What do we call the "middle" variables in a complex research model?
I've always found it difficult to pack up real-world data into the neat boxes of methodology textbooks, but if you really need a label for these, I'd go with mediator variable.

From a famous article on these variables:


  Specifically, we differentiate between two often-confused functions of
  third  variables: (a) the moderator function of third variables, which
  partitions a focal independent variable into subgroups that establish
  its domains of maximal effectiveness in regard to a given dependent
  variable, and (b) the mediator function of a third  variable, which
  represents the generative mechanism through which the focal
  independent variable is able to influence the dependent variable of
  interest.


Reference:

Baron, R. M., &amp; Kenny, D. A. (1986). The moderator–mediator variable distinction in social psychological research: Conceptual, strategic, and statistical considerations. Journal of personality and social psychology, 51(6), 1173.