I did a little research and found Tenebrio obscurus (mini mealworm) they reach 12-19 mm. Also the beetles (adults) of the mealworm, superworm and mini mealworm are all members of the family Darkling beetle, and it is estimated that this family of bugs consists of nearly 20.000 members, so I guess there are 20.000 larvae, but these three are the only ones that are used as food for reptiles and arachnids.