There are many species of Aphid, all regarded as pests by gardeners! They are mostly named after the plant they feed on, these look like Knapweed Aphids, although they are on a thistle, so are more likely to be Thistle Aphids (Uroleucon cirsii). Adult Thistle Aphids are 4-5mm long, which would be the size of the one at the base of the second photo - the others are young ones. Judging by the proximity of the ladybird, they won't last long...