Sawfly larvae - Croesus septentrionalis

[goosey]

This I find interesting .
For years I have had Sawfly larvae - Croesus septentrionalis in the garden and at one time or other recorded the egg laying, hatching larvae and the larvae at various stages. But this is the first time I have ever come across a shed skin .
I was pretty pleased and posted the image on Friday and thought no more about it. Then it dawned on me that the larvae which had recently shed its skin has a different colour head. All the other larvae I see had a black head but this one doesn't although the black head bit is very noticable on the skin.

Is this usual when larve go through the instar stages?

1. A larva which has just shed the skin with a green head.
2. Larvae of the same species with black heads.(no clearasil jokes )
(old image from gallery)

1. 2.

[goosey]

Today I saw a female Croesus septentrionalis sawfly searching the garden for the "right place" I suspected she was wanting to lay eggs and kept an eye on her knowing (well I was 99% sure) she would lay her eggs on a willow leaf. I have seen this other years.
Once she settled down I was lucky to be able to get in really close and even twist the leaf out of the shade to get some pictures.
In the end she laid around 80-100 eggs by the look of it.