Are there differences in the components of saliva among species? Also, do the functions of saliva differ between species?
It could be better answered if you specified which species you were interested in. Broadly from Wikipedia, snakes have venom, some swifts have gummy saliva to help in the building of nests, spiders produce their web from salivary glands etc. This is too broad a topic. The human salivary composition is found on the Wikipedia page I have linked. Salivary compositions can change with habits which have been detailed in this article. To compare, I am also providing the link to an article on the saliva composition of an assassin bug.