Spiders

They may not be true 'spiders' per say, but Shepard was rounding them up to that. Giant, over-sized spiders are how she saw the Rachni. The thought of something with multiple legs crawling across her skin made her itch. Even if Grunt was there to kill them all, they still made Shepard shudder at every turn.

Finding the Queen was somewhat surprising. The Rachni that were on this planet had to of come from somewhere though. She was trapped, captured by the Reapers. If Shepard let her go, she promised to oppose the Reapers.

It was a hard decision, but in the end Shepard chose to save her once again. If these damn spiders were going to help build the Crucible and harass the Reapers, by all means, let them.