RESULTS OF CROSSING. 327 



A comparison of the above record with those of pure variolarius 

 shows that in this back-cross the relation between mating and 

 deposition of eggs is not normal, for, as a rule, eggs are deposited 

 not oftener than once or twice between two matings, whereas 

 in this cross, eggs were deposited eleven times between the 

 first two matings and six times between the last two matings. 

 The number of eggs, however, that were deposited between 

 June 21 and August 28 is above our average for pure variolarius 

 and a normal proportion of these eggs developed. 195 eggs 

 were deposited and 177 of these hatched. 149 were reared to 

 the winged stage. (760? and 73?.) All of these females are 

 preserved as pinned specimens and the males are preserved 

 either as pinned specimens or in glycerine. 



Intromittent Organ. This organ is a chitinous spiral easily 

 dissected from the genital segment of the males (photos I to 60). 



In accordance with Dr. Eltringham's suggestion, the entire 

 male segment was first softened in warm caustic potash, until 

 the intromittent organ was sufficiently pliable to be dissected 

 out without breaking. 



Only a single organ was mounted on a slide, and the slide was 

 so indexed that the insect from which the organ was taken could 

 be identified and therefore, the relation of the intromittent organ 

 to any other character could be satisfactorily studied in one 

 and the same insect. This was possible in the case of 'the genital 

 spot, for the genital segment of all the specimens which were pre- 

 served in glycerine had been photographed before the segment was 

 removed for dissection. 



Each intromittent organ being mounted in balsam, it was 

 photographed at a magnification of 20 diameters, and all measure- 

 ments were made from these photographs. The actual length, 

 therefore, of each organ is one-twentieth of the length recorded 

 in this paper. 



Measurements from such photographs of the intromittent 

 organs of E. variolarius, E. ictericus and of the offspring from 

 the back-cross are given in the following three tables. 



Table I. gives the lengths of the intromittent organs of 62 

 specimens of E. variolarius. This table is quoted from our 

 report of results from the cross between E. variolarius and E. 

 servus. (Foot and Strobell, '15, see footnote, p. 322.) 



