59 



A CONSIDERATION OF THE VALIDITY OF THE OLD RECORDS BEAR- 

 ING ON THE DISTRIBUTION OF THE BROODS OF THE PERIODICAL 

 CICADA, WITH PARTICULAR REFERENCE TO THE OCCURRENCE 

 OF BROODS VI AND XXIII IN 1898. 



By C.'L. Marlatt. 



SOURCES OF ERRORS IN THE OLD RECORDS. 



Iii examining the records of the distribution of the twenty-one broods 

 of the Periodical Cicada hitherto accepted, it is seen that considerable 

 uncertainty attaches to the data of certain broods, not only from the 

 fact of their covering- in greater or less degree territory occupied by 

 both races, but more particularly because the records are frequently 

 based on years in which broods so overlapping have appeared in con- 

 junction-. Examining the 21 broods hitherto studied, it will be seen 

 that in each period of 17 years between six and nine years are signal- 

 ized by the joint occurrence of a 17 and 13 year brood. Owing to the 

 difference in the x^eriods between the recurrences of the southern race 

 and northern race, different broods of both races are being constantly 

 brought into relationship with each other, and in fact the same two 

 broods can come together only once in 221 years. For example, the 

 broods which unite in appearance the present year were last in conjunc- 

 tion in 1697 and will not come together again until the year 2119. 



The overlapping of broods thus appearing in codj unction, including 

 some of the more important ones of both races, has given much uncer- 

 tainty to some of the records. In the case of the broods of the 17-year 

 race, the following extend on their southern boundaries into the terri- 

 tory of the 13-year race, and hence the records of the southern localities 

 are open to some question : Broods VI, X, XIY, XVI, I, IV, to a slight 

 extent also in the case of Broods II and III, and doubtfully in the case 

 of Brood IX, the possibility of confusion in this last brood depending 

 on the accuracy of the extreme northeastern extension of the 13-year 

 Brood XIX.* 



The following broods of the 13-year race extend northward into the 

 territory occupied by the 17-year race, and are hence open to some 

 question : Broods XXIII, XVIII, XIX, and XX. 



The records can not be questioned of the 17-year Broods VII, VIII, 

 XI, XIII, and V, and of the 13-year Broods XXIV, XXI, and XXII, 

 because these broods are limited in distribution to the territory of a 

 single race. 



The most notable instance of the overlapping and consequent prob- 

 able confusion of the records is seen in the case of Brood X of the 

 17 year race with Broods XXIII and XIX of the 13-year race. The 

 remarkable feature in the distribution of the broods named is the not- 



* See map of races and broods given in Bull. 44, new ser., U. S. Dept. Agric., figs. 

 2-19 (pp. 25-49). 



