Omma stanleyi, copyright David Maddison.

Belongs within: Coleoptera.

The Archostemata are a distinctive group of wood-boring beetles with the labrum fused to the head capsule in adults, and a distinctive wing-tucking mechanism where the wing tips are rolled rather than folded under the elytra (Grimaldi & Engel 2005). The most widespread family in the modern fauna is the Cupedidae, members of which have the the antennal insertions dorsal and more or less approximate, fore coxal cavities separated by a prosternal process, lobed fourth tarsomeres, and grooves on the underside of the thorax for reception of the legs. The Ommatidae of South America and Australia have shorter, laterally inserted antennae, contiguous fore coxal cavities and simple tarsomeres. Micromalthus debilis is a native of North America whose females may either metamorphose fully as in other beetles or attain reproductive maturity as paedogenetic larvae (Philips & Young 2001).

A long beetle life
Published 21 February 2008
Tenomerga mucida, from Wikipedia.

Coleoptera fossils are known since the Lower Permian, but carry problems all of their own. Almost the entire early fossil record of beetles is composed of isolated elytra, which offer few diagnostic characters and which may be suspected of rampant homoplasy (Ponomarenko 2002). While representatives of recent families or their close relatives have been described from early on, a lot of doubt must hang over the accuracy of these identifications. A classic example is the Triassic family Obrieniidae, originally identified as the earliest representatives of the weevils (Curculionoidea), but now regarded as merely convergent (Kuschel 2003). Krzeminski & Lombardo (2001) assigned a single elytron to the genus Notocupes in the family Cupedidae. The Cupedidae survive to this day—an example of a living species is shown just above. Generally found in rotten wood, the are one of the few survivors of the basal (paraphyletic?) beetle suborder Archostemata, making them very interesting in understanding beetle evolution. The genus Notocupes (recently regarded as a synonym of Zygadenia by Ponomarenko 2006) is a fossil genus known from Triassic to the Palaeocene—a really quite spectacular length of time, and, in light of the problems I’ve just mentioned, really worth a further look.

Eating Mum from the inside out
Published 20 November 2008
Micromalthus debilis feeding on rotting wood. The large specimen on the left may or may not be a mature female. Photo courtesy of Alex Wild.

Life doesn’t always go with the easiest way to do things. Sometimes things get complicated. But sometimes things get so complicated, so seemingly unnecessarily confusing and baroque, that one can’t help wondering that someone, somewhere, is taking the piss.

Micromalthus debilis is a tiny, wood-boring beetle that has the dubious claim of going through what is perhaps the most complicated life-cycle in the animal kingdom. You have to look at obscure parasites such as Buddenbrockia and Mesozoa before you find possible competitors. Only one species of Micromalthus is currently recognised. This species is found more or less pan-tropically and pan-subtropically in rotten wood, but its range may have been increased by human transport (Pollock & Normark 2002). Within the beetles, Micromalthus debilis holds a decidedly isolated phylogenetic position—it belongs to the small relictual group known as Archostemata which comprises the sister group to all other beetles, and while fossils of the Micromalthus lineage are only known from amber (dating back to the Cretaceous), the possible sister-group of Micromalthus, the Cupedidae, has a fossil record dating back to the Triassic (Beutel & Hörnschemeyer 2002; Grimaldi & Engel 2005).

Most Micromalthus don’t even look like beetles. Micromalthus are usually female and become mature while still effectively larvae (Pollock & Normark 2002). Such females produce eggs asexually by parthenogenesis. Such egg production is usually thelytokous—asexual eggs produced by diploid females hatch into diploid females. Like some other insect lineages (including, most famously, Hymenoptera), Micromalthus has haplodiploid sex determination—females are diploid while males are haploid. Production of males in Micromalthus is rare (more on that in a moment). Like Strepsiptera, Micromalthus is hypermetamorphic (it goes through multiple larval stages). When thelytokous Micromalthus eggs first hatch, out come highly mobile, legged larvae called triungulins. The triungulins feed for a few weeks, then moult to become legless cerambycoid larvae which live a few months more. Remarkably, the ovaries begin to develop in the cerambycoid larva, which then moults directly into the mature paedogenetic adult without going through a pupal stage, making the adult a true reproductive larva. In most holometabolous insects such as beetles, flies and wasps, the reproductive organs don’t begin to develop until the pupal stage.

