The fauna discussed here includes some remarkable features, both biogeographical and ecological. Two of the smallest Thysanoptera families worldwide, the Stenurothripidae (=Adiheterothripidae) and the Fauriellidae, are both represented in California. The first of these derives its name from fossil species, and apart from the two genera in California includes only Holarthrothrips with four species found between the Mediterranean and India in the flowers of Phoenix (date palm) trees. The Fauriellidae, with one genus in California, is known otherwise only from two species in the Mediterranean area and two in southeast Africa. Similarly, the genus Orothrips in the Aeolothripidae is known otherwise only from the Mediterranean. Currently there is no explanation for these discontinuous distributions between western North America and the European Mediterranean.
In contrast to these distant relationships, the Californian thrips fauna is noticeably different from, and less diverse than, the thrips fauna of the eastern USA. In particular, the large suite of leaf-litter species related to the genus Eurythrips that is so abundant in the eastern States (Mound, 1976, 1977) is not known to be represented in California. Moreover, in Florida (Diffie et al., 2008), possibly as a result of the more humid climate, the thrips fauna includes a much larger proportion of Neotropical species that are presumably windborne northwards from the Caribbean. In California, there appears to be a considerable faunal discontinuity southwards, with increasing numbers of large fungus-feeding thrips being found in Mexico. Moreover, the very large and worldwide genus Thrips shows a startling discontinuity with the Neotropics, such that only one native species of this genus is known from south of the border between the USA and Mexico.
The genus Ankothrips (Melanthripidae), also Dactuliothrips (Aeolothripidae), include an interestingly large number of spectacular species, but as yet there is no ecological explanation for their diversity in California, nor many observations on their behavior and biology. More problematic is the presence of over 20 species of the genus Aeolothrips (Aeolothripidae). The members of this genus are commonly regarded as facultative predators, and thus might be expected to exhibit limited host specificity. Judging from the available descriptions, particularly of males, the diversity of Aeolothrips species in California is probably real, and thus constitutes an interesting biological phenomenon. In contrast, the presence in California of eleven species of the genus Leptothrips (Phlaeothripidae) requires further substantiation, both morphological and biological. Species in this genus are considered to be predatory on mites or other small insects, and at least one species is found on a wide range of native and introduced plants. The apparent diversity within Leptothrips is possibly not real, but a reflection of inadequate morphological taxonomy, and genetic studies could help to confirm or refute this suggestion. Two other genera are also species rich in California, Thrips (Thripidae) and Liothrips (Phlaeothripidae). Both of these are known to have species with a high level of host-specificity, and thus both genera may yet prove to be even more diverse than is currently recognized.
It should be emphasized how little is known of the biology of the endemic Thysanoptera fauna of California. Moreover, recent collecting has suggested that more species live in this area than are currently recorded.