Etymology
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insectarium (n.)
1872, from insect + -arium, abstracted from aquarium, etc.
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terrarium (n.)
1877, from Latin terra "land" (from PIE root *ters- "to dry") + -arium, abstracted from aquarium.
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planetarium (n.)

1734, "orrery, astronomical machine which by the movements of its parts represents the motions and orbits of the planets," Modern Latin, from Late Latin planeta (see planet) + Latin -arium "a place for." Sense of "device for projecting the night sky onto the interior of a dome," developed by Zeiss in Germany, debuted in Munich in 1923 and is attested by the name in English from 1929.

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-ary 
adjective and noun word-forming element, in most cases from Latin -arius, -aria, -arium "connected with, pertaining to; the man engaged in," from PIE relational adjective suffix *-yo- "of or belonging to." The neuter of the adjectives in Latin also were often used as nouns (solarium "sundial," vivarium, honorarium, etc.). It appears in words borrowed from Latin in Middle English. In later borrowings from Latin to French, it became -aire and passed into Middle English as -arie, subsequently -ary.
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