幼虫 and 羽化: a larva and emergence as an adult
Compare 幼虫 in discovery and picture-guide comparisons with 羽化 in a butterfly's adult emergence and a recorded time. Separate the stage noun from the change.
幼虫 as a found or compared larva
幼虫 (ようちゅう) means larva or caterpillar. 「葉の裏に小さな幼虫を見つけた。」 means “I found a small caterpillar on the underside of a leaf.” It names the found object and the underside of a leaf as its location.
「図鑑で昆虫の幼虫と成虫を見比べた。」 means “I compared insect larvae with adults in an illustrated guide.” The speaker compared larvae and adults using an illustrated guide. Comparing them does not report a larva changing into an adult at that moment.
羽化 as emergence and a recorded time
羽化 (うか) means adult emergence from a pupa. 「飼育箱の中で蝶が羽化した。」 means “A butterfly emerged from its chrysalis in the breeding box.” The butterfly in the breeding box is the object undergoing the change.
「研究員は昆虫が羽化する時刻を記録した。」 means “The researcher recorded when the insect emerged as an adult.” Recording this time does not establish one fixed emergence time for every insect.
A life-stage object versus a change
幼虫 names larvae found or compared; 羽化した describes the change from a pupa to an adult. Replacing the butterfly with a larva newly hatched from an egg would change the stated event.
The underside of the leaf and the butterfly's breeding box belong to separate examples. Discovery, guide comparison, emergence and time recording are not one continuous observation or completion of every growth process.
Does 飼育箱の中で蝶が羽化した describe a larva that has just hatched from an egg?
Show the answer and why
No. It describes the butterfly emerging from its chrysalis as an adult in the breeding box. Keep that change separate from finding 幼虫 or comparing larvae and adults in a guide. 羽化 is not replaced by egg hatching or completion of every growth process.