Trictenotomid beetles (Trictenotomidae)

Trictenotoma davidi	Autocrates aeneus

Dmitry Telnov

Even some specialist coleopterists, when they hear the name "trictenotomids", don't quite know what to say: "Who?+ Is that a fact?!" Some might take them for a variety of longhorn beetle, or stag beetle. There are only a few people around with a "personal" knowledge of these interesting representatives of the order of Coleoptera.

DIVERSITY AND BIOGEOGRAPHY

The family of trictenotomids is a classic example of a small family with a distinct and restricted area of distribution. There are currently just 14 recent representatives of this family known to science (see table 1). As yet, no fossil species of trictenotomids have been discovered. All the contemporary species have been encountered in the Oriental zoogeographical region, and only a few of them have distribution areas that extend into the South-Eastern Palaearctic.

Note: In the table, genera are set out in systematic order, and species in alphabetic order.

MORPHOLOGY AND ANATOMY

Trictenotomids are large and striking beetles, but the body colouration of most of them is rather uniform. Some of them have a shiny metallic colouring to the body surface (Autocrates aeneus, A. vitalisi), while others are covered to a greater or lesser extent with a thick hairy coating, and have a "plush" appearance (species of the Trictenotoma genus). Autocrates oberthueri has several light-coloured stripes on its wing cases, and looks as if lines have been drawn lengthwise along it.

The main feature that marks out trictenotomids from all other representatives of the superfamily Tenebrionoidea is their size. Even in dwarf individuals, the body length is never less than 4.0 cm, and in many species it is 8.0 cm or more. Furthermore, trictenotomids have a very distinctive appearance. In terms of habitus, they are very reminiscent of some longhorn beetles and stag beetles (in the latter case, because of the triarticulate clava of antennae). This similarity is further enhanced by the large size of all of these groups. Why are these beetles so large, and, most importantly, what is the reason for the convergent similarity in body shape with longhorn beetles and stag beetles? This looks like remaining one of the numerous mysteries of beetle evolution for some considerable time to come+

The body in trictenotomids is slightly flattened dorso-ventrally, which seems to be in order to help the imago to crawl along under the bark of trees. The body integument in trictenotomids is very hard.

The head is usually large, but always narrower than the prothorax, flattened and prognathical. The eyes are large, vertical, fine-faceted, and notched on the frontal edge near the base of the antenna. The labrum is short and transverse. The mandibles are large and powerful, with a more or less dense rugulose surface, the structure of the grooves being a systematic characteristic. Their external surface is smooth, while the internal surface has uneven asymmetrical jags. In males of the genus Autocrates, the mandibles are very large and long. The maxillar palpes are hatchet-shaped. The antennae are powerful and long. Three of their apical segments are serrated, and the main joint is very large and long. The antenna attachment points are open. The external maxillar lobe is not very prominent, while the internal lobe is long, narrow and downy.

The prothorax is powerful and transverse, and always broader than the head. It is notched or angular along the sides, and its posterior edge has a bend in the middle. In representatives of the genus Trictenotoma, there are round-shaped areas on the disc of the prothorax, varying in size and without any downy covering, the purpose of which is not as yet clear. The frontal coxae are transverse, and divided by a broad prosternal process. The anterior coxal fossae are open from behind and inside. The epimeres of the mesothorax reach as far as the anterior coxal fossae. The central coxae are moderately far apart, and their fossae are open laterally. The scutellum is readily visible and large.

The elytres taper slightly from the middle towards the apices. Their stippling is confused. The epipleures are incomplete. The hind wings are folded for about a third of their length. The venation of the hind wings in trictenotomids is similar to that found in representatives of the sub-family Prioninae of the family of longhorn beetles, without the subcubital spot. Because of their large size and weight, they are not very good flyers. The abdomen has seven dorsal and five ventral segments. The two frontal ventrites are coadunate.

The legs are long and the tarsi are heteromerous (the tarsal formula for both sexes is 5:5:4). All the tibiae on the inside have two minor apical spurs. The tarsal joints are cylindrical, and the lower part of each of them is coated with thick hairs.

The aedeagus is false-trilobed, with articulated parameres.

The body surface is shiny or usually coated with thick (though not in all species) light-coloured (beige, reddish, orange, gray) hairs, beneath which the basic background is sometimes not distinguishable.

Sexual dimorphism is clearly marked. The males have powerful mandibles. This characteristic matches that of stag beetles, but the degree of prominence in trictenotomids depends on the overall body dimensions - in dwarf males it is not fully manifested. Also in males, the final segments of the abdomen are modified: the hypopygidium and the pygidium, with notches and appendages of various shapes (as in representatives of the tribe Formicomini of the family of ant-like flower-beetles (Anthicidae) and in some other groups of Tenebrionoidea). In females the pygidium is extended and conical in shape, and notched, while the hypopygidium is simple and whole. The antennae are generally longer in the males than in the females. The males are also usually larger and more powerful than the females.

