I don't consciously do symbolism in my stories. I always got super pissed off in school English when *I* wanted to learn how to write a novel, and they kept assigning things where I had to pick apart stories for something I wasn't sure was even there.
That said, as I got older I do understand much more about symbolism than I did then. Here's what comes to mind on the ones you listed:
1. the Scythe - My first thought is death, grim reaper. My secondary thought is the scythe's traditional purpose...harvest. But it's a distant second, since as a reader I'm much more familiar with it as a symbol of death, and not harvest and life, since we don't tend to use scythes in modern farming these days. Scythes also have a dramatic shape.
2. A centipede - Ew! Squish it! I wouldn't really assign much meaning by default to a centipede. Doesn't mean it's there, just that I wouldn't pick up on it.
3. a spinal cord, - Major component of the nervous system of vertebrates, second only to the brain. The spinal cord or "backbone" of something symbolizes the essential part or center of an organism or organization.
4. Moths - Creatures of the night. Creatures that take up residence in empty places. In the case of Luna Moths, have a relationship with the moon. The default idea is that moths are ugly, but frankly I personally think some types are as beautiful or more beautiful than butterflies. Moths can also stand for transformation, as a butterfly can, but probably a less appealing one.