Objectives and limitations

The available information on the Thysanoptera of California is essentially superficial. Most studies on these insects in this State have focused on either pest control strategies or on descriptive taxonomy, with a few observations on the biology of pest species (see Bailey, 1938). Knowledge of thrips biology, particularly the biology of native species, thus remains remarkably poor. Many publications include lists of "host plants", but such lists are largely records of plant species from which one or more adult has been swept. Such records ignore the fact that thrips are readily distributed by winds and will commonly alight on plant species on which they are unable to breed, or possibly even to feed. The base-line element of biological knowledge—where does a species maintain its populations—is thus not available for most thrips species. Other aspects of thrips biology have been equally ignored in California, such as population structure, seasonality, geographical distribution, and the significance of variable body size to sexual behavior.

Equally important is the weak evidence on which some species are distinguished from each other, usually by museum-based taxonomists. For example, Chirothrips falsus and Chirothrips simplex are regarded here as representing a single species, but are considered to be distinguishable by some taxonomists on the basis of obscure and minute differences. The two names are used for specimens apparently co-existing on various grasses widely across North America, but with no evidence of biological differences in host plants or breeding patterns.

The studies presented here do not constitute a formal "taxonomic revision" of the Californian thrips fauna. Such a product will require extensive field work, recording the breeding host-plants of species, together with preparation onto microscope slides of large numbers of well-cleared specimens. This study is based on the currently available museum collections. These generally comprise specimens that have been poorly mounted onto microscope slides, with little or no attempt at the clearing that is required to reveal the many structural features used in current studies. Moreover, the available host-data is commonly imprecise, with no indication of the host on which breeding occurs, or even if the species is associated with leaves or with flowers.

Our limited objective here has been to draw together much of the available data on the thrips fauna of California in the hope of stimulating and facilitating future studies on these curious insects. We also provide information and identification details about a considerable number of foreign species that we consider likely to be introduced into California in the future. This revised system is thus intended to be of increased value to quarantine entomologists, not only in California but in other parts of the USA and indeed of much of the world.

Illustrations of thrips

Taxonomic descriptions have traditionally been illustrated with line drawings, although for most thrips species no illustrations have ever been published. Partly this was because, until the late 20th century, scientific journals charged high prices to include drawings. Thus most of the papers published in North America by J. D. Hood are un-illustrated, despite the fact that that author prepared many outstanding line drawings (Hoebeke, 1994). Even into the second half of the 20th century some entomological journals in North America still insisted that illustrations should be a limited proportion of the total pagination of a paper. Thus the text of Stannard & Mitri (1962) is curiously repetitive, because the editor of the journal demanded a particular ratio of words to illustrations.

Preparation of high quality line drawings is expensive in artist's time. Thus in recent years, photomicrography has been used increasingly, particularly through the use of images with enhanced depth of field. The image processing software, Automontage, has been used in the production of the images presented here. This software collates pixels that are in focus in a series of optical slices. This can produce outstandingly informative images of slide mounted specimens, but there are two important limitations. Black specimens are very difficult to photograph, unless individuals are specially prepared through gentle bleaching. Similarly, pale but non-macerated specimens are difficult to photograph, because iridescence associated with the body contents obscures surface detail.

Photomicrographs of slide-mounted thrips require specimens to have been fully cleared chemically, using a technique that causes minimal change to the color of the cuticle, as indicated in the section Preparing Thrips for Study. However, many of the species illustrated here are known only from old museum specimens that have never been cleared or bleached. As a result, many of the images presented are less than satisfactory, such as those of pale species of Scirtothrips and Xerothrips, and black species of Bolothrips and Megalothrips.

Three different illumination systems have been used in the preparation of the images. DIC (Nomarski) illumination can produce outstanding images of well-prepared specimens. However, many of the images were taken using either phase-contrast or bright field. Because phase-contrast often introduces color aberrations, some images of pale thrips have been reproduced here in monochrome.