Bug type moves are super effective against Dark, and Psychic.
ummmmmmmmmmm nobody knows?
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I guess just to make it good against something other than grass.
But if you wanted to get all serious about it, then you could say that bugs fly around in the dark just as much or more than in the day, or at least stuff like moths do. And maybe their underdeveloped brains aren't easily manipulated so that would explain why it's strong against psycic.
But to be honest I doubt either of these theories are true.
Typings just work out that way.
Why does Dark not effect Psychic at all? It just works out that way.
-Bugs bother you, so they interfere with Psychic's concentration.
-Bugs like the dark and shade and stuff.
But most of all, those two needed a second weakness.
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Dark's strengths and weaknesses are dodgy no matter how you try to work it out. "Dark" being such an abstract concept, not a "physical" thing with obvious weaknesses like the Fire or Grass types. The same could be said for the Dragon type, really.
Hell, if you wanted to get really logical, it should be weak to Fire if anything, it being a light source and all...
It evens out the metagame
To build upon the 'interfering with concentration' theory, I think that the Bug type (and, while we're at it, the Dark type) represent types of common fear. If you're brain's overloaded with fear, rational or otherwise, it can't function psychically at all. At least, that's my take on it.
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A lot of people are afraid of bugs.
I already mentioned this but the Psychic type weaknesses correlate to some common fears (Bug, Dark, Ghost).
As for why Dark is weak against Bug... Cheap shots are really kind of useless against bugs.
Also, you may want to check this topic out: How is (insert Pokémon type) weak to (insert Pokémon type)
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I was about to joke that Psychics fear pesky bugs but Kibago-Mayfan's reason works. And of course, I just figured that bugs love the dark. I think a better question is why is Dark stong against Ghost when the same logic for bugs can be applied? Even moreso for than in fact since they "hide in the shadows" as it goes.
I thought the reason was like Jabberwocky: Circular strength/weakness -
Fighting < Psychic < Dark < Bug, repeat.
Because Bug had to be Super Effective against something, and i guess Grass is a logical time sice bugs eat plants and often love the dark, which is why they could be super effective against dark types
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