Ulidiidae, the "picture-winged flies,'' are a family of true flies belonging to the superfamily Tephritoidea. Over 130 of the family's approximately 700 species, and 40 of its 128 genera, have been documented north of Mexico in the United States and Canada. Although the family is primarily saprophagous and commonly found in association with feces or decaying plant tissue, several species are known to be pestiferous on crops significant to agribusiness, primarily in the American southeast. Here the taxonomic key provided in the "Otitidae" chapter of the Manual of Nearctic Diptera (Steyskal 1987) is updated with color photographs, including notes on taxonomy and distribution of species. Specimens from collections at North Carolina State University, the Florida State Collection of Arthropods, and the National Museum of Natural history were photographed using a Canon DSLR and macro lens, and focus stacked using Zerene Stacker. Information on distribution of species was taken from collection records, online databases, and previous publications.

Steyskal, G.C. 1987. Otitidae. In Manual of Nearctic Diptera. Volume 2. Edited by J.F. McAlpine. Agriculture Canada, Hull, Quebec, Canada. Pp: 799-808.

Tritoxa cuneata species group (photo by C. Wallace)

The "picture-winged flies" (Ulidiidae) are one of the larger families of the Diptera superfamily Tephritoidea, numbering approximately 800 species total, about 135 of which are found in the United States or Canada (Steyskal 1987, Diaz-Fleischer et al. 2000) (some uncertainty remains regarding exact number, due to limited research, taxonomic revisions, and high number of Euxesta Loew). They are a family of acalyptrate flies, generally small to medium, and broadly recognizable by the distinctively patterned wings that grant the family its common name, though this trait is neither universal among nor exclusive to them. Traits common to the family are illustrated in Figure 1.

Currently, Ulidiidae is divided into two subfamilies: Ulidiinae and Otitinae, each with three tribes (and an additional incertae sedis group of genera in the latter) (Kameneva and Korneyev 2006). These subfamilies are unequally distributed geographically; Otitinae is primarily Holarctic, and represents the slight majority of species documented north of Mexico, whereas Ulidiinae is primarily concentrated in the Neotropical region of Central and South America (Arnett 2000, Kameneva and Korneyev 2006, Galinskaya et al. 2014).

Unlike its close relative, Tephritidae, the ulidiids are primarily a saprophagous group (Ferrar 1987; Arnett 2000). The greater number of picture winged flies can be found in association with feces or, more typically, rotting vegetables and fruits, where they commonly oviposit. Some publications (e.g. Arnett 2000) present the family as exclusively associated with plants, but, although there is a clear preference throughout the family for plant tissue, dead or alive, it is not exclusive (Ferrar 1987, Sivinski 2000, Marshall 2012). There is, however, a repeated trend within the Ulidiidae of true and/or opportunistic phytophagy, in which larvae act as primary, secondary, or tertiary invaders on living plant tissue (Kameneva and Korneyev 2010; Goyal et al. 2012). This is particularly well documented in those species that act as pests on plants such as sugar beet, onions, or corn (Ferrar 1987; Bjerke et al. 1992; Goyal et al. 2010, 2011, 2012).

Like the Tephritidae, adult ulidiids commonly use their distinctive wings to engage in often complex mating rituals. Male members of Callopistromyia Hendel raise their dappled wings at a 90 degree angle over their abdomen and "strut," while others such as Delphinia picta Fabricius make a "rowing" motion while walking over leaves (Sivinski 2000, Marshall 2012). Even more "extreme" examples of novel reproductive strategies can be found in species outside of a Nearctic distribution, as in the stalk-eyed Plagiocephalus Wiedemann, or the post-copulatory ejaculate expulsion in females of Euxesta bilimeki Hendel (Brunel and Rull 2010, Luis Rodriguez-Enriquez et al. 2013).

Unfortunately, the biology of many ulidiids, even in the relatively well-documented North American taxa, remains unknown or understudied. One potential barrier is the near absence of taxonomic tools. There is so far no comprehensive illustrated guide to the taxa of ulidiids of any biogeographical region, and many described species and genera remain to be photographed or otherwise imaged, or else are unavailable to a broad audience. This update to the key will fill some of this void by providing new images of many more taxa than have previously been available.

Specimens and data records were accessed from insect collections at three institutions: the Insect Museum in the Department of Entomology and Plant Pathology at North Carolina State University (NCSU), the Florida State Collection of Arthropods (FSCA) at the University of Florida at Gainesville, and the Diptera Collection in the U.S. National Entomological Collection (USNM) at the National Museum of Natural History (NMNH) of the Smithsonian Institution in Washington, DC.

