This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems.
ðɪs ɪz bɪkˈʌz ðə lˈɑːɹviː ɑːɹ ɔːlɹˌɛdi kɑːɹnˈɪvɚɹəs ænd kəntˈɪnjuː tə fˈiːd æz pɹˈɛdəɾɚz wɛn ðeɪ ɑːɹ ɐdˈʌlts sˌoʊ fjˈuː tʃˈeɪndʒᵻz ɑːɹ nˈiːdᵻd tə ðɛɹ daɪdʒˈɛstɪv sˈɪstəmz.