the redback spider is from Australia, so I'm not sure if their hyphenation rules are different, but in American English, if you're putting a color before a noun as a descriptor, you link the two with a hyphen: i.e. "the hawk with a red tail" is "the red-tailed hawk," or "the boy with blue eyes" is "the blue-eyed boy." If I hadn't seen "redback" first written that way, my assumption would have been to write is as "red-back." It's definitely a grammatical exception.

Miss Alexis is absolutely right - in your example 'redback' is a noun (the name of the species of spider). Most compound adjectives have hyphens to make their meaning clear, but not all do. EXAMPLE: We have two metre long planks of wood. (The meaning is unclear, because you could have two planks of wood that are a metre long each, or you could have an unspecified amount of planks that are two metres long each.) We have two metre-long planks of wood. (two planks) We have two-metre-long planks of wood. (each plank measures 2 metres) That's generally why we use hyphens with compound adjectives (adjectives made up of more than one word). BUT If the compound adjective FOLLOWS the noun, we don't have to use a hyphen, because the meaning is clear. EXAMPLE: High-speed trains (the adjective comes before the noun and a hyphen is needed) The trains are high speed (the adjective comes after the noun and no hyphen is needed) Some compound adjectives (especially ones with suffixes or prefixes) can be written as one word: "The machine has a RINGLIKE structure." It might be useful for you to look up 'compound adjectives' and learn more about them. Good luck, and enjoy your language learning.

Hello Raena! In this instance, those are actually the proper names of these animals. Redback spiders are a specific type of spider species, so you would not call them Red-tailed spiders. :-] It is not a matter of grammatical correctness here! Redback and Red-tailed are both adjectives.