The Outer Hebrides, because of their apparent isolation, attracted zoologists in the past who looked for new species and subspecies and some sought to explain the origin of the invertebrate fauna by exotic land connections in preglacial times. These expectations were largely unfulfilled and the speculations are unsupported by geologists or geomorphologists. The non-marine invertebrate fauna is examined from published records, museum collections and special field surveys and the lists total over 2500 species, of which approximately one-third are of general occurrence in the Outer Hebrides. Three species of mites are new to the British fauna and over 700 species, mainly insects, are additions to the Hebridean fauna. The fauna is impoverished compared with the mainland and this may be due to climate and the lack of variety of habitats and the low structure of the vegetative cover. Upland species occur on the moors and some descend to sea level, possibly on account of the low mean temperature in summer. Species with a north western distribution occur in the Odonata, Hemiptera, Lepidoptera, Hymenoptera and Diptera. Species with a discontinuous distribution are discussed and it is suggested that some reached the islands by natural means and others by human agency and there is no evidence of refugia for a glacial relict fauna. Examples are given of insects with a south western distribution arriving by sea in driftwood and five amphiatlantic species, including a freshwater sponge, which may have been brought by migrating birds, a wrack-fly, two strand-line beetles and a mite, which may have been distributed by the Gulf Stream and the North Atlantic Drift.
There are no endemic species, but a few so-called subspecies of insects may be restricted to the Outer Hebrides. Changes in the fauna are traceable in a stratified Flandrian shell deposit in Harris, where two species have become extinct, and some insects have also become extinct in St Kilda during the last century. Some of the most conspicuous land snails have arrived in historic times and the most rapid coloniser has been a brackish-water snail which has spread throughout the islands since 1933.