"Butterflies are insects."
Translation:Le farfalle sono insetti.
Degli is a combination of 'di', meaning of, by or from (sometimes). Gli is of course the plural male article. What you wrote translates to 'The butterflies are the insects'. While duolingo's 'degli insetti' was more right (Butterflies are of the insects), just dropping the articles altogether and saying 'Farfalle sono insetti' would be the best option, I think.
Maybe I could help here : )
just dropping the articles altogether and saying 'Farfalle sono insetti' would be the best option
Farfalle sono insetti. is wrong because you can't drop definite article in front of farfalle as in English.
Articles are also used speaking in general, in particular about general categories of things like here, that can be subjects or objects of sentences e.g.
La farfalla è un insetto.
(A butterfly is an insect.)
Le farfalle sono insetti.
([The] Butterflies are insects).
Mi piacciono le farfalle.
(I like [the] butterflies.)
la feminine singular / le feminine plural
When the article is coupled to the subject it's never dropped except when we have two or more countable nouns /uncountable nouns (= e.g. names of languages, colors, sports, food/material ect.) e.g.
L'oro e l'argento sono metalli preziosi.
Oro e argento sono metalli preziosi.
(Gold and silver are precious metals.)
L' inglese è una lingua universale.
Inglese è una lingua universale.
(English is an universal language.)
Gli studenti e gli insegnanti sciopereranno domani in piazza.
Studenti e insegnanti sciopereranno domani in piazza.
(Students and teachers will strike tomorrow in the square.)
however the phrases without article are used for informal speeches;
when the article is coupled to the object instead you could omit it sometimes, but it depends on the case anyway, for instance with the verb to like in my example the article needs.
it marked me wrong for typing "Le farfalle sono gli insetti"
About Philster's phrase, in Le farfalle sono gli insetti., gli (the) is wrong because in this type of sentences only the subject that belogs to the general category takes the article (butterflies).
To be correct, Philster's phrase should continue, making the speech specific and having to add an article, e.g.
Le farfalle sono gli insetti più belli del mondo.
(Butterflies are the most beautiful insects in the world.)
Concluding, I wrote when it's possible to omit the article in detail here:
Degli is a combination of 'di', meaning of, by or from (sometimes).
delle is a partitive article, it serves to indicate an indenfite quantity or, like here, a part of all (subcategory of insects), so it's not the articulated of , that is it only has its form but a different function. Literally it means something like some type of in this specific sentence.
Continue to read my replies here for further information:
The 'degli' used here would be the partitive article, meaning 'some'. We don't use it in English in an expression like this but I think Italian requires it, so their expression would translate as 'Butterflies are some insects'. This is obviously true, though not how English would phrase it.
I got this wrong because I hadn't been taught "degli" yet. I wrote "Le farfalle sono gli insetti".