I noted a very large number of cabbage white (the yellow and black ones) caterpillars on the side of my house recently - the house bounds a field currently growing cauliflower. The caterpillars look fat and healthy, I'm not sure why they crossed from the field over my garden to the house - maybe to find somewhere to pupate?
The weird thing is that many of these caterpillars appear to be attached to, or sitting on, sacks of small yellow things which look very much like eggs. This is rather confusing - these are not new-born caterpillars next to eggs, they've clearly been eating in the field and come to the house later. I don't think butterflies would lay their eggs on the house rather than the plants, and even if they did why would the caterpillars return to the same place?
It looks like these healthy caterpillars are creating the sacs of yellow 'eggs' but that does not fit at all with the life-cycle of butterflies as I know it.
Is there any chance caterpillars can lay eggs in some cases and I never knew this? Or must something else be happening?