Answer: Mosquito, not further identified
I apologize for not providing a genus and species level identification - the mosquito started falling apart when I went to remove it from the tube and I'd like to keep it intact as a display. However, one of my viewers cleverly suggested that this was the previously undescribed Culex salinenis!
To answer the question of how it got into the tube to begin with, I had to investigate how the saline tubes are made. First, empty glass tubes are received in bulk from the manufacturer in our media prep lab. They arrive shrink-wrapped in plastic, and are stored like this until needed. The tubes are then unwrapped in the media lab and immediately filled with sterile saline (also purchased in bulk) using an automated pipetting device. This particular lot of saline-filled tubes was prepared in March, which is not a time when mosquitoes are known to be out and about in my neck of the woods (there is still snow on the ground). Also, I don't think we've ever seen a mosquito in our lab, even in the height of summer. So our best guess is that the mosquito was in the tube when it arrived in the lab, and we then unknowingly added the saline on top of it. I doubt that the mosquito was in the saline, since I wouldn't expect it to survive intact through our pipettes.
After the tubes are filled, they were sealed and autoclaved, so technically, I guess the saline was still sterile(!) The tubes were then inspected prior to being put into use, leading to this interesting discovery.
So regardless of how it got there, it was a great pick-up by our technologist and a good reminder of why quality control is so important in the laboratory.