28 THE PERIODICAL CICADA. 
section of Brood XVIII retarded one year, just as Brood XVI is an 
accelerated swarm of the same. Both, curiously enough, represent 
eastern extensions of the parent brood. 
Brood IV, separated from Brood XVIII by two years, seems to bear 
little relationship to the latter, and it is possible that a more logical 
arrangement consists in connecting it with Brood VII through Brood 
Y.I, of which last it may be considered as an eastern and northern 
extension. Brood VI, as indicated elsewhere, is a very notable instance 
of the formation of a new brood by what is undoubtedly an acceleration 
in time of appearance of a portion of a larger and older brood. Its 
relationship with Brood YII is very marked and can not be questioned. 
As shown above, the 13-year broods seem to divide themselves nat- 
urally into two sections, one related closely to Brood XVIII, and the 
other connected with Brood VII. 
Tne relationships of ^io broods of the 17-year race are somewhat 
more obscure, but here also it is seen that if the enumeration begins 
with Brood XI the broods follow each other in regular succession for 
eleven consecutive years. Then, after a break of one year, follows 
Broods V and VIII, and after another break of one year Brood IX, 
which last, however, is a very doubtful and uuiinportant brood and 
may not belong to the 17-year race. Taking up these broods in order, 
beginning with Brood XI, their relationships seem to be as follows: 
The main body of Brood XI occupies territory immediately west of 
the more important Brood XII, and also presents a number of colonies 
extending westward to Colorado. Broods XI and XII seem, therefore, 
closely allied in point of origin. 
Brood XIII presents little, if any, relationship to the last in point of 
location and distribution, but is closely allied to the following Brood 
XIV, which seems a western and southern extension of XIII. 
Brood XV* presents little relationship with Brood XIV in point of 
distribution and covers a very compact territory. 
Brood XVII, being a widely scattered one, and occurring usually in 
small numbers, does not seem to present any particular relationship 
with any of the preceding or following broods. 
Brood XIX is local in distribution and not very important, and is 
divided into two sections by the territory occupied by the following 
Brood XX, with which it thus seems to be closely allied. Brood XXI 
is very distinctly a southern extension of Broods XX and XIX, These 
three broods seem, therefore, to be closely allied in their origin, and, 
curiously enough, occupy territory which divides the two main sections 
of the great 17-year Brood XXII, which next follows in regular succes- 
sion. Brood I, following XXII, is evidently an extreme northeastern 
extension of the latter. 
Brood V, which follows Brood XXII after an interval of two years, 
would seem naturally in point of distribution to be a western extension 
of the latter, but the two years' difference in time gives it a rather 
