bluestocking, on the contrary, is a person, whereas bluebottle may denote both a flower and an insect but never a bottle.
Similar enigmas are encoded in such words as man-of-war ("warship"), merry-to-round ("carousel"), mother-of-pearl ("irridescent substance forming the inner layer of certain shells"), horse-marine ("a person who is unsuitable for his job or position"), butter-fingers ("clumsy person; one who is apt to drop things"), wall-flower ("a girl who is not invited to dance at a party"), whodunit ("detective story"), straphanger (1. "a passenger who stands in a crowded bus or underground train and holds onto a strap or other support suspended from above"; 2. "a book of light genre, trash; the kind of book one is likely to read when travelling in buses or trains").
The compounds whose meanings do not correspond to the separate meanings of their constituent parts (2nd and 3rd group listed above) are called idiomatic compounds, in contrast to the first group known as non-idiomatic compounds.
The suggested subdivision into three groups is based on the degree of semantic cohesion of the constituent parts, the third group representing the extreme case of cohesion where the constituent meanings blend to produce an entirely new meaning.
The following joke rather vividly shows what happens if an idiomatic compound is misunderstood as non-idiomatic.
Patient: They tell me, doctor, you are a perfect lady-killer.
Doctor: Oh, no, no! I assure you, my dear madam, I make no distinction between the sexes.
In this joke, while the woman patient means to compliment the doctor on his being a handsome and irresistible man, he takes or pretends to take the word lady-killer literally, as a sum of the direct meanings of its constituents.