cicada, continuous
蟬 centers on the cicada insect, with the extended sense of continuous sound or activity coming from the cicada's long, unbroken buzzing.
蟬 combines 虫 (insect) with 單, which likely contributes the sound and may also suggest the idea of single or simple, though the exact historical connection to cicadas is uncertain.
The insect radical 虫 beside 單 (which looks like a simple, single unit) can remind you of a cicada's simple, continuous drone.
For セン, imagine a cicada sending out its continuous sound: send -> セン, and the insect sends its call through the summer air.