I am assuming that you are referring to Ambystoma maculatum. I have personally never kept this species but it sounds like that you are on the right track. Loose earthworms may disappear into the substrate so it may be better to try and offer them directly in front of the animals via forceps. For more detailed information check the postings here http://www.caudata.org/forum/forumdisplay.php?f=38

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