Early 18th century; earliest use found in Philosophical Transactions of the Royal Society of London. From classical Latin ōstiolum little door from ōstium + -olum.
Definition of ostiolum in US English:
ostiolum
nounˌɒstɪˈəʊləm
Mycology Botany
An ostiole, especially of a fungus.
Origin
Early 18th century; earliest use found in Philosophical Transactions of the Royal Society of London. From classical Latin ōstiolum little door from ōstium + -olum.