The question I saw most this year (2021) was “what are the black spots on the back of cicadas for”? The people asking this question are specifically talking about Magicicada cicadas that have recently molted and are still white/cream colored and soft (teneral from the Latin word “tenen” meaning soft).
The area of the cicada where the black spots appear is called the pronotum — “pro”, meaning before in Greek, and “notum”, meaning the back, also in Greek. Before the back.
The spots contain a pigment that will gradually spread throughout a cicada body as it hardens, and transforms from white to black.
People speculate that the two black spots resemble eyes, and that might scare away predators. This might be possible, but I haven’t read anything to substantiate the hypothesis.
Wow thanks! I knew what they were called but had no idea they contained the black pigment! Truly amazing!!
I have been looking for an explanation for the two black spots on the prothorax on the emerging 17-year cicadas. This explanation of the spots being pigment reserves seems surprising to me. Do you have a source for this? I found a paper from 1911 (Studies on Melanin. II. The Pigmentation of the Adult Periodical Cicada; Ross Aiken Gortner, 1911) that indicates the cicadas darken as a result of oxidation. I recognize the two processes aren’t necessarily mutually exclusive but am unclear about how they would relate. Any more information on the source would really be appreciated. Thanks!!!!