THK XU-A[ERICAL RELATIONSHIP OF THK SEXES OF LKPIDOPTERA. IBlH 



The numerical relationship of the sexes of Lepidoptera. 



By .). W. TUTT, F.E.S. 



A recent remark made by Dr. Chapman [antea, p. 139) as to the 

 numerical relationship of the sexes of lepidoptera in nature leads me 

 to call attention to an important series of tabulations by Standfuss 

 [Handhiich der PalaearkU'xchen (iroKi^-SrhDirtteiiiniic, pp. 190-191), in 

 which it will be seen that, as the result of breedini>- above 32176 insects 

 of 40 species, between the years 1877 and 1893, he obtained results in 

 which, in almost every case, the males were in excess of the females, but 

 to such a small extent that — eliminating the well-known factors of the 

 greater ditticulty of breeding ? s of some species owing to their longer 

 larval stage and their requirement of a greater quantity of food (resulting 

 in the larger size of the abdomen and the formation of ova), as a con- 

 tingency consequent upon the artificial conditions — one may safely say 

 that the evidence goes to prove that, in almost all broods, the sexes are, 

 to a great extent, fairly equal, the total numbers being 16524 <? s to 

 15652 ? s. Itisquite true that when one is out in the fields one frequently 

 sees more <y s than $ s, and observes that this appears to be particularly 

 the case when one hurries from place to place, spending a day here and 

 there, but the difference gradually fades as the length of stay in a given 

 place increases, and when one makes observations on a species through- 

 out the full period of its appearance in a given locality, and more 

 particularly when one has learned the difference of the habits of the 

 sexes, for, in many species, especially among the butterflies, the habits 

 are exceedingly different, and the much more retired and less active 

 $ comes into much less prominence than the J , and is comparatively 

 rarely seen by the superficial observer who covers ground and does not 

 remain for a fair length of time to study the species. A case in point 

 will illustrate my meaning. On my arrival at Arolla towards the end of 

 July, 1903, Kvi'bia mnestra was just appearing, and 1 captured possibly 30 

 or 40 (^ s (out of a very much larger number observed) per day for 3 or 

 4 days, to 1 or 2 ? s; had I left the locality then 1 might have assumed 

 that, in this species, at Arolla the S « largely outnumbered the 9 s, but, 

 having obtained a fair number of specimens, I only captured, until 

 August 9th, such specimens as came in my way, without attempt- 

 ing to step aside for them ; these again were largely <? s. However, 

 on August 10th, recognising that my series after all was a very poor 

 one, owing to the comparative absence of J s, I collected the species 

 carefully as it flew about the long grass round the juniper bushes and 

 netted those on the flowers on the slopes, with the result that I took 

 home some 50 ? s, and only saw about half-a-dozen <? s worth netting. 

 No doubt at Arolla the $ s are a little later, their habits are very 

 different and more retired than those of the J s, but that there is no 

 real difference in the numbers of the sexes is certain. Similarly with 

 Krehia ijoante and E. nevinc. It is possible, in their respective 

 localities, to take up a position on an exposed path with rocks beetling 

 up, and capture hundreds of ^ s without ever seeing a J , but find the 

 flower-clad slopes where the ? s lay their eggs, and learn the habits of 

 the latter, and you will soon discovei- that the apparent difference in 

 numbers is due almost entirely to a difference in liabit and not, in fact, 

 the result of any real numerical difference. Illustrating the same 

 point among the moths I may say that, in climbing the slopes at Arolla 



July 15th, 1904. 



