In real life, mantises are prime nightmare fuel for many insects. The prospect of a mantis large enough to tower over buildings would be very bad news for us.
The mantis itself is actually pretty effective for the time. Young kids are particularly likely to be disturbed by the monstrosity.
The ending, where its arm begins to move, after we've been led to believe that the monster is dead. While it turns out to be a simple case of the mantis' nerves acting up even after its death, it still makes for a surprisingly tense moment.
