Dec, 1902.] Variations in the Periodical Cicada, 325 



normal forms, seems subject to wider variation than the other 

 structures. 



Measurements ha\-e been made of eight hundred specimens 

 taken at random from various localities, the only point of selec- 

 tion being to get an equal number of both forms (in each sex ) in 

 order that the frequencies for each form should be fairl}- repre- 

 sented. All intermediate forms are represented in due proportion 

 to their occurrence in the lots examined. 



Results of the measurements show a very decided constancy 

 for each variety and for each sex of each variety, the wing length 

 of cassifii -moles averaging 27.4 mm., width 10.9 nun. and body 

 length 22.9 mm.; while for normal form the wing length is 

 33.1 m., width 12.5 mm. and body length 28 mm. For the 

 females, cassini ioxm, wing length 30 mm., width 11.3 mm., body 

 length 24.1 mm., while normal 17-decem are wing length 34, 

 width 12.8, bod}' length 29.3 mm. The constancy of each can best 

 be shown by curves of frequency for certain measurements, and 

 this is shown in accompanying plate. Taken b}- constancy of 

 each form and it must be recognized that their forms are well 

 established. 



I may add that measurements, so far as made, indicate same 

 proportion in length of beak and ovipositor and in width of 

 thorax. It is believed that the wing length and width is as good 

 a criterion of variation for the species as any other measurement 

 that could be selected. The length of the entire bod}- varies 

 somewhat with degree of contraction, and for females with con- 

 dition of ovaries, but in the measurements given this was elimi- 

 nated, as far as possible, by taking the specimens in same condition 

 of maturity ; most of them were dr}-, but some were killed iu 

 formalin. 



Color variation is also very constant. In rare instances certain 

 forms showed a narrow orange border to abdominal segments. 



There is a difference in genitalia, but apparent!}- not enough to 

 exclude the idea of crossing, and Riley says the difference is not 

 constant. I have not examined a sufficient number to pronounce 

 upon the constancy, but from the method of coition I should 

 think a pairing of opposite varieties, while not impossible, would 

 be difficult. There is a very decided difference in note — , a fact 

 recorded as early as 1830 — and this, if the note is a mating call, 

 would certainly have an influence in maintaining this isolation. 



A special effort was made to note copulations and determine 

 whether in any case the varieties crossed, or an}- efforts were 

 made toward crossing, but out of seventy pairs taken in antjc 

 not a single instance of cassini paired with normal 17-decem form 

 has been seen. There is here, then, a very evident case of isola- 

 tion due to sexual selection, and it would appear on this basis, 

 every opportunity for perpetuation of the ^-ariety. 



