VERY odd observation!

[Thamnophis]

I would not rule out that ants (or other insects) are sometimes eaten.

Some thirty years ago I had a terrarium (200 x 50 x 50 cm) in which different species of Thamnophis lived. There also was a Podarcis sicula (Italian wall lizard) in this terrarium because there was no other room for him. He always sat on top of the snakes under the lightbulb so he could get the most warmth.
There was never a problem between them.

But when I put a handful of mealworms in his foodbowl it happened that they were gone very fast (they could not escape). And it were too much mealworms for that one lizard to eat in one time.
I suspected a Thamnophis radix to eat them. Have never caught him in the act, but the times that it happened he was the only snake that was active on that moment.
Unfortunately I have never examined the stool of the snakes for chitineparts of the mealworms.

[zooplan]

Martin Hallmen reported that garters may eat thier own (sheded) skin because of the smell of feeder fish, so the garter may have eaten some ants, carrying worm pieces.

[aSnakeLovinBabe]

i have seen some of my baby garters eat their own skin! like, grab it, and start eating it. I wouldn't entire rule out that some garters have taken up a liking for insects... I've got one that'll eat waxworms... but then again she will eat anything I put in front of her if I present it like food.