Dec., 1902.] Variations in the Periodical Cicada. 
3 2 5 
normal forms, seems subject to wider variation than the other 
structures. 
Measurements have 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 fairly 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 cassini males averaging 27.4 mm., width 10.9 mm. 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 form, wing length 30 mm., width 11.3 mm., body 
length 24.1 mm., while normal 17-decem are wing length 34, 
width 12.8, body 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 by 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 body 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 dry, but some were killed in 
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 apparently 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 any efforts were 
made toward crossing, but out of seventy pairs taken in coitu 
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 variety. 
