Why isn't "there is an insect" accepted here. I before understood it as "el/ea este un/o ..." would mean "it's an ..." and "este un/o ..." = "there is a ..."
They're not actually the same thing.
"Insect" specifically means an invertebrate arthropod belonging to the class Insecta, characterized by three-segmented bodies, six jointed legs (although not six-jointed), a single pair of antennae and compound eyes. This classification includes such orders as Lepidoptera (moths, butterflies), Diptera (house flies, fruit flies, etc.), Odonata (dragonflies, damselflies) and Coleoptera (beetles), among others.
"Bugs", by contrast, is way less specific. It just generally means any kind of small, creepy-crawly thing out in nature, including non-insects like worms, centipedes, snails and spiders. Which is to say, all insects are bugs, but not all bugs are insects.
On the other extreme, in Romanian, every "gândac" is an "insectă", but not the other way around. The DEX and wikipedia claim that "gândac" is a coleoptera, but we also use it for cockroaches and some other insects.
The best translations for "gândac" are "beetle" or "roach".
The first part of you comment I would agree with, but I would limit "bug" to the true bugs in the order Hemiptera.
Thus all bugs are insects but not all insects are bugs.
It should still be accepted, though. Colloquially in English, "bug" and "insect" are used interchangeably.
Yes, I agree with Arcaeca; but WorseHalf is correct as to his mentioned "true bugs".