22 THE PERIODICAL CICADA. 
of these measurements indicated a decided constancy for each variety 
and for each sex of each variety in wing lengths and widths and l)ody 
lengths. The color Aariation was also very constant. Tlie ahdomen 
of the cassinii form is normally entirely black beneath, only rare 
specimens showmg a narr()\\' hind border of yellowish or orange 
3'ellow. The cross veins also on the wing forming the W mark are 
commonly less black, and the W therefore shortened. This point, 
however, as in the normal form, seems subject to wider variation 
than the other features. 
There is a difference in genitalia, but apparently not enough to 
exclude the idea of crossing, and, according to Riley, this difl'erence 
is not constant. In the mating, out of seventy pairs observed there 
was no instance of cassinii pairing with the normal large form, 
evidencmg an apparently complete isolation by sexual selection. 
Professor Osborn shows, therefore, that there is no ground for con- 
sidering the small form as a dimorphic or seasonal stage of the large. 
Professor Osborn infers that the cassinii is a derived form, since it 
appears less commonly than the other and probably has a more 
restricted range, and suggests that it may be possibly a ^'depatiperate 
variet}^" which has become in the course of ages fulh' established, 
especiall}' vdih Brood X, being very rare wdth Broods III and XIII, 
which he had also studied. He concludes: '^Whether this form be 
called a variety, subspecies, or species, is, it seems to me, of less 
importance than a recognition of its distmctness, and the determina- 
tion, if possible, of its phylogenetic relationsliip." 
The nomenclature of the species, variet}^, and races of the periodical 
Cicada adopted by the \\Titer is the same as that followed in Bulletin 
14, namely, the Linnaean species Tihicen (Cicada) septendecim, with 
the tredecim race of Walsh and Rile}", and the variety cassimi of Fisher. 
THE BROODS OF THE PERIODICAL CICADA. 
The subject of the broods of the periodical Cicada presents a number 
of mteresting fields of inquiry, such as the consideration of the origin 
of the broods, their chronological historv and classification, and their 
exact geographical limits or distribution. These topics Avill be taken 
up somewhat in detail, with the exception of the chronological liistory 
of the appearances during the last two hundred years and accompany- 
ing voluminous historical records, which, for reasons to be later noted, 
have been largely omitted. 
The Origix of the Broods. 
It is not necessaril}" true, but it is a reasonable inference, that in the 
early period of the existence of the periodical Cica(h\ on this continent 
it was represented l)y a single brood. Assuming tliis to have been the 
