What do you call clearly different sub-groups within a species?
In eusocial insects, especially ants and bees, these groups are called "castes" (see e.g. Hölldobler &amp; Wilson, 1990. This is the same term that is used for social stratifications in some human societies e.g. in India.

One definition of caste is:


  ...the physical or the behavioural/physiological phenotype of a eusocial individual, or both.  


(from Hölldobler &amp; Wilson, see also antwiki.org)

Some division of labour as well as collective rearing of young can occur in prosocial animals as well, but there the term caste is not used.