Well, even if it's not a larvae it would be nice to know what it is please

Several of these on my Quince bush, they looked like a dropping at first glance. They are glossy wet, about 4-8 mm long and leave a black stain when touched. From above they are dark brown and during the time I observed them they remained motionless, even when gently nudged.
When you look beneath the outer watery coating it looks to have caterpillar type feet, whatever it is I'd be glad of an ID please.
Thanks in advance
Jasej