As if the production of parthenogenetic, reproductive larvae was not remarkable itself, that’s not actually where the process becomes complicated. Some females at the end of the cerambycoid stage, instead of moulting straight through to adults, actually do go into a pupal stage. When those pupae reach maturity, instead of becoming larviform like their sisters, they produce a fully-developed, winged adult beetle. These winged females are presumably able to disperse to new feeding grounds, though what determines whether a larva becomes a paedogenetic or winged adult seems to be unknown.

One of the rare fully developed adults of Micromalthus debilis. Photo again courtesy of Alex Wild.

An few female larvae differ further from their sisters in that they don’t start developing ovaries while still larva, but not until they reach the final instar. These females, instead of producing numerous thelytokous eggs, produce just one arrhenotokous egg—a parthenogenetically produced egg that will develop into a haploid male. When she lays this single egg, it remains attached to her until it hatches into yet another larval type, a legless curculionoid larva. The instant the male larva is hatched, it plunges its head back into its mother, and proceeds to devour the contents of her body. Once it has finished consuming its hapless mother, the cannibalistic offspring will go through a series of moults culminating in a winged adult male. Some arrhenotokous females, if the male egg fails to develop properly or is lost before it hatches, may switch to producing thelytokous eggs that hatch into other females.

Ironically, in light of the terminal cost to the female of producing a male offspring, no matings between males and females and sexually-produced eggs have been observed in Micromalthus, and some authors have suggested that the males produced in this way are all sterile. Reproduction in Micromalthus would then be entirely parthenogenetic. This seems very unlikely—as females that produce males only ever produce a single offspring, surely there would be a strong selective pressure for eliminating production of males entirely if they were completely non-functional. Arrhenotokous females make no attempt to elude the attentions of their hungry sons, but submit readily to their fates. Male production is also more likely when resources become stretched. It seems much more likely that sexual reproduction does occur, probably between the males and the rare winged females, though the two are generally not produced by a colony at the same time and inbreeding between males and females of the same colony would be unlikely.

The full life cycle of Micromalthus debilis, taken from Pollock & Normark (2002).

Why does Micromalthus have such an obscenely complicated and sordid life cycle? Like other wood-living insects, Micromalthus rely on endosymbiotic bacteria to digest the wood they feed, and these endosymbionts may be transmitted to offspring through the ovarian tissue. For bacteria transmitted in such a way, males represent a reproductive dead end, and many such bacteria in insects have been shown to negatively affect male production in order to increase the ratio of female offspring and improve their own chances of transmission (Hurst & Jiggins 2000; the most famous examples are species of Wolbachia). Pollock & Normark (2002) suggest that endosymbiotic bacteria may be transmitted to female offspring but not to males, and that male cannibalism may be a means of circumventing this handicap. The high cost of producing males would have resulted in selective pressure to keep the number produced to a bare minimum, explaining their rarity. Unfortunately, while this is an intriguing idea, it currently suffers from a dire shortage of evidence. The Micromalthus will not give up their secrets easily.

