Summoned beings, especially those of the [Spirit] type, weren't sentient as they were merely formed from mana and were only fighting off basic combat instincts and orders.
Hence, Seira's insult didn't affect the Spirit Serpent one bit.
However, who it did affect was the owner of the summoned being, the Goblin High Shaman.
Due to its high intelligence, it was able to learn a bit of human language and although it cannot speak the language, it can understand significant bits of conversation.
Instinctively, the monstrous shaman grasped that the young female human was referring to its summoned Spirit Serpent as a worm, utterly enraging it.
"GHARARRK!" The Goblin High Shaman shrieked, ordering the Spirit Serpent to devour the impudent young woman.