How To: Create a linesymbol with the arrow at the midpoint
Instructions provided describe how to create a linesymbol in ArcMap style, which has an arrow in the midpoint of the line. These steps only work for cases where the line is rendered over predefined, one-color backgrounds.
Start ArcMap. Navigate to Tools > Styles > Style Manager. If necessary, it is possible to create a unique style - Styles button > Create New; the new style can be saved anywhere on the disk.
Copy 'Arrow at End' line from ESRI.style to the created style into the 'Line' folder. Right-click on the symbol just copied, and click Properties.
Add a new layer by clicking on the + (plus sign) at the lower left corner of the menu. Make sure that this new layer, Cartographic Line Symbol (plain), is the first layer in the Layers window.
Click on the second (original) line symbol layer, which is now the second layer in the Layers window. Click on the Line Properties tab. Under Line Decorations, the third radio button should be checked. Click on the Properties tab.
In the new menu, under Line Decorations tab, click the Symbol button - write down the Size, the Properties button, the Length and Width of the arrow, and click Cancel twice, to get back to the Line Decorations menu.
Increase Number of Positions to 3 - notice the arrow at the beginning, end, and midpoint of the line.
Click on the + (plus sign) in the lower left corner to add another layer of line decorations. This layer is added on the top of existing layers, so it needs to be grabbed and dragged so that it is below the existing layer. There is nothing in this layer, so it's invisible.
Click on the new, empty layer and increase the Number of Positions to 2 - two arrows at the beginning and the end display. They might be a different size than the arrows on the original layer, so go to Symbol Properties on this layer, and adjust the arrow size (Size, Length and Width) to the numbers previously written down.
While in Properties menu, change the color of these arrows to match the color of the background, which the line symbol will be rendered upon. For example, if the background is white paper, change it to white. The two arrows at the beginning and end of the line are going to mask the black arrows on the original decoration layer.
Click OK twice to close.
Back in the line decoration editor, the second layer is not visible again (unless it's highlighted) because the arrows are now white. However, in the Preview, the linesymbol with a black arrow in the middle should be visible, as well as the white arrows at the beginning and end.
Back in the Symbol Properties Menu, the linesymbol with the arrows as described above is visible, and the line itself is extended through the white arrows.
Rename the line symbol (right-click, Rename). Close the style manager.