Looking up R objects by name

Just use the get function:

for (sample in c("larva", "pupa_0h","pupa_3h","pupa_6h","pupa_12h","pupa_72h","adult")){

# Convert regions
DB <- convertToTargetRegions(queryRegions = get(paste0("DAR_btwn_stages_KC_subset_", sample)),
                             targetRegions = dbRegionsLoc,
                             minOverlap = 0.4,
                             overlapType = "any")