Nasuka

(from ebookjapan):

Toshiro Oyama, a poor young man living an ordinary life in a ramshackle apartment, is attacked one night by a blood-sucking insect kept by Mayu Yano, a beautiful woman in the next room.

Mayu saves him from a near-death attack, but Toshiro's resistance to insect bites is recognized, and he is offered a job at the "Insect Development Company. There, Toshiro becomes embroiled in a series of suspicious events...?