There two shall I bee

This is a part two of my previous blog post. In that post I had wondered whether there was a preferred size for the nibbles taken from a Cercis occidentalis tree in my yard by some leafcutter bees. Consequently, my son and I went into the garden and measured the opening at the outside edge of leaf cuts on 17 different leaves of my tree. Our measurement of the opening at the outer edge of the leaf serves as a proxy for the total size of the piece cut from the leaf. The Cercis has many unmolested leaves, so I don't begrudge the bees their share.

Here's the distribution of measurements:


Factoring in the dim light and 4th grade skill factors, I think that each measurement we made had a plus or minus 1 mm accuracy.

As I surmised previously, there appears to be a distinct small, medium, and large size preferences centered at about 8, 14.5, and 20 mm, respectively. As the pieces get larger, the width of the distribution about the local mean decreases. What can explain this? Perhaps, if small pieces are used for larval food and the larger for structural parts of the nest then this makes sense: The holes I've drilled for nests are of a uniform diameter and might require specific sizes of cut leaf to line appropriately, whereas food can come in many tasty, bite-sized, pieces.

There are also a few giant pieces that seemed not to be the result of two adjacent, smaller, cuts. We made an effort not to record what appeared to be adjacent cuts that had merged into one, but the remaining few giant cuts didn't have any features that indicated they were the result of merged smaller cuts.