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What sort of beetle is this? I live in a suburb of Detroit and found a whole bunch of these living under my lawn when I was replacing a section of it. I'm assuming these are also the source of the grubs I found as well. Are these harmful?

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You found a stag beetle (family Lucanidae).

Although the males are much more readily recognized with their large "stag antler" shaped mandibles, these species are often sexually dimorphic with females having much smaller mandibles (e.g., see here). However, in some species the males' mandibles are likewise diminished and not stag shaped.

According to the Wikipedia page:

Currently, the only means of identification of world species is the illustrated text by Mizunuma, T. and S. Nagai. 1994. The Lucanid Beetles of the World. Mushi-sha, Tokyo. 337 pp. based on the work Catalogue illustré des lucanides du globe in Encyclopédie Entomologique (series A 27: 1-223) by Robert Didier and Eugene Seguy.

So without this guide (and without known dimensions of your specimen), an exact identification will be very difficult.

However, your species looks very similar to a male specimen of Lucanus placidus.

Lucanus placidus - http://museum.unl.edu

Additional pictures (such as the one below) can be found here.

Lucanus placidus

According to Wikipedia, L placidus is 22–35 mm long, has two or more teeth on its jaws and has dark femora. The species is sometimes found in soil and it does live in Michigan:

Lucanus placidus map - http://museum.unl.edu

  • We can rule out Lucanus capreolus because the color is wrong:

    [L. capreolus is] dark reddish brown in color with a smooth, shiny elytra; [their] femora are orange-brown.

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It's a Lucanus. Don't know the concrete species.

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  • $\begingroup$ It is not clear to me that the two insects are in fact from the same genus, since the Wikipedia stag beetle has much larger mandibles than the one pictured. $\endgroup$
    – March Ho
    Commented Jun 6, 2016 at 5:35
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    $\begingroup$ @MarchHo Mandible size varies between male and female, and between species in the genus, so it might be a type of lucanus. That said, this answer is probably more appropriate as a comment until something more definitive can be provided. $\endgroup$
    – user137
    Commented Jun 6, 2016 at 7:10
  • $\begingroup$ So, after all, it can really be a Lucanus, so why the downvote ? For a new user like me, it's very discouraging to get one. $\endgroup$
    – digfish
    Commented Jun 8, 2016 at 18:53
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    $\begingroup$ A downvote just means the answer isn't useful. You haven't provided any evidence to support your answer at present. I'd suggest adding what key you used and ideally a pointer to some resources that would allow the OP to follow it up to species level (or if it's too difficult to go to species level, why it's too difficult). For what it's worth I think you're right about it being Lucanus (probably capreolus). Please check out the tour for advice on how to write a good answer. And welcome to Biology.SE! $\endgroup$
    – arboviral
    Commented Aug 11, 2016 at 16:24

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