It is not only in horror movies, giant bugs did exist on planet earth, but back then there was something special in the air.
A higher concentration of oxygen in the air allowed dragonflies to sometimes get as big as hawks and some millipede-like bugs grew as big as two meters in length.
The study now shows that the proportion of oxygen has drastically decreased in the earth's atmosphere and as the result, the bugs can not get any bigger than they are right now.
The bigger an insect, the bigger the proportion of its body devoted to its tracheal system, which functions like a lung but is far less efficient at large sizes, according to the study.