BugGuide's "Remarks" on aphis nerii note that they "pick up deadly cardiac glucosides from the host plant and deposit them in their bodies. Their bright orange color serves as a warning to predators- at best they taste awful, at worst they can kill."

I was observing them last week for a good while, and saw them repel very large carpenter ants. When I placed one on a tiny spider's web, the spider was repelled each time it advanced on the aphid. After about six attempts, the spider vacated its web until the aphid eventually wandered off.

The two black tubes on its abdomen are the source of the noxious cardiac glycosides that warded the spider and ants off, sort of like the aimable nozzles of bombadier beetles and such. The substance is derived from the plants that host them.

Being so small (no loupe on hand), it was hard to tell exactly, but if I watched a larger one I thought to see it aim them. A shoot-out on a micron scale under every leaf, that's insects for you.

Thanks, UFL.: http://creatures.ifas.ufl.edu/orn/shrubs/oleander_aphid.htm

I've added the point about the cornicle secretions to the info page.

Thank you for confirming their ID.