OF WASHINGTON. 117 



an extensive 1 3-year brood, the brood which, in 1868, first gave 

 me the opportunity of establishing and defining the tredecim as 

 contradistinguished from the septendecim race. 



The fact that thirteen years and seventeen years, respectively, 

 are required for the full development of this insect, according to 

 the race, is thoroughly established on chronological data, one of 

 the septendecim broods having been observed every seventeen years 

 since 1715. Nevertheless, there is chronic skepticism as to the facts, 

 especially on the part of Europeans not familiar with the Ameri 

 can literature upon the subject. Anomalous and exceptional 

 facts in natural history very generally provoke such skepticism. 

 \Vhile, therefore, there has never been any doubt in the minds of 

 intelligent entomologists, it has always struck me as desirable to 

 give experimental proof of the fact that this insect remains under 

 ground during the seventeen-year and thirteen-year periods allotted 

 to it, according to race, minus the two brief months of its imaginal 

 or winged existence. This was all the more necessary because 

 of the difficulty of rearing the larvas in confinement, and of fol 

 lowing any one particular individual throughout its development. 

 Nevertheless, a number of individuals are easily traced by repeated 

 diggings under certain trees where the larvae are known to have 

 entered the ground abundantly in any particular year. This ex 

 perimental proof I have endeavored to obtain ever since the year 

 1868, when I followed in St. Louis county, Missouri, the exten 

 sive tredecim brood which appeared there that year. The ob 

 servations which I made there, and others made by an agent 

 specially instructed, Mr. J. G. Barlow, of Cadet, Mo., for the 

 tredecim race, and similar observations made here at Washington 

 for the septendecim race, with the assistance of Mr. Th. Per- 

 gande and Mr. C. L. Marlatt, have permitted me to follow the 

 larval life from year to year with great care, so far as the first 

 twelve years are concerned, and with less care and continuity for 

 the subsequent years. The development during these later years, 

 however, has been followed with sufficient accuracy by the study 

 of individuals from different broods, the age of which was chro 

 nologically known. 



The cases of retarded or accelerated development in this species 

 are remarkably few, considering the immense numbers in which 

 the insects appear during their stated years. A few stragglers 

 are sometimes heard the year before or the year' after the regular 

 appearance, but so rarely as to make the regularity all the more 

 striking. It is, indeed, difficult to explain these exceptionally 

 long periods of larval life in this species, or the great regularity 

 in development through some eight degrees of latitude, on any 

 theory of advantage to the species not possessed by other species 



