (when the larvae did not perish young or in the 

 cocoons) that only males were produced. Mr. J. E. 

 Fletcher has likewise successfully experimented with 

 the species just named, with the same result, save that 

 in one experiment with N. curtispinaheTeKteA 21 3 $ 

 and 1 ?. The rearing of a ? from an unimpregnated 

 ? is certainly very rare, and contrary to the results 

 obtained with other species and by myself with the 

 same species, yet from the care with which Mr. Fletcher 

 conducted his investigation there can be no doubt of 

 the correctness of his statement. t 



The same gentleman got an unimpregnated ? of 

 Nematus gallicola\ to lay eggs, but owing to the 

 weakness of the plant (a potted one) did not rear the 



* Scot. Nat., iv, 157; Trans. Ent. Soc., 1880, 77. 



t Ent, M. M., 1880, p. 269. J Trans. Ent. Soc., 1880, 77. 



