Tragidion confusion

Back in October, I discussed a recent review of the cerambycid genus Tragidion, authored by Ian Swift and Ann Ray and published in the online journal Zootaxa.  These gorgeous beetles mimic the so-called “tarantula hawks” (a group of large, predatory wasps in the family Pompilidae) and have been difficult to identify due to poorly-defined species limits, wide range of geographic variation, unusually high sexual dimorphism, and apparent potential for hybridization in areas of geographic overlap. Swift and Ray (2008) recognized seven North American and four Mexican species, including two newly described species and another raised from synonymy. It was an excellent work that provided much needed clarity based on examination of types and included detailed descriptions and dorsal habitus photographs of all species and separate keys to males and females to facilitate their identification. Unfortunately, my summary caused some confusion regarding species that occur in the deserts of southern Arizona, southern New Mexico and western Texas. In this post, I’ll clarify that confusion and provide details for distinguishing these species.

Formerly, it was thought that two species of Tragidion inhabited this region, with populations exhibiting smooth elytra and breeding in dead stalks of Agave and Yucca (Agavaceae) representing T. armatum and those exhibiting ribbed elytra and breeding in dead branches of a variety of woody plants representing T. annulatum. This concept dates back to the landmark monograph of the Cerambycidae of North America by Linsley (1962). Swift and Ray (2008) noted that Linsley’s concept of T. annulatum was based on an erroneously labeled type specimen, and that true T. annulatum referred to populations in California and Baja California (for which other names – now suppressed – were being used). This left the AZ/NM/TX populations attributed to T. annulatum without a name. The previously suppressed name T. densiventre was found to refer to populations inhabiting lowland habitats and breeding in Prosopis and Acacia (Fabaceae), but those occurring in montane habitats and breeding in Quercus (Fagaceae) represented an as yet undescribed species, for which the name T. deceptum was given. I included Swift and Ray’s figure of T. deceptum in my post – but mistakenly included the male of T. densiventre alongside the female of T. deceptum!

This error may never had been noticed, had it not been for the discriminating eyes of BugGuide contributor, Margarethe Brummermann. Margarethe is currently collecting photographs for a field guide to Arizona beetles and had photographed a male and female of a “ribbed” species in Montosa Canyon. Using the illustration of T. deceptum” in my post, Margarethe concluded her specimens represented T. deceptum and asked me to confirm her ID. When I told her the specimens represented T. densiventre, her confusion was understandable (given that her male appeared identical to the T. deceptum” male in my post). Further query on her part prompted me to do a little digging, and I discovered my error. The figure in my post has since been corrected – both that figure and a figure from Swift and Ray (2008) showing the male and female of T. densiventre are included below, along with additional information to allow their identification.


Tragidion densiventre Casey, 1912

Tragidion densiventre was formerly synonymized under T. auripenne (a rare species known from the four corners region of northern Arizona, southern Utah, southwestern Colorado, and northwestern New Mexico). Males of T. densiventre can be distinguished by their longer antennae, tawny-tan elytra and distinctly red-brown head, legs, and scape, while females have shorter antennae and the elytra red-orange. Both males and females of this species are distinguished from T. deceptum by their five elytral costae that curve inward toward the suture and extend to near the elytral apices, as well as their relatively narrower basal black band. Females of this species may be further distinguished from T. deceptum by their all black (or nearly so) antennae. Tragidion densiventre is found predominantly in xeric lowland desert habitats in Arizona, New Mexico, and west Texas (as well as northern Mexico). Larvae have been recorded developing in dead Prosopis glandulosa and Acacia greggii, and adults have been observed aggregating on sap oozing from stems of Baccharis sarothroides (Asteraceae) and flowers of larval host plants. Although the biology of this species has not been described in detail, it is likely that the observations of Cope (1984) for T. auripenne refer to this species. This is the classic T.annulatum” commonly observed in the desert southwest.

Tragidion deceptum

Tragidion deceptum Swift & Ray, 2008

Tragidion deceptum superficially resembles T. densiventre due to its ribbed elytra; however, it is actually more closely related to the Mexican species T. carinatum. Like T. densiventre, the males exhibit longer antennae and tawny-tan elytra, while females have shorter antennae and orange-red elytra. However, the head, legs and scape of males are black, as in females of the species, rather than red-brown as in males of T. densiventre. Females exhibit distinctly annulated antennae, in contrast to the all black antennae of T. densiventre. Both males and females are distinguished from T. densiventre by the elytral costae – only four rather than five, not incurved towards the suture and extending only to the apical one-third of the elytra. In addition, the basal black band is very broad – exceeding the scutellum by 2 × its length. This species is similarly distributed across the desert southwest as T. densiventre but occurs in more montane habitats, where it breeds in recently dead branches of several species of Quercus. Like T. densiventre, adults are often found feeding and aggregating on Baccharis sarothroides, and in a few lower canyons bordering desert habitats in the Huachuca Mountains of southeastern Arizona this species and T. densiventre have been collected feeding alongside each other on the same Baccharis plants. Tragidion deceptum is one of several species in the genus (along with T. coquus in eastern North America) that have been collected using fermenting molasses traps (more on this in a future post).


Cope, J. 1986. Notes on the ecology of western Cerambycidae. The Coleopterists Bulletin, 38:27–36.

Linsley, E. G. 1962. The Cerambycidae of North America. Part III. Taxonomy and classification of the subfamily Cerambycinae, tribes Opsimini through Megaderini. University of California Publicatons in Entomology, 20:1-188, 56 figs.

