a blackish spot marks the place where a centipede was squashed last week, at the beginning of the month, perhaps the month before, or later

The first view of the centipede is described in the quote above. A... sees the centipede as a stain on the wall as she and Franck sit down to eat dinner. We learn at this point that Franck killed the centipede at some previous time.

She says “Hello” in the playful tone of someone who has slept well and awakened in a good mood; or of someone who prefers not to show what she is thinking about—if anything—and always flashes the same smile, on principle; the same smile, which can be interpreted as derision just as well as affection, of the total absence of any feeling whatever.

A..., as the quote above suggests, is inscrutable and unknowable. Her demeanor, especially her smile, is a form of defense that is always the same and thus is not a window to her thoughts and feelings but a barrier. It could mean anything—or nothing. It is perhaps this inscrutability that feeds the narrator's jealousy, or perhaps it is a play on the double entendre of the French word “jalousie,” meaning Venetian blind as well as jealousy. Because of how she presents herself, the narrator can only see glimpses of A..., bits and pieces, as if seen through the slats of a Venetian blind.

The body is curved toward the bottom: its anterior part is twisted toward the baseboard, while the last joints keep their original orientation -- that of a straight line cutting diagonally across the panel from the hall doorway to the corner of the ceiling above the closed pantry door.

Once again the centipede is described. The centipede stain grows, eerily. Possibly it is a sign that the narrator's sense of being stained has grown.