It starts with a girl, sleeping peacefully in her bed.  Except there's a small mark, like a bug bite, on her left forearm.  She scratches it absently, but continues sleeping.

Then she wakes up.  She goes about her morning routine, walks into the bathroom, goes to brush her teeth, pushes her hair away from her face...  and notices the mark on her arm.  Only now it's bigger, about a centimeter wide and a couple centimeters long, running along the middle of her left arm.  She gives it a WTF look, rubs it briefly, but doesn't pay it any more attention.

A bit later, she's walking down a city street, headphones on, occasionally rubbing her arm, but otherwise preoccupied.  She window shops for a bit, flips through some books in a store, finally ends up in a cafe.  While reading a book and sipping some coffee, she gets a pained look on her face.  She pulls back her sleeve to reveal a long, undulating welt running the length of her forearm, from the back of her hand to nearly her elbow.  She quickly pulls the sleeve back down, casts a glance around the room to make sure no one else noticed (they didn't -- the bystanders don't pay her any notice throughout), picks up her stuff, and rushes out of the cafe.

She hurries back down the city streets, this time with her attention focused on her arm, which she grips with the opposite hand.  When she reaches her apartment, she throws off her coat and bookbag, pulls back her sleeve, races into the kitchen, and pulls out a knife.  She lays her arm across the counter, hesitates for a fearful moment, then draws the knife quickly up the back of her arm, wincing.  As the skin splits, a long, segmented insect with dripping wings and snapping mandibles forces its way out.  It flops on the counter, and both girl and bug gasp for air.  She stares at the insect in shock as it waves its wings dry and writhes around on the countertop.  Then suddenly, it leaps up, wings carrying it into the air, buzzing around the kitchen before zipping out of the room to parts unknown.

The girl finally realizes she's got a gaping, bleeding wound down her arm, and digs through a drawer with her good hand, pulling out a needle and thread.  She quickly stutures the wound shut, pulling the knots tight with the loose end of the thread in her teeth.  Then she seizes the knife and begins to stalk through the apartment.  She cocks her head to one side, listening for the bug.  Thinking she hears something, she turns the corner, walks through a door, and in that room is the insect, now grown to a couple meters in length, snapping its jaws and glaring at her with giant compound eyes.  She stares back, obviously afraid, but tightens her grip on the knife.  Girl and bug lunge simultaneously, one twisting past the other, the girl managing to avoid the serrated jaws by inches, and plunging the knife into the weak spot between the beast's segments.  The bug rears back, but she maintains her grip on the knife, and wrenches it around the circumfrence of the segment, cutting the bug's throax cleanly off its torso.

She stands, panting, in a puddle of blood and ichor, with the head and legs twitching on one side of her and the back end flopping on the other.  She slides open the balcony doors, hauls both parts out, and drops them off into the dumpster below.  She then sits on the balcony, rests her head against the rails, and looks exhaustedly up at the moon.