In the earliest concepts for the Mortasheen world it was exactly like "The Fly;" you just get zapped in a machine and instantly fused. It kind of still works that way but it's more complicated.
It still works on normal humans, but it's more commonly a Sectilian, a lineage of metahumans who are the keepers of the tech itself and already born with countless Arthropod genomes.
When they near adulthood they begin to undergo a nasty metamorphosis and even form into a fleshy cocoon, which will die unless it can be fused with just the right species of Arthropod. They know which one they're "meant" to fuse with as it begins to haunt their dreams more and more throughout their youth. If successful they'll emerge from the process with part of themselves "swapped" with the bug, like the original Fly.
This retains their entire mind, and they have a permanent mental link with the counterpart bug, which now has some of their parts, and naturally lives as long as they do. It's just kind of a remote extension of their body.
If they were not a Sectilian (some other metahuman, or a regular human) or they're paired with the "wrong" Arthropod, then they become a fully merged hybrid monster, like the newer Fly
Arthropoid monsters are mentally much less human, with only fragmented/dim memories of their former life overwhelmed by appropriate new directives such as "spin webs to catch food" or "hide inside dead trees," though they'll generally still be loyal to anyone they were close to before.
Arthropoids can also mate, reproduce and make little baby versions of themselves, so the vast majority that exist are not the products of fusion, but fusion IS necessary to create an entirely new "species line" for them.
Mortasheen is a setting full of mad biologists so there are plenty of people who would do this for the sheer scientific discovery of it or because being a big bug under a big rock just sounds like a cool life. Becoming the first member of an entirely new monster variety also has some mystique to it, so even among Sectilians it's not always seen as a "failure." They may resort to it in order to live when they can't find the right personal bug, or they may prefer it because full Arthropoids are physically a lot more powerful/dangerous and they want to be able to protect their family clans.
Current official art of 2 Sectilians compared with equivalent Arthropoids:
