Meaning of BUG in English
commonly, any insect. In entomology, however, it refers specifically to any member of the insect order Heteroptera (e.g., chinch bug, bedbug). When the word bug is part of the common name for a member of the true bug order Heteroptera, it is usually a separate word (exceptions are bedbug and stinkbug). When used as part of the common name for an organism that is not a heteropteran, the word bug is usually not separatede.g., ladybug (a member of the beetle order Coleoptera), mealybug (a homopteran), doodlebug (a neuropteran larva), and sowbug (a crustacean). There are many exceptions to this convention. In the British Isles the name applies solely to one member of the heteropteran order, the bedbug Cimex lectularius.
Britannica English vocabulary. Английский словарь Британика. 2012