570 HEMIPTERA-HOMOPTERA CHAP. 



By so doing it has been found possible to establish the existence 

 of twenty-two broods which are distinguished by consecutive 

 numeration. .This being done, the evidence as to the years 

 during which Cicadas have appeared in any given locality is 

 examined, and the result is believed to bear out the view that the 

 life-cycle of the individual Insect is really one of seventeen years. 

 According to this view there are, underground, in certain localities 

 individuals of different ages that will appear on the surface as 

 mature individuals in different years. Thus in 1885 it was 

 understood that there were underground in Alabama two broods, 

 viz. brood xviii. that would appear on the surface in 1894, and 

 brood iv. that would appear on the surface in 1896. The pre- 

 dictions made as to the years in which Cicadas would appear in 

 some given locality are considered to have proved correct. More- 

 ever, particular entomologists have in certain localities verified 

 by personal examination the appearance of the Insects for several 

 consecutive periods of seventeen years. These facts appear fairly 

 conclusive, but they are much complicated by another point, viz. 

 that in certain localities the period is one of thirteen, not of 

 seventeen, years. This is to some extent a question of climate, 

 the thirteen-year interval being chiefly characteristic of the 

 Southern States. It is not, however, entirely so, for there 

 are localities in which the broods have an interval of either 

 thirteen years or seventeen years. Another fact should be 

 remembered, viz. that it is admitted that not quite all the 

 individuals of a particular brood are true to their proper time of 

 appearance ; in other words, a few specimens may appear pre- 

 cociously a year or two before their comrades, while some may 

 lag behind to a considerable extent. It is therefore a matter 

 for great surprise that, under these circumstances, the broods 

 should keep distinct at all, for one would suppose that time- 

 variation of this kind would lead to completely obscuring the 

 distinctness of the broods. We must also call attention to the 

 fact that both the seventeen-year and the thirteen-year broods 

 have a dimorphic form, or sub-species, called C. cassinii which 

 accompanies the ordinary form, with which it is apparently as a 

 rule not connected by intermediates. 1 



1 We must refer those who may wish for further information as to this complex 

 and difficult question to the writings of the late Professor Riley, especially to 

 Bulletin No. 8, 1885, U.S. Department of Agriculture, division of entomology ; 

 and to the more recent report by Marlatt, Bull. Dep. Agric. Ent., N.S. No. 14, 1898. 



