for the 13-year broods. Apparently no differences 

 exist between the matching pairs of species of the 

 13- and 17-year broods; they are merely more or 

 less isolated from each other for 13-17 generations. 



In Illinois, at least, one 13-year brood, the Great 

 Southern Brood (Marlatt's XIX), is not as isolated 

 from 17-year broods as might be expected. Along 

 the border of the range of this brood, contacts with 

 all of three 17-year broods may occur within 221 

 years, and contact with one of these three 17-year 

 broods could and possibly does occur every 65 years 

 (five generations) or 78 years (six generations), as 

 indicated in Table 1. If such contacts are made, 

 enough hybridization may occur to prevent isolation 

 of the "incipient species." 



Table 1. — Dates of simultaneous emergences of a 13- 

 year brood and each of three 17-year broods of periodical 

 cicadas in Illinois. (Mathematically, 65 and 78 are functions 

 of 13.) 



If the matching pairs of 13- and 17-year cicadas 

 are merely phases of the same species, three full 

 species might remain. Each would have its own 

 niche, the small M. cassini living on roots of bottom- 

 land trees, the large M. septendecim living on roots 

 of upland trees, and A/, septendecula. intciTnediate 

 between the first two and rarer, developing priinarii\' 

 on hickory and walnut roots (Dybas & Davis 1962; 

 Dybas & Lloyd 1974). 



It seems hardly possible that M. septendecim is 

 a single species with three forms, resulting from 

 development in separate and different habitats, and 

 with two phases — 13-year and 17-year — primarih- 

 resulting from environmental influences. Howc\'cr. 

 the order Ilomoptera, to which these cicadas belong, 

 is well known for having species that develop highly 



divergent forms in response to theii feeding sub- 

 strates. Some scale insects, for example Aspidiotm 

 ancylus (Putnam), produce divergent forms according 

 to whether they feed on bark or lea\es or on certain 

 trees and vines; these forms once were considered 

 to belong to separate genera (Stannard 196-5). Some 

 leafhoppers produce forms with radically different 

 types of genitalia, depending on temperature in- 

 fluences (Miiller 1957). Even so, the possibility that 

 there is just one species with forms having their own 

 songs, their own times of day for singing, preferences 

 for mating with their own kinds, and their o\\-n feed- 

 ing niches appears remote in the light of our present 

 knowledge and contradicts generally accepted con- 

 cepts of species. 



In this report only the broods will be considered; 

 the nature of the species and their evolution have 

 been well discussed by Llovd & Dvbas (1966a and 

 1966fo) and Dybas & Lloyd '(1974). 



STRAGGLER CICADAS 



Stragglers from the 13- and 17-year sequence do 

 occur with some frequency. Usually, according to 

 most records, onh- a tew indi\iduals emerge at these 

 off times, and there seems little chance for mating 

 and production of progeny. In 1969. however, a large 

 number of stragglers emerged in the Chicago area. 

 This emergence was of further singularitv' because it 

 occurred at the close of a 13-year interval in a 

 region where only a 17-year brood was known. Dybas 

 (1969) made an extensive study of these "13-year 

 stragglers" and concluded that he may have had a 

 "ringside seat" along the evolutionan,' pathway on 

 \\hich 13-year cicadas were derived from 17-year 

 cicadas. He assumed that the cicadas of this par- 

 ticular emergence probably would not develop into 

 a new brood because of the usually cold climate of 

 Chicago. Dybas postulated that the heat effect of the 

 city may have had some influence on these particular 

 stragglers, causing them to emerge so earl\-. 



Stragglers that emerge in the fall of the year can 

 be positively identified as deviates, but those that 

 emerge in the regular season (May and June) need 

 to be anaKzed with considerable care. Tliese spring 

 stragglers seem to have provided the basis for all 

 tlie false broods listed b\- Marlatt and others. Most 

 stragglers emerge early, in the year before the main 

 brood is due. 



Supposed Illinois Stragglers 



I'nless otherwise stated, data refer to single speci- 

 mens in the INHS collection. 



1904. Riverside, Cook Count\-. Possibly not 1907, 

 as given b\- .\lexandcr 6c Moore (1962"). The 

 last digit appears to be a 4 rather than a 7. This 



