Feeling Grubby

Changing the plants in the windowbox, Mrs ph found one of these, screamed, ran and demanded I investigate further. I found lots more! I have no idea what they will turn into but they are big. The pot they're in is a 2litre measuring jug and some of these are almost as big as my thumb! Any suggestions what they might be?

(Sorry Sollergirl, there was no way Mrs ph would let me keep these over the weekend to bring in for your lab)