As Fritz said, impaction is a blockage - there would be no feces. It is possible one of your new leopard geckos regurgitated the mealworms. Were they in a slimey looking mucous coated mass? Could be a) due to parasites b) due to incorrect temperatures in your enclosure c) they simply ate too much. Sometimes babies and juveniles do this, but rarely more than once.
Do take a fresh stool sample with you to the vet if you can. Dried out ones are no good. You can keep a sample in the fridge (not freezer) overnight if necessary. That's the best and easiest way to check for parasites.
You might ask your vet about giving a little dab of yogurt to the new leos too. Yogurt with live cultures can go a long ways towards restoring healthy gut bacteria. Doesn't take much, either and my vet recommends it after any round of antibiotics.
Good luck, and let us know how it goes!
>>I found some undigested mealworm in the feces of my newly aquired baby geckos. They are both painfully thin, and I've only seen one of them eat. At the store I purchased them from they were kept on sand. I'm not ruling out other medical problems-- they're going to the vet today-- but I think they may be impacted. I know from caring for humans that impactions are very serious, and I imagine with tiny geckos the process just goes more quickly. What can I do now to start treating the impaction?
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Andrea A. 