Its blades are its  spears. 
( ( Its blades ) ( ( are ( its spears ) ) . ) )
(ROOT (S (NP (PRP$ Its) (NNS blades)) (VP (VBP are) (NP (PRP$ its) (NNS spears))) (. .)))
Blades are an inferior substitute for spears.
( Blades ( ( are ( ( an ( inferior substitute ) ) ( for spears ) ) ) . ) )
(ROOT (S (NP (NNS Blades)) (VP (VBP are) (NP (NP (DT an) (JJ inferior) (NN substitute)) (PP (IN for) (NP (NNS spears))))) (. .)))