14 THE PERIODICAL CICADA. 
other thirteen; hence the impropriety of the specific name septendecim. * * * 
The anatomy of the insects of both divisions is precisely the same, but septendecim 
does not of course apply to the Southern division, whose lives are but thirteen years. 
Shall we call the latter Cicada trededmf Why there is this difference in the periods 
of lives of the two tribes Ave can not explain. It is not the climate that causes it, 
as a moment's reflection will prove. If that were the cause the difference would be 
more gradual. For example, in northern New York they would have been, say, seven- 
teen years; in Pennsylvania, sixteen; iu Maryland and Virginia, iifteen; in North 
Carolina and Tennessee, fourteen, and in South Carolina, etc., thirteen years in 
completing their existence. But that is not the case. The difference of years takes 
place abruptly, on and about the line of 34° and 35^ of north latitude, on the north 
side of which the period is seventeen years and on the south thirteen years. 
While Dr. Smith is hardly justified iu the last statement, it is never- 
theless true that the 17-year race is northern and the 13-year race is 
southern. The territory of the two races is graphically shown in figs. 
2 and 3, and is described in detail and mapped for all the broods in a 
later section. 
In this bulletin the two forms of the periodical Cicada have been 
designated as "races," adopting the position taken by Professor Eiley 
and the majority of the writers on this insect, rather than considering 
them to be distinct species, as is held by some specialists. Professor 
Eiley and others opposed the idea of their being specifically distinct 
not only because of their practical identity in general characteristics 
and habits, but also on the ground of external structure, no material 
difference in this respect having been noted between the two races, 
although it was known that the individuals did not cross when they 
appeared together. Dr. Walsh was very firmly of the opinion, on the 
other hand, that they represent two distinct species, yet in a letter to Mr. 
Darwin he described the 13-year race as an incipient species, to which, 
for convenience, it is desirable to give a distinctive name. 1 His pub- 
lished views on the subject, given in a posthumous paper, are quoted 
below. 2 Referring to the impossibility of distinguishing species in cer- 
tain genera by a mere comparison of the perfect specimens, he says : 
Upon the same principle I strongly incline to believe that the 17-year form of the 
periodical Cicada (C. septendecim Linn.) is a distinct species from the 13-year form 
(C.tredecim (Walsh and Eiley :3 ) Riley), although it has been impossible for me, ou 
the closest examination of very numerous specimens, to detect any specific difference 
between these two forms. It is very true that the 13-year form is confined to the 
more southerly regions of the United States, while the 17-year form is generally, but 
not universally, peculiar to the Northern States ; whence it has been, with some show 
of plausibility, inferred that the 13-year form is nothing but the 17-year form 
accelerated in its metamorphosis by the influence of a hot southern climate. But, 
1 See Index to Missouri Entomological Reports, Bull. 6, U. S. E. C, p. 58. 
-American Entomologist, Vol. II, p. 335. 
3 Taking the ground that Dr. Phares can not be credited with the race name 
"tredeciin" on account of the ephemeral character of the journal in which he 
employed it, the credit should go to Walsh- Riley, since the article in the American 
Entomologist of December, 1868, where it was next suggested, was a joint or edi- 
torial one, as is sanctioned by Professor Riley himself in the Bibliography of Eco- 
nomic Entomology, Part II, p. 61, No. 474. 
