First Insects, 405 - 375 mya, first hexapods and insects

The Devonian saw the flourishing and diversification of arthropods as well as plants, fish, and the evolution of amphibians. Before we could examine the biochemistry and DNA of arthropods, it seemed clear, based on body design and physical similarities, that the hexapods (the insects and their closest arthropod cousins such as springtails and proturans) evolved from the myriapods (centipedes and millipedes, etc.), but in the early 21st century more careful analysis of biochemical relationships revealed that hexapods more likely split off from the crustaceans, or even the largest and most diverse class of crustaceans, the Malacostraca. The similarities between myriapods and hexapods that made people believe they were more closely related than hexapods and crustacea are probably examples of convergent evolution. Hexapods may have split off from crustaceans (or possibly the myriapods) as far back as the Silurian era, but we only begin to see hexapod fossils (so far) in the Devonian rocks.

The earliest identifiable hexapod fossil yet found belongs to a springtail (a collembolan) from Rhynie chert of Scotland, dated to about 400 million years ago (mya). Springtails are not technically insects; they are insect cousins in the hexapod clade. Devonian rocks from about 390mya in the Gaspé peninsula of Quebec show a bristletail (an archaeognathan), and represent the first fossil that can be clearly identified as belonging to something in the class Insecta. There are also some insect remains from 378mya found in Gilboa, New York, but fossils of early dragonflies (The Protodonata—the precursors of the true dragonflies, Odonata, which didn’t appear until the Triassic) or other insects (Neoptera, insects that aren’t dragonflies or bristletails), appear mainly in the Carboniferous and Permian Eras that followed the Devonian.

One problem with identifying Devonian hexapod and insect fossils is that most of the fossils are fragmentary, and the primitive insects of the Devonian were probably all in the Order Thysanura, which includes the Archaeognatha (bristletails) and Zygentoma (silverfish), insects that look very much like centipedes, so that fragments can’t clearly be determined as belonging to primitive insects or primitive centipedes. Even the non-insect hexapods aside from the springtails (which are represented in the Devonian fossil record) such as the proturans and diplurans were probably on land in the Devonian, but no good (fairly complete and well-preserved) fossils of these hexapods exist until more recently in the geologic record.

The Devonian is the period when hexapods that aren’t true insects such as the springtails appear in the fossil record, and the most primitive true insects, the bristletails (Archaeognatha) and silverfish (Zygentoma) first appear, and these were established by 400mya to 390mya. Insects probably evolved from crustaceans on land, and did so around the same time period (early-mid Devonian) that terrestrial scorpions and spiders evolved. The early terrestrial arthropods of the Devonian seem to appear in the fossil record around the same time as plants begin their radiation (diversification) on land. But the Devonian was only the earliest stage for insects, and the sort of diversity of insects we see around us today came later, in the Carboniferous era, when insect fossils became much more common and diverse.

  Links about the first insects:

1. North Carolina State’s John Meyer has a good page for his class on General Entomology about the non-insect hexapods called Protura (or “Coneheads”).

2. Univeristy of Florida's Entomology & Nematology department also offers good information about proturans.

3. The Discover Life website offers some useful links and aggregated information about the order Thysanura.

4. Carolin Haug and Joachim T. Haug offer a useful overview of eight claimed Devonian insect fossil examples in their article: The presumed oldest flying insect: more likely a myriapod?

5. If you have access to an academic library, so you can read full-text articles from Palaeontology, we recommend: Fayers, S., & Trewin, N. (2005). A hexapod from the early Devonian Windyfield chert, Rhynie, Scotland. Palaeontology, 48(5), 1117-1130. doi:10.1111/j.1475-4983.2005.00501.x

This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.