What are the main differences between 蝉 (chán) and 知了 (zhīliǎo) both meaning cicada.
Do they both refer to the same species?
Will either saying be understood by the majority of native speakers?
Is there a particular region where one is more common?
Is one archaic?
Yes, they both refer to cicada.
Most people use 知了 in both oral and written forms in mainland China. People almost never use 蝉 when they speak.
The reason cicada is called 知了 is because they make the noises which sound like they are saying (zhī liǎo... zhī liǎo... zhī liǎo...). Therefore, 知了 is also used to describe the sound cicada makes.
I'm not sure if one is more archaic than the other. As the word 知了 was used in classic Chinese literature 《月令七十二候集解》, which was written by 吳澄 in 元 dynasty (AD 1271–1368)
蜩，蟬之大而黑色者，蜣螂脫殼而成，雄者能鳴，雌者無聲，今俗稱知了是也。
I agree that both 蝉 and 知了 refer to cicada, but I don't think people will speak 知了 across China. Here, in Chengdu ,people don't say 知了,but 蝉子.However， 知了is a mandarin word, so an educated man can understand if you say it.
Some names for plants and animals vary from one place to another. One good exmple I would like to take is the "corn"，玉米 in mandarin. In the areas near Chengdu, people would say:"苞谷" or “玉麦", and in the city of Nanjing, people would say “苞芦".
I think 知了 is based on the sound a cicada makes (aka onomatopoeia), but both are reasonable terms to call a cicada. I don't think one is more right or better than another.
This page on Nciku has a little more information.
both of them are cicada
蝉 is used as it's formal name， and it's more archaic
知了 is used more in spoken or literature.
