Introduction

Overview

            Tenebrionidae is a large and diverse beetle family. Members are represented from all major biogeographical regions of the world and from widely varied habitats. At present there are approximately 19,000 species described worldwide in more than 2000 genera. The family is the largest in Tenebrionoidea, and is currently fifth in number of beetle species overall. Most of the approximately 200 genera and well over 1,000 North American species inhabit arid regions of the Mountain and Western states. Approximately 225 species are recognized east of the Mississippi River, with undoubtedly more to be discovered and described. In recent years, to more clearly represent our hypothesis of phylogenetic relationships, the family has grown by the inclusion of traditionally segregated families such as the Alleculidae and Lagriidae in North America . Several genera have been excluded, including Boros Herbst (to Boridae) and Phellopsis LeConte (to Zopheridae) (Crowson 1955).

            Tenebrionidae has in the past proved difficult to partition into meaningful supraspecific classifications. The subfamilial, tribal and generic phylogenetic arrangement of the group adopted herein is based primarily on the work of Aalbu et al. (2002), and is a gross approximation of what is currently hypothesized to represent monophyletic clades. These recent assessments of tenebrionid classification utilize characters of both adults and larvae, and have clarified some of the taxonomic arrangements of the group proposed by earlier workers looking exclusively at adults (Lacordaire 1859, LeConte and Horn 1883, Arnett 1963).  

            Adult tenebrionids are highly variable in form and size, from broadly ovate to elongate, strongly convex to flattened, with vestiture variable, and ranging in length from 1-60 mm worldwide (more often 2.5-20 mm in North America north of Mexico). They are often referred to as darkling beetles because of their usually black or brown coloration; they are primarily nocturnally active, substrate dwellers. Diagnostic characters for adults include emarginate eyes, definite epistomal margin, heteromerous tarsi (5-5-4), closed prothoracic coxal cavities, concealed antennal insertions, usually stout, moniliform or incrassate antennae (occasionally serrate or clubbed), and paired abdominal defensive glandular reservoirs.

Life History

            Tenebrionid larvae, often referred to as “mealworms” or “false wireworms,” are usually cylindrical to slightly flattened, occasionally short and broad, fusiform, or strongly flattened. The head and all visible tergites or only the head and abdominal apex are heavily sclerotized. Diagnostic characters for larvae include the presence of a frontoclypeal suture, flat and dome-like antennal sensorium, simple malar apex which is not cleft, simple ninth sternum, annular or annular- multiforous spiracles, and the absence of an endocarina, mandibular prostheca, hypostomal rods, ventral prolegs, and patches or rows of tergal asperites. Böving and Craighead (1931) and Lawrence and Spilman (1987) treated larvae generally for the entire family, including illustrations of select species. Steiner (1995) presented descriptions and illustrations of selected tenebrionid pupae.    

            Tenebrionid larvae and adults are primarily saprophagous or phytophagous; some are specialized fungus feeders; a few may be facultative predators (Corticeus spp.). They are readily found in a wide variety of dead plant and animal matter, including but not limited to humus, leaf litter, rotten wood, carrion and dung. The availability of plant and animal matter, soil types, and shrub cover are often conducive to microhabitat conditions that dictate the size of members of one species as well as the overall spatial heterogeneity within and between species of some adult tenebrionids (Doyen and Tschinkel 1973, Doyen and Slobodchikoff 1984, Stapp 1997). Among tenebrionids that are associated with trees, different areas are exploited as larvae and adults. Those associated with dead, rotten wood often softened and chemically altered by the action of various fungi include: Penetini, Tenebrionini, Amarygmini, Ulomini, Strongyliini, Coelometopini and some Diaperini, Alleculini, and Helopini. Those found associated with trees include: Coelometopini, Centronopini, Ulomini, Tenebrionini and some Diaperini, Alleculini, and Helopini (Aalbu et al. 2002). Some Diaperini are found in fermenting tree wounds. Large proportions of tenebrionids are also associated with fruiting bodies of Basidiomycetes (polypores). These include the Bolitophagini, some Diaperini and Alleculinae (some Bolitophagini being restricted to certain species of polypores). Surface grazers on lichens, algae and mosses growing on bark and rock surfaces include some Helopini, Amarygmini, and Alleculinae. Those found on leaves and flowers include some Alleculinae, Lagriinae, and Epitragini (Aalbu et al. 2002).

Epitragodes tomentosus (Epitragini) on Sycamore tree branch in Alachua Co., Florida

            Many tenebrionids are found on or in the soil, both as adults and larvae. These usually take shelter in leaf litter under rocks or logs. These include most Pimeliinae, Opatrinae, some Goniaderini, Lagriinae, Blaptini, Eleodini, and some Alleculinae. Many Tenebrionidae have adapted to life on sand dunes, both on the surface and in the sand. These include many Pimeliinae (almost all tribes have sand adapted species) and Opatrinae and a number of Eleodini and Blaptini. Many tenebrionids adapted for life in very hot, arid regions have sealed subelytral cavities, which reduces transpiratory water loss. The Phaleriini are found on sand at beaches. A few species have also become adapted to life in caves. These include members of the Eleodini, Tenebrionini, and Hypophloeini (Aalbu et al. 2002).

