More than 1,000 insect species have been identified on Ruhnu. As some insect groups have not been explored very thoroughly, more species are expected to be found. The insect fauna here is constantly changing because of immigration and fading of species. The most common insects on Ruhnu are beetles (539 species) and Lepidoptera (369 species). Out of the 42 families of beetles on Ruhnu, rove beetles with 117 species are the most common ones, followed by ground beetles with 85 species and snout beetles (Curculionidae) with 51 species. There are several species of beetles and large Lepidoptera who cannot be found anywhere else in Estonia or who have been first discovered on Ruhnu, of beetles, for example, Stenagostus rufus, Lacobius decorus, Ampedus erythrogonus and Stenus palposus, of Lepidoptera, for example, the Lesser Treble-bar (Aplocera efformata) and Epilecta linogrisea. Ruhnu was the first of the western islands where the Large Copper (Lycaena dispar; III conservation class) was found.

Of Hymenopetra on Ruhnu, 84 species of Aculeata and 22 species of ants have been found; of those, all bumblebees as well as Formica truncorum, Formica polyctena and Formica rufa are protected species. There are 15 species of Odonata, of those the Migrant Hawker (Aeshna mixta) and the Common Darter (Sympetrum striolatum) are not very common on the mainland and the Green Hawker is a protected species (III conservation class). 10 species of Orthoptera have been found, of those the European Mole Cricket was last seen in the 1930s. 27 species of Heteroptera, 10 species of caddiesflies (8 of them Limnephilidae) are protected. Scientists who explored Ruhnu before 1940 have also described many species of fleas.

Created in 2014