Location data for taxa represented in the United States and Canada was based primarily on Steyskal's (1987) descriptions and supplemented with documentation of occurrences in published literature (Arnett 2000, Marshall 2012), the Catalog of Life (Roskov et al. 2017), the website Systema Dipterorum (Pape and Thompson 2017), the Global Biodiversity Information Facility (GBIF), and collection information from the three institutions listed above. Based on available information, none of the represented species are definitively known from Alaska.

In total, Ulidiidae is represented in the United States and Canada by 40 genera and 132 species. Members of all recognized subfamilial, suprageneric categories (Otitinae: Cephaliini, Myennidini, and Otitini; Ulidiinae: Lipsanini, Pterocallini, Seiopterini, Ulidiini), as of the most recent comprehensive taxonomic consideration of the family (Kameneva and Korneyev 2006), are represented by at least one genus, with a bias towards the primarily Holarctic subfamily Otitinae. A list of genera and species with a documentation in North America north of Mexico is available in Table 1, which also specifies the source (i.e. NCSU) of the specimens photographed for this paper.

Additional Notes

In Arnett (2000), Pseudotephritina Malloch is treated as a sub-genus of Pseudotephritis Johnson. Herein, however, I agree with Steyskal (1987) and Kameneva and Korneyev (2006), in treating it as a genus.

One additional genus, Prionella Robineau-Desvoidy, is indicated as appearing in the United States in Arnett (2000), and on Systema Dipterorum (diptera.org). However, it is a genus of unrecognizable identity and is therefore excluded.

The terminology used follows the standard set in the "Adult Morphology and Terminology" chapter of the Manual of Afrotropical Diptera (Cumming and Wood 2017). "Frontal index" refers to the ratio of frons height to frons width at base. Figures 2-7 provide pictures of ulidiids from the key with relevant morphological traits labeled.

The bulk of the text in the original dichotomous key is preserved from Steyskal (1987). Where relevant, terms are updated to comply with the terminology from the Manual of Afrotropical Diptera, as in the replacement of "bristle" with "seta." Additional small changes to the original text were made to clarify, correct, or remove redundant and potentially confusing traits. For example, in couplet #30, 30' stated "tibiae usually distinctly banded," but this is contradicted by couplet 32, which clarifies that neither Pseudotephritina Malloch nor Xanthacrona Wulp possess distinctly banded tibiae.

Specimen Imaging

With the exception of Hiatus fulvipes Cresson (represented by non-photographic illustrations) and Texasa chaetifrons Steyskal (type: United States Natural Museum, image: Dr. Allen Norrbom), all images in this key were taken by the author. Each final image was produced through focus stacking. Initial photographs were taken using a Canon EOS 6D camera with either the Canon MP-E 65mm 1-5x macro lens or the Laowa 60mm f/2.8 2X Ultra-Macro (used for some of the specimens from the FSCA), at variable magnification and aperture depending on specimen, mounted on a Cognisys StackShot standard macro rail. Images were stacked using the Zerene Stacker system. Additional images of Texasa chaetifrons, Physiphora clausa Macquart, and Tritoxa flexa Wiedemann were taken with Canon EOS 5DS and MP-E 65mm lens. Edits, to remove blemishes and to correct for color and clarity, were performed using Adobe Photoshop CS6. The specimen identified as Tritoxa cuneata Loew was photographed before the publication by Sinclair et al. (2021) describing two new species in the T. cuneata species complex; locality information and male genitalia are unavailable to determine whether it is actually T. californica Sinclair et al. or T. decipiens Sinclair et al. rather than T. cuneata. One species, Hiatus fulvipes, is known exclusively by several type specimens and was unavailable for photography; it is represented with drawn illustrations. These illustrations were completed by an artist using Plate VI of Cresson (1906) as a reference.

For each trait used diagnostically in the key, an image was attempted; however, several traits were not visible from the available specimens due to the position in which they were pinned.

Table 1. List of all contemporary species of Ulidiidae with a Nearctic distribution. Additional columns indicate species presence in Canada or the United States, as well as origin of specimen photographed ("---" indicates absence of photograph). Abbreviations for repositories: NCSU - Insect Museum in the Department of Entomology and Plant Pathology at North Carolina State University; FSCA - the Florida State Collection of Arthropods at the University of Florida at Gainesville; USNM - Diptera Collection in the U.S. National Entomological Collection at the National Museum of Natural History of the Smithsonian Institution in Washington, DC.