[rogertb]

Looks like some kind of 'droppings' ?

[sunshineband]

Caterpillar frass. (droppings)

If you search directly above where it has fallen you should find the culprit

[TheWhiteRabbit]

Yup, definitely caterpillar poo. Pulled 18 of the critters off one cauli the other day...couldn't see them at first but kept looking and there they were.

[Nobbie]

Looks like it's in a greenhouse and so I suspect it is caterpillars on your tomatoes. Check the leaves as you may well have an infestation. Easy to deal with, just shake the plants each day and pick the caterpillars off the ground and dispose of. As it's late in the season, I'd also remove a lot of the lower leaves. If you leave them, they will start nibbling the fruit as well.

[Aunt Sally]

It looks like a cucumber rather than a tomato, Nobbie

[Nobbie]

It looks like a cucumber rather than a tomato, Nobbie

I would draw your attention to the dark leaf on the left of the picture with a nibble mark on the side

[Aunt Sally]

Ah... yes, you're right!

Tomatoes grown beside cucumbers.

[Headgardener22]

Tomato Moth caterpillar. Google it, you'll see what you're looking for. Find them because they'll eat into the tomatoes (and peppers if there are any).

[rogertb]

Yes it's a cucumber leaf entwined with the toms and yes they've been nibbling the toms ... I'll get in there and have a shake !