The best way of distinguishing trictenotomids from longhorn beetles (Cerambycidae) and stag beetles (Lucanidae) is by the tarsal formula. It is 5:5:4 for all trictenotomids, while in stag beetles and longhorn beetles of sub-family Parandrinae it is 5:5:5; in longhorn beetles of sub-family Prioninae, the tarsae appear to be 4-jointed, but then the formula is again different - 4:4:4.

PREIMAGINAL STAGES AND ONTHOGENESE

Currently we only know of the imago of one of the species (Trictenotoma davidi). The imagines of this, and clearly of other species too, develop beneath tree bark. There is no other information concerning the preimaginal stages of trictenotomids.

The imago is slightly flattened, the epicranial carina is short, the 3rd segment of the antennae is reduced, and the stigmata are not developed. The mola of the mandibles is transverse and costate, while the mala is truncated and segmented at the frontal edge. The meso- and metathorax, and also abdominal tergites I-VII, have a series of longitudinal ribs. Segment IX of the abdomen has a pair of contiguous and backwardly protruding urogomfes. Segment X is transverse, and is located on the ventral side of the abdomen. The spiracles are annular in shape, with an oval peritreme.

The imagines probably live under bark or in rotting wood.

BIOLOGY AND MODE OF LIFE

All the species in the family inhabit damp and seasonal forests in the tropical and sub-tropical belt. Unfortunately we have extremely meagre information concerning the biology of trictenotomids. They fly well to the light, but the adult beetles spend a considerable portion of their lives underneath the bark of trees.

THE PLACE OF TRICTENOTOMIDS IN THE SYSTEM OF THE ORDER OF COLEOPTERA

In the actual interpretation of the system of the order of Coleoptera (Lawrence, Newton, 1995), the Trictenotomidae family is assigned to the superfamily Tenebrionoidea, infra-order Cucujiformia, sub-order Polyphaga. At the present time, there are two main versions of the exact position of the family trictenotomids in the system of Tenebrionoidea (fig. 1), based on two versions of the phylogeny of similar families.

Fig. 1. Phylogeny of Trictenotomidae and some related groups.

1 - Relationship between Trictenotomidae, Salpingidae, Boridae, Pythidae ? Pyrochroidae according to Watt (1987).

2 - Relationship between Trictenotomidae, Salpingidae, Boridae, Pythidae ? Pyrochroidae according to Pollock (1994).

Personally I go along with the viewpoint stated by my colleague Mr. Darren Pollock in his work; but this is only my own subjective opinion.

REFERENCES

Unfortunately, despite the small number of species, there are so far no summarising systematic monographs concerning representatives of this family. The only catalogue of species (Gebien, 1911) is now out-of-date. The descriptions of the family in the well-known manuals (Crowson, 1955; Lawrence, 1982; Nikitskii, 1992) are very brief. The species may only be defined using individual works (which are for the most part original descriptions), but without any comparative material to hand, this is a difficult task. The main sources in the literature which might provide assistance to anyone who is interested in this family are listed below.

Nikitskii N.B. 1992. The Trictenotomidae family - trictenotomids. In: Opredelitel' nasekomykh Dal'nego Vostoka SSSR [Handbook of insects of the Far East of the USSR]. Vol. 3, part 2. St Petersburg: 493 [in Russian].

Crowson R.A. 1955. The Natural Classification of the families of Coleoptera. London: 131.

Gebien H., 1911. Trictenotomidae. In: W. Junk, S. Schenkling (eds.). Coleopterorum Catalogus, 37: 741-742.

Lawrence J.F., 1982. Trictenotomidae. In: S.P. Parker (ed.). Synopsis and Classification of Living Organisms. New York: 544.

Lawrence J.F., Newton A.F., 1995. Families and Subfamilies of Coleoptera (with Selected Genera, Notes, References and Data on Family-Group Names). In: J. Pakaluk, A. Slipinski (eds.). Biology, Phylogeny, and Classification of Coleoptera. Warszawa: 779-1006.

Pollock D.A., 1994. Systematic Position of Pilipalpinae (Coleoptera: Tenebrionoidea) and Composition of Pyrochroidae. - Canad. Entomol. 126: 515-532.

Telnov D., 1999. Zoogeographie der Trictenotomidae BLANCHARD, 1845 (Coleoptera: Heteromera). - 7. DPU zinatniskas konferences rakstu krajums, A9: 95-97.

Watt J.C., 1987. The Family and Subfamily Classification and New Zealand Genera of Pythidae and Scraptiidae (Coleoptera). - Systemat. Entom. 12: 111-136.

Additional information in Internet:

Author: Dmitry Telnov, August, 2000.

Editors: A.G. Kirejtshuk, A.L. Lobanov, September of 2000.

Last updated: April, 2007.