Using a feeding insert simplifies the way we supply and remove food, also helps protect the colony from rot, mold or decaying matter as it can be removed quickly, they are designed to fit flush into our test tubes and seal with a tiny cotton ball at the front.
 
Four size options available to fit the inner diameter of the most common test tubes available, the inside hole grows proportionally with the larger inserts. 
 
For 15 mm tubes, insert measures 12.7 mm diameter x 25 mm length
For 16 mm tubes, insert measures 13.7 mm diameter x 25 mm length
For 18 mm tubes, insert measures 15.7 mm diameter x 25 mm length
For 20 mm tubes, insert measures 17.7 mm diameter x 25 mm length
 
Alternatively you can use a "test tube adapter" to connect it directly to an "outworld" as you can place the food there, often this is a less stressful way of providing food  as it requires moving the test tube less or disturbing the colony less by not being so close to their chamber.