36 
THE PERIODICAL CICADA. 
although the Cicada occurs in the nortli western portion of the State. 
The western records reported in Bulletin 14 in Colorado, and doubt- 
ful occurrences along the northern slope of the Big Horn Mountains 
of western Wyoming and Montana, have been shown, with very little 
doubt, to belong to another species of Cicada (Tihicen cruentlfem 
\J[\\. and allies), very possibly also similarly periodic in reappearance. 
The territory covered by the periodical Cicada is graphically illus- 
trated by the two maps showing the range of the 13-year and the 
17-year races, respectively (figs. 2 and 3). A brief examination of 
these maps develops the very interesting and suggestive fact that if 
superimposed the areas occupied by the two races would, in a gen- 
FlG. 
-Map showing distribution of the broods of tlu' Li-year ran 
eral way, lit together along their adjoining sides. This was to have 
IxHMi expected, but one would hardly have predicted the notal)le 
northern extension of the 13-year race in Missouri and Illinois in the 
Mississippi Valley, following, however, in an exaggerated way, the 
isothermal lines of this region. The extension northward of the 13- 
year race very greatly exceeds the limits of the Lower Austral zone, 
as marked on ^lerriam's map, and if this insect were taken as a basis 
this zone would have to be very greatly extended northward in the 
two States named. With this important exception, the 13-year race 
is confined pretty closely to the Lower Austral and the 17-year race 
covers the l^pper Austral, with large extensions northward into the 
