writeChanges( index ) {

		_matrix.makeTranslation(
			this.curveLengthArray[ this.whichCurve[ index ] ],
			this.whichCurve[ index ],
			this.offsets[ index ]
		);
		this.object3D.setMatrixAt( index, _matrix );
		this.object3D.instanceMatrix.needsUpdate = true;

	}
The extra information about which curve and curve position is stored in the translation components of the matrix for the instanced objects
This writes that information to the matrix and marks it as needing update.

@param {number} index - The index of tge instanced element to update.