We can all agree that for the most part, its pre-evolution Paras is in control of its body, but is the host of two parasitic Mushrooms who leeches off its life force. In exchange, it gains a wide array of moves to choose from. Most of its natural moveset seem to be the result of those two mushrooms, except for Slash, X-Scissor, and Slash. Even Leech Life could be argued as a gift from the Mushroom.
Even its ability seems to be the result of those tochukaso mushrooms. Effect Spore is obviously from the Mushrooms itself, while Dry Skin seems to be a result of the Mushroom draining the nutrients from the bug. Now it may seem like a disadvantage, but it gives Paras and its parasites an easy source of relief and energy in the rain.
As it grows its obvious that one of its Mushrooms is taking over and forcing the other one out and becomes completely dominant upon evolution. Now this leads to the question: "Who is it that we should be reffering to when we call out our Parasects?"
Many of you will probably say that it is the insect that is Parasect, I however believe that at this point in its life, the Mushroom has completely taken over the mind of the creature that was once known as Paras, to become Parasect. The expression on a Parasect's face looks more like the face of a zombie.
It is likely that the roots of the Mushroom has intertwined with the insect's central nervous system controlling it's body, blocking any signals from the brain. It is also possible that the roots have linked up with the brain itself creating a completely new being. Though the body itself is still controlled by the Mushroom, it still may exhibit some characteristics of the old Paras due to its connection with the brain. This leads to the second question: "What happens if the Mushroom is removed or dies?"
We could go with the "Slowbro Theory" where it will simply de-evolve back to Paras. However, the flaw I find in that idea is that both the Slowpoke and Shellder were completely different creatures, and their minds were separate. For Parasect, the Mushroom is too intertwined with the body that if ti dies, the body itself is most likely to die, becoming too dependant on the Mushroom. However, if the Mushroom is removed, I believe that it can be replanted and live, while the Insect half of Parasect would have suffered extensive nerve damage and die off. I even think that if the body dies, the <ushroom can live off it's corpse and eventually implant itself on the earth after the Insect has decomposed.
Now onto the third question: "Why don't the Paras remove the Musrooms before it consumes their body, and are they born with it?"
Like I had mentioned before, it gives them an extensive moveset that allows them to live, and I doubt that it would even have the capacity to evolve without the Mushrooms. By letting the Mushrooms stay on their backs rather than removing them (and we've seen instances in the anime where this has happened), they are ensuring their survival.
First and foremost, they are given better defenses thanks to the mutitide of spores the Mushrooms canc reate. It can paralyze or poison its enemies and its prey, and thanks to the mushrooms are most likely immune to their own spores. It even has a spore which can set its enemies to sleep 100% of the time. This makes it fairly easy for it to run away from predetors, and finish off its prey.
Secondly, I believe that once the species evolve into Parasect, that the Mushrooms will actually go out of their lengths merely to breed the insect half of their bodies. Why? For the survival of the Tochukaso Mushroom species. The Musrooms could easily just send spores and have its offspring grow on the ground, however, by letting the Insect breed, they are creating a new host body for their children.
"So what's the advantage of being on an insect's back?" Well it gives the Mushrooms transport, their spores can only do so much, but they aren't fatal. They only delay the enevitable. By being on a host isnect, it allows the Mushrooms a chance to escape
I'm pretty sure we're wondering then, "Why two Mushrooms on Paras only?"
At birth, I believe multiple spores are grown on the Paras's back. However, only two of them become dominant enough to grow to the mushrooms we usually see on them. The struggle for dominacne doesn't stop there, since only one can take controll of the bug and become Parasect. Even if only one Mushroom per Insect survives to adulthood, it is still enough to guarantee the survival f the Tochukaso's species.
So the final question I'd like to raise is "If Paras weren't born with Mushrooms on them, how did this partnership start?"
The answer is fairly obvious. Without the Mushrooms, the ancestors of the paras were limited in their movesets, and were most likely easy targets. Their ancestors had probably observed when other creatures tried to consume the Mushrooms, they often fell ill, or fell asleep. This led to a "lightbulb" moment in the heads of the Paras ancestors, that if they were to carry those Mushrooms around on their backs, that they too would have those defensive capabilities, although I think they only carried one small mushroom.
I don't believe that the Parasect evolution came instantaneous, nor did the Musrooms taking over the mind. The Paras ancestors most likely started off with their offspring searching for their own Mushrooms rather then having it given to them at birth. Eventually the Paras ancestors learned they could shake the spores onto their child's back and have it grow as they grew. Since the spores were planted, two were able to grow to its full strength, and were able to share the extra mushrooms with each other if theirs died off.
Over hundreds of thousands of years, evolution (Darwin evolution not Pokemon evolution) took place. Something mutated int he Mushrooms which allowed them to controll their host bugs. No longer was it the insect using the Musroom, but the Musroom using the Insect. The modern Paras and Parasect was born.