i wouldnt return your earthworms, as long as you keep the soil moist and refrigerate them they will last for months, im pretty sure your pacman will be feeding by then, and i'm sure when he does he would like to have a nice worm to chew on, looks like your pacman is about the size of mine when i bought him that one glorious day at petsmart, i hate chain stores by the way, so you may want to cut the worm in half if its of the larger variety, yes earth worms tend to burrow fast, but my frog has usually made up its mind if it wants it before i even put it in the enclosure, if my worm isnt snatched up within about a minute, i pull the worm and put back into the refrigerator and then try again in a day or two, and john may be right about your substrate, looks pretty dry, hygrometers are recommended, but it only gauges air humidity and doesnt necessarily gauge your how damp your substrate is, if my substrate doesnt look wet i mist, which in my enclosures are every 3 days, but i also keep organic peat under my substrate which helps the cause of retaining substrate moisture, but i also keep live plants in my enclosures which also raise o2 levels and feeds off your frogs waist which in return keeps your tank a little cleaner, but doesnt mean to skimp on maintenance as there is really nothing to rid your tank of frog urine, but there are lots of other enviormental factors that play into how moist your substrate stays, but a good top will make all the difference in keeping your substrate moist, if you are using a screen top you will probaly need to mist at leat twice a day, im using a glass top with the plastic strip on the back and it has holes drilled in it on my suriname tank, my temp is 84 in tthat tank right now and humidity now up to 90 percent, half the glass on that tank is fogged over, on my cranwelli tank i use the same glass top but removed the back plastic panel and am now using wired screen, this tank is in the same room and the added ventilation drops the ambient air temp to 81 and humidity at 85 percent and none of the glass is fogged over, you may want to experiment a little bit to find the right combo