The key to the solution of the mystery of Julia Stoner's death is the poisonous snake which Dr. Roylott sent through the ventilator between their rooms and had trained to come back up the bell-rope and through the ventilator when he blew on a whistle. The author, Sir Arthur Conan Doyle, obviously did not want, to use the word "snake," however, especially in his title. That would have given everything away. The big question in this "locked room murder mystery" is: How could a girl be murdered when she is sleeping inside a room with the door locked and the window sealed by iron shutters? Doyle coined the term "speckled band" to avoid using the word "snake" in the title and in the text. In fact, the word "snake" is only used near the very end.
“It is a swamp adder!” cried Holmes; “the deadliest snake in India."
"The discovery that this was a dummy, and that the bed was clamped to the floor, instantly gave rise to the suspicion that the rope was there as a bridge for something passing through the hole and coming to the bed. The idea of a snake instantly occurred to me, and when I coupled it with my knowledge that the doctor was furnished with a supply of creatures from India, I felt that I was probably on the right track."
Julia Stoner must have seen the snake that bit her and caused her to die in agony in her sister Helen's arms. But Julia was dying, and her speech was incoherent. Instead of using the word "snake," she told her sister:
‘Oh, my God! Helen! It was the band! The speckled band!’
The term "speckled band" not only enables the author to avoid using the word "snake" but it serves as a red herring to keep the reader guessing. Dr. Roylott is obviously the prime suspect, but there seems to be a possibility that the "band" of gipsies may have been somehow responsible for Julia's death. The "band" of gipsies still spends some time on the land around Stoke Moran, so they might conceivably have some reason for wanting to kill Helen Stoner too. Dr. Roylott might still be involved, but he might have hired the gipsies to do his killing for him. The mystery is not completely resolved until Holmes and Watson hide in Helen's room and Holmes drives the snake back through the ventilator by whipping it with his cane.
Holmes confesses his mistake to his friend Watson near the end of the tale:
“I had,” said he, “come to an entirely erroneous conclusion which shows, my dear Watson, how dangerous it always is to reason from insufficient data. The presence of the gipsies, and the use of the word ‘band,’ which was used by the poor girl, no doubt, to explain the appearance which she had caught a hurried glimpse of by the light of her match, were sufficient to put me upon an entirely wrong scent.