"of seventeen years," 1855, originally and usually in reference to cicadas, from Latin septemdecim "seventeen" (see seven, ten) + -al (1). Related: Septemdecimally. Septemdecenary "occurring once in 17 years," also of cicadas, is attested by 1843 (in William Kirby and William Spence, "An Introduction to Entomology," an etymology/entomology contact).
They never descend very deeply into the earth, but remain about the tender fibres of the roots. The only change they appear to undergo in their subterranean pilgrimage of seventeen long years is increase of size. As the time of their transformation approaches, they rise to near the surface of the earth, form cemented cells, in which they pass their pupa state, after which they burst the skin on the back and ascend the first tree, and again perform their septemdecimal offices of generation. [John Hooper, report on the seventeen-year locust, in "Annual Report of the American Institute," New York, 1855]
updated on May 15, 2022