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 body 
lengths. The color variation was also very constant. The abdomen 
of the cassinii form is normally entirely black beneath, only rare 
specimens showing a narrow hind border of yellowish or orange 
yellow. 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 difference 
is not constant. In the mating, out of seventy pairs observed there 
g, 
was no instance of cassini pairing with the normal large form, 
evidencing 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 cassiniz 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 ‘‘depauperate 
variety’? which has become in the course of ages fully established, 
especially with Brood X, being very rare with 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 distinctness, and the deterniina- 
tion, if possible, of its phylogenetic relationship.” 
The nomenclature of the species, variety, and races of the periodical 
Cicada adopted by the writer is the same as that followed in Bulletin 
14, namely, the Linnaean species Tibicen (Cicada) septendecim, with 
the tredecim race of Walsh and Riley, and the variety cassinii of Fisher. 
THE BROODS OF THE PERIODICAL CICADA. 
The subject of the broods of the periodical Cicada presents a number 
of interesting fields of inquiry, such as the consideration of the origin 
of the broods, their chronological history and classification, and their 
exact geographical limits or distribution. These topics will be taken 
up somewhat in detail, with the exception of the chronological history 
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 ORIGIN OF THE BRoops. 
It is not necessarily true, but it is a reasonable inference, that in the 
early period of the existence of the periodical Cicada on this continent 
it was represented by a single brood. Assuming this to have been the 
