How do I differentiate these two Tenebrio?

I have been raising Tenebrio for a while now, but I keep finding larvae that are much darker than the rest, but as soon as they pupate, I can no longer figure out which were the oddities.

And I was wondering if there is an easy way to differentiate between the adult Tenebrio molitor and the adult Tenebrio obscurus? Same thing with their larvae.
If anyone can answer, it would really help, because I keep between four and six adult specimens from every generation of mealworm beetles that I raise/breed, and because Tenebrionidae is not one of my beetle strong points, I don’t really know if I should be labeling all of them, or just a few of them as T.molitor.

So far I’ve preserved/pinned 29 adult Tenebrio, and I’m currently on generation 7, I’m not sure why there is one random one since I usually only take an even number of reference specimens from each generation.

Some of the adults have pronotums which are very rounded, which I think are T.molitor, and some of the adults are thinner, thinner/less rounded pronotums(or prothorax, depends on who taught you), are darker coloured, and have narrower bodies and smaller heads. These I think may be T.obscurus because of this, their legs are also thicker.

@borisb Can you assist at all?

This might help - Beetles Identification Guide ( It’s specific to stored product pests. The couplet separating the two species is on page 30. The differences are not great. I use ctrl f to find things in PDF’s.

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@mamestraconfigurata Thanks Ian, the article actually helped me ID several other beetle stowaways that I commonly find in with my Tenebrio, as well as with differentiating the two species. Thank you!

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