18 
THE PERIODICAL CICADA. 
is supposed to represent a dimorphic form of the larger and entitled at 
most to a varietal rank only. 
Certain divergences, however, may be noted in the dates of appear- 
ance and the habits of the two forms. The larger one appears some- 
what earlier, from eight to ten 
days; correspondingly also the 
smaller form disappears some- 
what later in the season than 
the larger. In fact, specimens 
of the former have been found 
as late as July 15 which pre- 
sented no appearance of age 
and were not at all frayed. 
The small Cicadas are also re- 
ported by various observers 
to be more or less gregarious 
in habit, not always inter- 
mingling with the larger ones, 
but collecting in small com- 
panies in orchards, or in 
thickets along the streams 
and moist places. It has also 
been noticed that the males of 
the small form have a somewhat different song note, and this last varia- 
tion seems to have been fully confirmed. 1 
The nomenclature of the species, variety, and races of the periodical 
Cicada adopted by the writer is as follows: The Linneau species, 
Cicada septendecim, with the tredecim race of Walsh and Riley, and the 
dimorphic variety cassinii of Fisher. 
Fig. 1. -The periodical Cicada: A, male of typical 
form — natural size; c, d, genital hooks — enlarged; g, 
singing apparatus — natural size; J?, male of the small 
form (cassinii)— natural size; e, /, genital hooks — en- 
larged (after Riley and Hagen). 
THE BROODS OF THE PERIODICAL CICADA. 
Tbe 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 BROODS. 
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 
See Proc. Acad. Nat. So., Phil., Sept., 1851, Vol. V, pp. 273-275. 
