Common Blue (Polyommatus icarus) Butterfly and Clouded Yellow (Colias croceus)

The Common Blue (Polyommatus icarus) is a small butterfly in the family Lycaenidae, widespread over much of the Palaearctic. Their uppersides and undersides have different patterns: male uppersides are irridescent blue, while females are rather brownish. Undersides are greyish colour in the males and more brownish in the females.

Common Blue (Polyommatus icarus) male. (by-nc-nd)

Common Blue (Polyommatus icarus) female. (by-nc-nd)

Common Blue (Polyommatus icarus) male on a yellow flower. (by-nc-nd)

Colias croceus is a small butterfly of the Pieridae family, that is, the Yellows and Whites. In most places it is simply known as "the clouded yellow".
The Common Clouded Yellow's breeding range is North Africa, southern Europe, and eastwards through Turkey into the Middle East, but it's common throughout much of Europe as a summer migrant, in good years individuals reaching even Scandinavia.

Common Clouded Yellow (Colias croceus) butterfly. (by-nc-nd)

Obviously this isn't a butterfly, but the colours match the colour palette of the other images — all the insects sit on yellow flowers.

Hoverfly (by-nc-nd)