Someone's been building a little home for their babies out of the spinach leaves.
I picked some spinach before to make pie later and among the little beasites I rescued from the rinsing bowl were a brown lacewing (one of my favourite little beneficial beasties), a cute little yellow spider (he got put back out with the aphids) and these leaves that show a leaf-cutter bee (Megachile spp) has been workig hard to make a nest for the next lot of youngsters. Native bees are quite happy to hang out in your garden if you provide the right habitat for them. Grow blue, violet or purple flowers for them as these can be more easily seen by insects, and provide homes by drilling holes (about 4-6 mm) in wood for them to nest in. Free pollination and increased biodiversity are the result.
Leafcutter bees at work.