184 



Increase in Number of Species 



produce genetically isolated populations. The most dramatic exam- 

 ples are the periodic cicadas of eastern North America comprising 

 a species flock of the genus Magicicada. These insects are about an 

 inch long and frequently occur in such numbers that their combined 

 calls make a deafening roar. A few of the species have seventeen- 

 year life cycles, the others thirteen-year cycles, all but a few 

 months spent as nymphs feeding on tree roots in the soil and only 

 about two months as adults. In spite of the fact that the seventeen- 

 year group always matures in exactly seventeen years, there is an 

 adult brood of these insects every year, not every seventeen years 

 as one would suppose. The ranges of some broods overlap in area 

 (Fig. 80), so that while the adults of one brood are active and 



Fig. 80. The periodical cicada (a) ranges of the nine abundant broods 

 having seventeen-year Hfe cycles. The broods are numbered from 1-17, one for 

 each year, hence five years elapse between the emergence of adult swarms of 

 Broods 14 and 2; (b) ranges of four broods having thirteen-year old cycles. 

 The broods are numbered from 18 to 30. Only broods 19 and 23 are abundant 

 and widespread. (Adapted from Marlatt.) 



mating above ground, nymphs of other broods may be growing 

 in the soil directly beneath. 



Eleven of the 17 seventeen-year broods are extensive, the others 

 are either scattered or small in extent. Brood 11 occurs only in a 

 small area of the New England states. Up to 1921 Brood 12 had 

 only six locality records scattered from Maryland to Illinois. The 



