dA/dt = -delta*A(t)                                  when t <  tau
        -delta*A(t) + P*A(t-tau)*e^-(A(t-tau)/A_0)   when t >= tau

Where A is the population of adult flies

      tau is the development time from eggs to adulthood

      P is the maximum adult fecundity rate multiplied by 
      the survival rate from egg to adult.

      A_o determining how quickly fecundity declines with 
      adult population. 