THE SPECIFIC DISTINCTNESS OF OPOKABIA CHKISTYI. 



81 



The same measurements are shown in the following diagram, in 

 which each x represents one specimen, and each division in the 

 scale -01 of a millimetre : — • 



In farther experiments I did not think it necessary to conceal from 

 myself the species of each specimen examined. I examined 8 more 

 specimens of (J. cliriiitiji and 15 more of (>. dilutata, bringing the total 

 number of each species up to 26. Among the (). dilutata were some 

 from the south of England. Further, as (). christyi is a somewhat 

 smaller insect than O. dilutata, I thought it necessary to denote the 

 measurement in each case by its ratio to some other measurement of 

 the specimen. For this purpose I selected wing-expanse, and for 

 convenience in handling the numbers, I multiplied the ratio in each 

 case by 10,000. Thus in a specimen having the distance between the 

 points •39mm. and wing-expanse 41mm., the ratio would be -ff or 

 •0095, but would be represented by 95, which I will call the index of 

 the specimen. In (>. christi/i the index varied from 40 to 81, and in 

 LK dilutata from 95 to 141. In the following diagram each division 

 in the scale represents a difference of 5 in the index, smaller differences 

 being neglected. 



An examination of some hybrid dilutata x cltrist>/i gave further 

 confirmation of the distinction. Twelve specimens were examined, all 

 bred from pairings obtained in captivity by myself. In eleven specimens 

 the distance between the points varied between -SOmm. and -SSmm., 

 thus bridging the gap between the two species. One specimen gave 

 a measurement of -26111111. 



The distinction may be observed without removing the abdomen, 

 and in favourable cases an ordinary pocket lens is sufficient, but a 

 higher magnifying power is desirable, as the points cannot always easily 

 be distinguished. The observer should hold the specimen in a good 

 light, underside upwards, tail pointing away from him. He should 

 then take a small camel-hair brush, and brush downwards, i.e., towards 

 himself, the hairs on the margin of the 8th segment. The points will 

 thus be exposed, and may be examined under a pocket lens, or the 

 specimen may be laid on the stage of a microscope. For determining 

 the species, accurate measurement is not necessary. With the points 

 properly exposed one can tell at a glance whether they are near or far 



