Definition
The dedicated role that delivers material, collects empty bins and returns kanban cards to the cells on a fixed route at a fixed frequency; with this role in place operators never have to leave the cell to look for parts.
In detail
The water spider is the circulation pump of a pull system: without it kanban cards never travel back, replenishment degrades into operators fetching parts themselves, and line-side inventory swells. The role needs standard work of its own — the route, the stops, what is carried on each run and how often — otherwise it turns into an on-call handyman. Two pieces of floor evidence show it is working: operators never leave the cell during the shift, and line-side inventory stays at the quantity set for the point of use. It does not add a person; it consolidates the material handling time that was previously scattered across every operator.
Example in practice
Operators in a cell used to make three trips to the warehouse per shift. After a water spider runs a fixed route every two hours delivering material and collecting empties, nobody leaves the cell and line-side inventory falls from two days to four hours.