(453/1. A short account of the Periodical Cicada (C. septendecim) is given
by Dr. Sharp in the Cambridge Natural History, Insects II., page 570. We
are indebted to Dr. Sharp for calling our attention to Mr. C.L. Marlatt's
full account of the insect in "Bulletin No. 14 [NS.] of the U.S. Department
of Agriculture," 1898. The Cicada lives for long periods underground as
larva and pupa, so that swarms of the adults of one race (septendecim)
appear at intervals of 17 years, while those of the southern form or race
(tredecim) appear at intervals of 13 years. This fact was first made out
by Phares in 1845, but was overlooked or forgotten, and was only re-
discovered by Walsh and Riley in 1868, who published a joint paper in the
"American Entomologist," Volume I., page 63. Walsh appears to have adhered
to the view that the 13- and 17-year forms are distinct species, though, as
we gather from Marlatt's paper (page 14), he published a letter to Mr.
Darwin in which he speaks of the 13-year form as an incipient species; see
"Index to Missouri Entomolog. Reports Bull. 6," U.S.E.C., page 58 (as given
by Marlatt). With regard to the cause of the difference in period of the
two forms, Marlatt (pages 15, 16) refers doubtfully to difference of
temperature as the determining factor. Experiments have been instituted by
moving 17-year eggs to the south, and vice versa with 13-year eggs. The
results were, however, not known at the time of publication of Marlatt's
paper.)

I am very much obliged for the extracts about the "drumming," which will be
of real use to me.

I do not at all know what to think of your extraordinary case of the
Cicadas. Professor Asa Gray and Dr. Hooker were staying here, and I told
them of the facts. They thought that the 13-year and the 17-year forms
ought not to be ranked as distinct species, unless other differences
besides the period of development could be discovered. They thought the