Discussion Bleach

Not a fly. A cricket, actually.

Specifically this one:

https://en.wikipedia.org/wiki/Meloimorpha_japonica

^ Yup...

The species he transformed into is literally the name of his zanpakuto (the name of the Japanese bell cricket in Japanese common parlance). And a zanpakuto, I will remind you, reflects the user's own soul. So it isn't Aizen's doing. Tosen is spiritually (and literally in resurreccion) a Japanese bell cricket.

His hollowfication and resurreccion were kind of subtly foreshadowed, actually.

- The name of his zanpakuto = Japanese bell cricket

- The tsuba on the zanpakuto of all (or most) shinigami is gold, brass, or bronze in colouration. Tosen's tsuba is bluish-silver, like the tsuba/handguard of all Arrancar zanpakuto. Another major clue.

- Tosen can use Descorrer (as seen when he retrieved Grimmjow) which is a technique unique to Menos-class Hollows and Arrancar, and which apocryphal material confirm hollowfied shinigami can also learn.