Why is it Honey Bee and not Honeybee?

Why Honey Bee is Two Words

Regardless of dictionaries, we have in entomology a rule for insect common names that can be followed. It says: If the insect is what the name implies, write the two words separately; otherwise run them together. Thus we have such names as house fly, blow fly, and robber fly contrasted with dragonfly, caddicefly, and butterfly, because the latter are not flies, just as an aphislion is not a lion and a silverfish is not a fish. The honey bee is an insect and is preeminently a bee; “honeybee” is equivalent to “Johnsmith.”--FromAnatomy of the Honey Bee by Robert E. Snodgrass