For all its isolation, St Kilda has had a fairly balanced scientific programme investigating its various invertebrate groups, making it one of the best known fauna of the Western Isles. The range of taxa that have been studied in detail is impressive, for example 37 parasitic ichneumons have been recorded in the entire Western Isles, of which 10 are known only from St Kilda.

Nearly 200 Diptera have been recorded on St Kilda, so flies are probably important pollinators of flowers on the islands. One of St Kilda’s dance flies, Rhamphomysis morio, is normally found at high altitudes on the mainland and the Western Isles. While the persistent strong winds might have been a crucial factors in the colonisation of the archipelago, the most successful insect colonists tend to be those that keep close to the ground. There are, for example nearly 150 species of beetles on the island list, including a Red Data Book weevil Ceutorhynchus insularis found nowhere else in Britain (and otherwise only at a few localities in Iceland). The paucity of available habitat types restricts the variety of those potential colonists that might have the capability of reaching such an isolated island group as St Kilda.

Although at least 367 Lepidoptera species are to be found in the Western Isles, representing about 14% of those on the British list, fewer than 25% of these have ever reached St Kilda. They include about 40 species of larger moths, the antler moth and dark arches moth being amongst the commonest. The least carpet moth is of note since it is otherwise known only from the south of England. There are a similar number of micro moths listed but this group is probably under-recorded. Only seven butterfly species have been recorded to date, all of them migrants, but only red admirals and painted ladies could be said to be almost annual in their occurrence.