Nowadays, the article "gli" (as well as the article "le") rarely (never) takes the apostrophe, also in front of words beginning with the same vowel.
gli idraulici - gli italiani - gli insetti - le erbe - le eliche - le entrate
you must use the article gli in front all masculine plural words (nouns and adjectives) starting with a vowel or s+consonant or z
- gli alberi (the trees), gli alti pini (the tall pines)
- gli studenti (the students), gli sporchi fiumi ( the dirty rivers)
- gli zaini (the backpacks)
Mario is stuck in a green pipe, while Luigi is eating some canoles.
That's a good question to wonder about. Whenever I think of one like that, I go to the English Wikipedia page for the English word, and then I find the link there to the language that I want. So I went to "Plumber" in English, clicked on "Italiano," and got this: https://it.wikipedia.org/wiki/Idraulico
It seems to answer your question and seems also to show what the word on the street is about the matter. I hope this helps!