Unfortunately it doesn't say much about either mindflayers or beholders. All I can say is Beholders most likely mate for procreation and are definately homospecies unless a mage gets a hold of them. As for how they mate... I'm guessing the male beholders' wing-wongs are tucked in until mating...
Mindflayers are a bit more sophisticated as they are a heck of a lot smarter. Definately a homospecies race judging from how they regard other races as slaves and food. According to one site I found, "Mind flayers have no family structure. Their social activities include eating, communicating with the elder-brain, and debating on the best tactics to conquer the Underdark. For amusement, they inflict pain on their captives and force slaves to fight in gladiatorial games. Mind flayers live about 125 years. They are warm-blooded amphibians, and spend the first 10 years of life as tadpoles, swimming in the elder-brain pool until they either die (which most do) or grow into adult illithids. On an irregular basis, adult illithids feed brains to the tadpoles, which do not molest the elder-brain. Illithids are hermaphroditic; each can produce one tadpole twice in its life."
Tadpoles they say? I though of them more as mating like octapi.
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