"A spider is not a good insect"
Translation:Buibui si mdudu mzuri
Assuming you misspoke and meant to ask if "mdudu" means strictly "insect" ("buibui" means spider):
According to the Swahili Wikipedia, "mdudu" is indeed used for just about any small arthropod, at least colloquially. To make a distinction when referring to "true" insects, you can literally say "wadudu wa kweli."