  The molecular clouds Lupus 1, 3 and 4 were mapped with the Mopra telescope at
3 and 12 mm. Emission lines from high density molecular tracers were detected,
i.e. NH$_3$ (1,1), NH$_3$ (2,2), N$_2$H$^+$ (1-0), HC$_3$N (3-2), HC$_3$N
(10-9), CS (2-1), CH$_3$OH (2$_0-1_0$)A$^+$ and CH$_3$OH (2$_{-1}-1_{-1}$)E.
Velocity gradients of more than 1 km s$^{-1}$ are present in Lupus 1 and 3 and
multiple gas components are present in these clouds along some lines of sight.
Lupus 1 is the cloud richest in high density cores, 8 cores were detected in
it, 5 cores were detected in Lupus 3 and only 2 in Lupus 4. The intensity of
the three species HC$_3$N, NH$_3$ and N$_2$H$^+$ changes significantly in the
various cores: cores that are brighter in HC$_3$N are fainter or undetected in
NH$_3$ and N$_2$H$^+$ and vice versa. We found that the column density ratios
HC$_3$N/N$_2$H$^+$ and HC$_3$N/NH$_3$ change by one order of magnitude between
the cores, indicating that also the chemical abundance of these species is
different. The time dependent chemical code that we used to model our cores
shows that the HC$_3$N/N$_2$H$^+$ and HC$_3$N/NH$_3$ ratios decrease with time
therefore the observed column density of these species can be used as an
indicator of the chemical evolution of dense cores. On this base we classified
5 out of 8 cores in Lupus 1 and 1 out of 5 cores in Lupus 3 as very young
protostars or prestellar cores. Comparing the millimetre cores population with
the population of the more evolved young stellar objects identified in the
Spitzer surveys, we conclude that in Lupus 3 the bulk of the star formation
activity has already passed and only a moderate number of stars are still
forming. On the contrary, in Lupus 1 star formation is on-going and several
dense cores are still in the pre--/proto--stellar phase. Lupus 4 is at an
intermediate stage, with a smaller number of individual objects.
