60 



able extension northward in Illinois and Missouri of the 13-year Broods 

 XXIII and XIX, which till almost exactly a district which would nat- 

 urally be supposed to belong to the 17-year race and probably to 

 Brood X. As pointed out elsewhere,* this circumstance has special 

 significance when it is remembered that the northward extension of the 

 13-year race is based on Broods XIX and XXIII, and that records of 

 the former were collected for the most part in 1868, when this brood 

 was in conjunction with Brood X, and of the latter in 1885, when Brood 

 XXIII was also in conjunction with Brood X, the limits of which curi 

 ously enough stop rather suddenly at or near the eastern State line of 

 Illinois. A possibility is immediately suggested that the northern 

 localities assigned to Broods XIX and XXIII properly belong to Brood 

 X. It is true, however, that records obtained the present year in the 

 main seem to sustain the accuracy of the older records, but there is 

 still sufficient doubt to warrant the taking of considerable pains in 

 future to obtain accurate and full records of the distribution on the 

 occasions of the recurrences of the several broods mentioned. Fortu- 

 nately, in 1902, the date of the next appearance of Brood X, there is no 

 13-year brood to confuse the records which may then be made. 



Many of the other scattering records of 13-year broods northward, 

 or of 17-year broods southward, may possibly be based on similar con- 

 fusions, arising from the overlapping of broods of the two races. 



The only way to accurately define the range of the different broods 

 is to undertake with each recurrence a thorough and systematic investi- 

 gation of all the territory open to the least doubt. Such work has been 

 repeatedly instituted, and particularly since 1868, and many of the more 

 strictly limited broods have been very carefully recorded and their dis- 

 tribution has been satisfactorily defined. Work of this kind has been 

 done for Brood III in Iowa by Professor Bessey, and for Brood V in 

 Ohio and West Virginia by Professors Webster and Hopkins. Similar 

 work has been done for Brood II in New York and New Jersey by Drs. 

 Lintner and Smith, and for X and XXIII by Riley in 1885, and Brood 

 XIX by Walsh and Riley in 1868. 



The value of a thorough and systematic canvass of the territory sup- 

 posed to be covered by any brood is exhibited in much of the work 

 referred to above, and notably in the case of Brood V studied by Pro 

 fessors Webster and Hopkins in Ohio and West Virginia. In the case 

 of this brood, however, there was no difficulty from an association with 

 any 13-year brood. 



WORK UNDERTAKEN FOR BROODS XXIII AND VI IN 1898. 



The present year a very careful investigation was undertaken by the 

 writer of the important 13-year Brood XXIII and the widely distributed 

 but less important 17-year Brood VI. By calling into requisition the 



* Bull. 14, etc., p. 26. 



