80 THE entomologist's record. 



On the underside of the 8th abdominal segment, on the posterior 

 margin of the segment, are two points or projections. The distinction 

 between the species lies in the distance between these points. In O. 

 christi/i the distance varies from (approximately) •12mm. to -SOmm. ; 

 in 0. Ulutata it varies from (approximately) -SSmm. to 'Slmm. 

 Perhaps the examination of a large number of specimens would some- 

 what extend these limits. At any rate it seems safe to say that any 

 specimen in which the distance is less than -SSmm. will be (>. christiji, 

 and any in which it is greater will be 0. (Ulutata. 



The material for my first experiment to test this distinction 

 consisted of a number of specimens of both species, captured near 

 Enniskillen last autumn. I first assigned the specimens to their 

 respective species by the wing-markings, which are in themselves a 

 satisfactory criterion to anyone properly acquainted with the two 

 species. I then detached the abdomen from each specimen and 

 mounted it on a double slip of white paper, writmg on the inside of 

 each slip a number to correspond to the number on a ticket attached 

 to the pin of the specimen. The number on the mount was thus 

 invisible to me when I was examining the abdomen. I then mixed 

 up the different abdomens, and wrote on the outside of each slip a 

 number independent of the one Avritten on the inside. My object was 

 to try whether I could determine the species of each abdomen without 

 reference to the wings. I next examined each abdomen under the 

 microscope, and recorded the distance between the points. In this 

 work I had the assistance of my colleague Mr. W. N. Tetley. We each 

 independently examined each abdomen, and in the few cases where we 

 disagreed we split the difference. A few specimens were rejected 

 iDecause we could not make out the points to our satisfaction, owing 

 probably to damage in mounting. I may mention that Mr. Tetley is 

 an experienced microscopist, but not an entomologist, and is quite 

 unable to distinguish O. dilntata and O. chuMyl by ordinary means. 

 It was evident to both of us that the measurements fell into two 

 groups separated by a decided interval. In one group, consisting of 

 eighteen specimens, the distance ranged from •12mm. to -BOmm., and 

 in the other, consisting of eleven specimens, from •36mm. to -Slmm. 

 After recording the measurements I looked at the original numbers 

 inside the doubled slips, and found that every specimen in the former 

 group was O. chriHtiji, and every one in the latter group (J. dilutata. 

 The measurements may be tabulated thus — 



0. christiji. 0. dilutata. 

 No. OF Specimens. Distance of Points. No. of SpEcniENs. Distance of Points. 



1 .. -12111111. 2 .. -Semm. 



1 .. -16111111. 1 .. -39mm. 



1 . . -ISmm. 1 . . •40mm. 

 3 . . •20mm. 1 . . -iSmm. 



2 . . •21mm. 2 . . -44miii. 

 1 . . •22mm. 2 . . •46mm. 



3 . . •23mm. 1 . . -47mm. 



1 . . -2.5mm. 1 . . -51mm. 



2 . . -26mm. 



1 . . •29mm. 



2 . . •30mm. 



The average distance in 0. christyi is between •22mm. and •23mm., 

 in 0. dilutata about •43mm. 



