  If Jaffe and Wilczek's diquark picture for $\Theta_5$ pentaquark is correct,
there should also exist a $SU_F$(3) pentaquark octet and singlet with no
orbital excitation between the diquark pair, hence $J^P={1/2}^-$. These states
are lighter than the $\Theta_5$ anti-decuplet and lie close to the orbitally
excited (L=1) three-quark states in the conventional quark model. We calculate
their masses and magnetic moments and discuss their possible strong decays
using the chiral Lagrangian formalism. Among them two pentaquarks with nucleon
quantum numbers may be narrow. Selection rules of strong decays are derived. We
propose the experimental search of these nine additional $J^P={1/2}^-$ baryon
states. Especially there are two additional $J^P={1/2}^-$ $\Lambda$ baryons
around $\Lambda (1405)$. We also discuss the interesting possibility of
interpreting $\Lambda (1405)$ as a pentaquark. The presence of these additional
states will provide strong support of the diquark picture for the pentaquarks.
If future experimental searches fail, one has to re-evaluate the relevance of
this picture for the pentaquarks.
