Bedbugs have been in the news in New York over the summer as a result of infestations throughout the city. An item about them on 30 August in Well, the online blog section of the New York Times, provoked this orthographical comment from a reader the following day:
Bed bugs is TWO words — not one. The general rule for writing out common names of insects is as follows. If the insect name is a misnomer (e.g., the dragonfly is NOT a fly and neither is a damselfly), then the whole name is written as one word. If it is not a misnomer, then it is written as two words (e.g., house fly, which is a real fly). The bed bug is a “true” bug and therefore is two words.
You may consider this to be an instance of what one wit has already described as folk entomology. I doubt that one person in a thousand has heard of this supposed rule, but other comments on the item argued the same point and they’re supported by discussions in numerous books on insect classification. A professor of entomology e-mailed me from South Africa to confirm that he had taught the rule to his students for the past 35 years.
The idea is to separate what are often called the “true flies” in the scientific order Diptera from other insects commonly but unscientifically called flies, and to draw a distinction between the Hemiptera or “true bugs” and bugs of other kinds. This is one reference of many:
Because of the aerial prowess of insects in general, a great many nonflies bear “fly” as part of the name, such as butterfly, firefly, stonefly, and mayfly. Notice that the names are spelled as all one word. True flies are described by two words, such as mydas fly, robber fly, and soldier fly.
The Kaufman Field Guide to Insects of North America, by Eric R Eaton and Kenn Kaufman, 2007.
Various books likewise argue that the same rule should apply to the bugs. To call them that in everyday speech is most commonly American. In Britain, for example, bugs are more typically microorganisms that cause disease, or the disease so caused. “I’ve gone down with some bug or other,” you might hear. Technically, the true bugs, such as the bedbug (or bed bug), are insects with mouthparts that are modified for piercing and sucking.
According to the rule, true bugs such as assassin bugs and shield bugs should have their names written as two words, but those that aren’t should be written as one, such as pillbug or ladybug (though what one should do with the British ladybird for the same creature is unstated, though as it isn’t a bird, presumably a similar rule applies).
Fly and bug are both old words of wide applicability and imprecise meaning that predate attempts at classifying the living world. Fly is by far the older; bug is recorded only from the seventeenth century as a generic term for various beetles or grubs. Nobody is sure where it came from, though a connection with the ancient sense of an object of terror that we retain in bugbear is suggested; the link might be directly with the bedbug, the first insect to be called a bug, which seems to have become a pest at around this time. The insect didn’t begin to be called a bedbug (at first as two words or as bed-bug) until the latter part of the eighteenth century.
The rule about inserting spaces in insect common names seems to be a modern creation, an informal way of using the spelling of these names as an aide memoire to distinguish Diptera or Hemiptera species from other little beasties. It’s highly unlikely ever to affect the usual spelling of bedbug, since the tendency in modern English is to amalgamate multi-word terms into single words, not split them apart. The spelling has long since become standard for everybody except professional entomologists.