Swift, I. and A. M. Ray. 2008. A review of the genus Tragidion Audinet-Serville, 1834 (Coleoptera: Cerambycidae: Cerambycinae: Trachyderini). Zootaxa, 1892:1-25.

Copyright © Ted C. MacRae 2009

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3 thoughts on “Tragidion confusion

  1. Hola!

    By training I am a geologist. However, I have many, many, scientific interests in the Sonoran Desert. I have been doing serious fieldwork there since age 10………..

    For the past 20 years I have been engaged in an intensive study of the saguaro cactus. These studies began with and continue to involve gathering detailed information about the blooming of this species. Along this path I’ve discovered a great number of other previously un-described relationships and aspects of this iconic species. Soon you will begin to see the results appearing in print!

    One of those is the relationship between a beetle, a “tarantula hawk” wasp, and the flowering of the saguaro.

    The wasp in question is the matt-black bodied Hemipepsis ustulata with all black antennae. To date I have found no lit. ref. of this wasp species feeding habits on the saguaro flower. John Alcorn from A.S.U. in Tempe, AZ is now retired. He has written extensively on the “lek behavior” of this wasp species as it uses palo verdes in the Phoenix environs.and has identified it from my images as the species that has been feeding upon the nectar of saguaro flowers.

    The female wasps begin to feed upon clear droplets of “nectar” provided by non-vascular nectaries
    of the petaloids of developing flower buds. I have never seen a male wasp engaging in this activity. However, when the saguaro flower opens the females, with their short mouth parts, are thwarted in their attempts to reach the nectary by the forest of anthers that fills the upper corolla of the saguaro flower……….

    Meanwhile, early on I began to notice the sometimes presence of a long-horned beetle with red-orange elytra that “always” lurks on the shaded side of a saguaro flower. It took me more than 8 years to observe one in the full sun of the east side of an open flower AND get an image of it! So, I, at long last, have a few images of one particular adult. It is definitely a Tragidion. I believe it to be armatum brevipenne – as every view I’ve ever had of them is of antennae that are significantly shorter than the distal end of their abdomens……

    In the immediate area of my saguaro bloom study plot there are Y. elata and Y. baccata along with A. palmeriand and A. schottii. The plot itself is at an avg. elevation of 1,070m/ 3,520ft + msl. and is in an Upper Sonoran Desert biome that has a few veg. elements from the Chihuahuan Desert.

    Unfortunately, the images that I have are low resolution as I was so excited to see one in the clear that I totally forgot to check the resolution of what I was taking!

    So, back on track with the “crux” of the relationships……..the beetles want to feed upon saguaro nectar too but, they ALSO have short mouth parts like the female wasps. However, the beetles have strong mouthparts and they chew a vertical slit in the corolla from its outside in order to get at the nectary! And, the female wasps have learned about this and regularly fly to the saguaro flowers whereupon they search the outer bases of the flowers for the slits and finding one they too feed! While they are searching and/ or feeding there they may be “joined”, figuratively and literally, by an ever watchful male that has used a nearby palo verde or even a saguaro as a “lek” !

    The “Batesian” Mimicry protects the beetles and the wasps get the benefit of the beetles feeding method!

    I have written you this so that it may serve to get some enthusiast to make the effort to either get a good telephoto image of this beetle AND/ OR “collect” one so that I can get a “positive” identification. I am also presently trying to get in contact with Ann Ray OR Ian Swift to see if they have any information that I don’t know about the relationships of these three species…….

    I hope you have enjoyed this little tale and I look forward to discussing this with you!

    Sincerely, & “Feliz Sendas” ! Bill Peachey/ Memo Durazno( en Sonora )
    14 January 2016

    • A very interesting tale, indeed! This seems to add considerable understanding to the nature of the mimetic relationship of these beetles to Pepsis and Hemipepsis wasps.

      Hopefully somebody will see this and be able to follow up with photos and a positive identification of the beetle.

      • Ted:
        There is more to this tale…I thought that it was “dragging on” a bit and so I cut it short! But, that is the “meat’ of it. The additional part has to do with the actual damage that this beetle “sometimes” wreaks. But, the final analysis of it is that it is, at its worst, not a significant factor in the saguaro’s reproduction. I have now learned what I have needed to know for my present needs. And the facts of the matter are that no individual OR combination of ALL of the saguaro pests constitute a significant negative factor in the reproduction of the saguaros at the sub-regional to regional scale!

        The major finding on my study plot is that it is extreme heat or cold, discrete, weather events are the major negative factors in this system.

        If I had the time, I think it would be quite interesting to “lurk” around all of the potential food species that grow there to see what they are laying eggs on. I have yet to see one flying. I believe that they are active at night based upon the way they avoid the sun during the day. I have done many hours of observation of saguaros and various species of agaves with helicopter-pilot grade “Litton”
        binocular night vision goggles at night studying bats but have never seen these beetles on any of the flowers. I’ve done a lot of work on both species of nectar-feeding bats that we have here in SE Arizona & Sonora. Their habit of being up on the outsides of the flowers would allow them to disappear into the relatively solid “clumps” of flowers that bloom many at once on saguaro stems AND the panicles of agaves.

        If you or any of the bloggers come down my way feel free to contact me and I’ll steer you to the plot. It is easily accessed during summer mornings in May & June and it is often quite exquisite to out there at those times!

        I could forward the sole good images that I have…….they are really frustrating to “blow up” as the needed resolution just isn’t there! But, I think that they are in their own right quite majestic little creatures!

        Let me know!

        I imagine that you were surprised to get that long “missive”… Ha!

        Thanks for maintaining this site! ———— Bill Peachey


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