             Some species are associated with nests of vertebrates or insects. These include mammals, especially rodents [Eleodini, many Pimeliinae and Opatrinae, some Alleculinae], birds [some Alphitobiini, Tenebrionini, Diaperini, and Alleculinae], bees [Triboliini], ants [a few Triboliini, many Pimeliinae and Opatrinae, some Alleculinae] and termites [many Amargymini, some Pimeliinae, Opatrinae and Alleculinae]. Predaceous tenebrionids exist mainly as larvae. These are found in bark beetle galleries [Hypophloeini], in nests of birds [Alphitobiini] or in palm fronds where they prey on other beetle larvae [Triboliini] (Triplehorn 1990, Aalbu et al. 2002). Some species in these same groups have evolved synanthropically with humans and are considered pests of stored grain or flour products in temperate climates. Tenebrio spp . and Tribolium spp . are secondary pests in granaries, while a few species of Eleodes Eschscholtz and Blapstinus Sturm are pests of cultivated plants in western North America (Campbell 1924, Sloderbeck 1995).

Tenebrio (mealworm) larva - length ~29mm.

            Kendall (1968, 1974) conducted studies on the diversity of form and function of the paired glandular reservoirs and their association with adult tenebrionid chemical defense systems. The taxonomic significance of these chemicals has also been reviewed (Brown et al 1992). Tenebrionids are able to secrete a wide array of chemical compounds, most commonly 1-alkenes and quinones (Tschinkel 1975). Many of these chemicals are secreted when adult tenebrionids are disturbed and have taken a defensive posture by raising the abdomen, often resulting in pungent, vile smelling aromas. In some parts of the United States , darkling beetles are often referred to as ‘stink bugs'. Müllerian mimicry, by the way of body silhouette, has been reported among certain genera that do exhibit defensive posturing and defense compounds to afford more comprehensive protection against predators (Doyen and Somerby 1974).

            Morphological, physiological, and behavioral adaptations to the surrounding environment have enabled members of this family to occupy a diverse array of niches. Sand walking and sand swimming, subelytral cavities and water catchment devices allow certain members of this family to dominate arid, or arid sandy environments. Fog basking and fog catchment trenches are two unique ways in which some desert inhabiting tenebrionids receive moisture (Hamilton and Seely 1976, Seely and Hamilton 1976). The ability of stored product as well as desert inhabiting tenebrionids to live in areas of low humidity is also attributed to exceptional powers of water economy through efficient cryptonephridial systems (Crowson 1981). Adaptations to mycophagous lifestyles also allow this family to occupy more humid conditions, utilizing mainly fungal growth as a food source (Triplehorn 1965).

Collecting Tenebrionidae

            Field work typically yields no single best way to attract or sample adult tenebrionids . They typically turn up in unlikely places and can be collected by using a variety of methods. Flight-mobile tenebrionids are taken in a number of different traps, including flight intercept, blacklight , mercury vapor, pitfall, sticky boards, beat sheets, Lingren funnel and Malaise traps. No trapping method, baited or unbaited , is typically more successful than another. Sweeping, especially where there are flowers, can yield members of the Alleculinae and a few other groups.

          Less vagile tenebrionids are best sampled at night by surface gleaning various substrates with headlamps. In the right habitat, several species can be observed on a single dead tree (esp. oak) at the right time. Similar observations by similar methods can be made on trees with fungus growth in mesic forest communities. Daytime sampling requires going through various substrates such as decaying plant, including tree, material and other debris. Careful inspection of various organic matter will often yield tenebrionid larvae.

Tenebrionid habitat at O’Leno State Park, FL	Coelometopine sp. under bark
Forked-fungus beetle, Bolitotherus cornutus (Panzer)	Typical Bolitotherus fungus habitat

Florida Tenebrionidae

          There are approximately 170 species of tenebrionids known in Florida, with additional species currently being described (Steiner in litt.) and exotic species reaching the state on occasion (Steiner et al. in prep). Compared to northern regions of the eastern United States, Florida apparently has greater tenebrionid diversity. Subtropical and tropical climatalogical influences probably increase species totals, while ample sandy environments provide suitable habitat for xeric adapted tenebrionids. In comparison, more northerly states such as Wisconsin and Ohio list approximately 95 and 85 species respectively. While the bulk of North American tenebrionid diversity remains in the arid west, many derived tenebrionid groups are represented in the mesic forests of Florida and eastern North America.

           Florida's unique geographical location and history had led to a high degree of precinctive (= "endemic" throughout this work) plant and animal species. The panhandle often represents the southern most distribution points of northern organisms and the Florida Keys the northern most point for West Indies and South American flora and fauna. Several early workers compiled lists of the beetle fauna present in Florida (Schwarz 1878, LeConte 1878), but none perhaps were as complete as Blatchley (1938), which listed nearly 2,000 species (Peck and Thomas 1998). We currently recognize well over 4,500 beetle species in the state; of that figure, approximately 12% are considered precinctive and 4.7% have been accidentally or intentionally introduced (Peck and Thomas 1998). The list of Florida darkling beetles currently contains 11 species considered to be known only in Florida , while six species appear to be fairly recently introduced. Florida also has approximately 14 cosmopolitan tenebrionids known to be stored products pests.

           While the scope of these checklists is based primarily on political boundaries, it should serve as an good indication of which species are present in Florida and the eastern United States and those that have ranges skewed either to the north or south on a more biologically natural scale. Many species have distributions that extend west beyond the Mississippi River, north into Canada and south into the Caribbean, Mexico, Central and South America. These lists and identification material should also be a means to readily identify pest species in various anthropogenic habitats. Tenebrionids represent a significant component of our regional entomofauna. Given that, insects are often overlooked in conservation practices. How do we begin to conserve when we barely know the true diversity of insects present in a given ecosystem? It is hoped that this work will provide some insight to that question, although a significant amount of distributional and life history information remains unknown. Perhaps others will use this work as a catalyst to contribute additional knowledge to the true diversity of tenebrionids, Coleoptera, and insects in general.

University of Florida Entomology and Entomology Dept. | Florida State Collection of Arthropods

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