Rose Sawfly larvae question (head colouring).

[Miss Caretaker]

Hi, as you can no doubt see, these are the larvae of the rose sawfly (I watched the female depositing the eggs and have watched their lives so far, do I need to get a life? ):

However, I was looking at them and saw that they have different coloured heads. Now, I know they are from the same batch of eggs as I have been watching this particular lot since they hatched (to see how long they survived until eaten by birds etc). Can anyone tell me what the difference in the head colour means? Is it just a slight difference in developmental stage or is it their sex or something else?
Thanks, Jo