Systematics of Archostemata
<==Archostemata [Cupedoidea]
|--+--Micromalthus LeConte 1879PY01 [MicromalthidaeMW15, Micromalthinae, Micromalthoidea]
| | `---M. debilis LeConte 1878PN02
| `--Ommatidae [Ommatinae, Ommatini, Tetraphaleridae, Tetraphalerini]MW15
| |--Notocupes Ponomarenko 1964KL01
| |--OmamimaGE05
| |--TetraphaleritesGE05
| |--Tetraphalerus Waterhouse 1901W01 [TetraphalerinaeMW15]
| | |--*T. wagneri Waterhouse 1901W01
| | |--T. bruchiMW15
| | `--T. verrucosusP02
| `--Omma Newman 1839LM87
| |--*O. stanleyi Newman 1839LM87
| |--O. mastersi Macleay 1871LM87
| `--O. sagittaB14
`--Cupedidae [Cupesidae, Cupesides, Triadocupedidae]MW15
| i. s.: Chengdecupes shilouense Hong 1985RJ93
| i. s.: Rhipsidegma raffrayi (Fairmaire 1884)B14
| Adinolepis Neboiss 1984LM87
| |--*A. eumana (Neboiss 1960) [=Cupes eumana]LM87
| |--A. mathesonae (Neboiss 1960) [=Cupes mathesonae]LM87
| |--A. scalena Neboiss 1984LM87
| `--A. youanga (Neboiss 1960) [=Cupes youanga]LM87
| AscioplagaY01
| Paracupes brasiliensisY01
|--Priacma LeConte 1874MW15, Y01 [Priacminae]
| `--P. serrata (LeConte 1861)B14
`--+--Prolixocupes Neboiss 1959MW15, Y01
| |--*P. latreillei (Solier in Gay 1849) [=Cupes latreillei]N60
| `--P. lobicepsMW15
`--+--Tenomerga Neboiss 1984MW15, Y01
| |--T. cinereusY01
| |--T. concolorLM87
| `--T. mucidaLM87
`--+--Distocupes Neboiss 1984MW15, LM87
| `--*D. varians (Lea 1902)LM87 [=Cupes variansLM87, Omma variansN60]
`--Cupes Fabricius 1801MW15, Y01
`--*C. capitatus Fabricius 1801N60, B14 [=Lymexylon (Cupes) capitatumG20]

Archostemata incertae sedis:
|--Sikhotealinia [Sikhotealiniidae]B14
| `--S. zhiltzovae Lafer 1996B14
|--J. ignoramus Ponomarenko 1985RJ93
`--J. minor Ponomarenko 1990RJ93
ObrieniaP02 [ObrieniidaeGE05]
`--O. kuscheliP02
Crowsoniella [Crowsoniellidae]Y01
`--C. relicta Pace 1975B14

*Type species of generic name indicated


Beutel, R. G., & T. Hörnschemeyer. 2002. Larval morphology and phylogenetic position of Micromalthus debilis LeConte (Coleoptera: Micromalthidae). Systematic Entomology 27 (2): 169–190.

[B14] Bouchard, P. (ed.) 2014. The Book of Beetles: A lifesize guide to six hundred of nature’s gems. Ivy Press: Lewes (United Kingdom).

[G20] Goldfuss, G. A. 1820. Handbuch der Naturgeschichte vol. 3. Handbuch der Zoologie pt 1. Johann Leonhard Schrag: Nürnberg.

[GE05] Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

Hurst, G. D. D., & F. M. Jiggins. 2000. Male-killing bacteria in insects: mechanisms, incidence, and implications. Emerging Infectious Diseases 6 (4): 329–336.

[KL01] Krzeminski, W., & C. Lombardo. 2001. New fossil Ephemeroptera and Coleoptera from the Ladinian (Middle Triassic) of Canton Ticino (Switzerland). Rivista Italiana di Paleontologia e Stratigrafia 107 (1): 69–78.

Kuschel, G. 2003. Nemonychidae, Belidae, Brentidae (Insecta: Coleoptera: Curculionoidea). Fauna of New Zealand 45.

[LM87] Lawrence, J. F., B. P. Moore, J. E. Pyke & T. A. Weir. 1987. Zoological Catalogue of Australia vol. 4. Coleoptera: Archostemata, Myxophaga and Adephaga. Australian Government Publishing Service: Canberra.

[MW15] McKenna, D. D., A. L. Wild, K. Kanda, C. L. Bellamy, R. G. Beutel, M. S. Caterino, C. W. Farnum, D. C. Hawks, M. A. Ivie, M. L. Jameson, R. A. B. Leschen, A. E. Marvaldi, J. V. McHugh, A. F. Newton, J. A. Robertson, M. K. Thayer, M. F. Whiting, J. F. Lawrence, A. Ślipiński, D. R. Maddison & B. D. Farrell. 2015. The beetle tree of life reveals that Coleoptera survived end-Permian mass extinction to diversify during the Cretaceous terrestrial revolution. Systematic Entomology 40 (4): 835–880.

[N60] Neboiss, A. 1960. On the family Cupedidae, Coleoptera. Proceedings of the Royal Society of Victoria 72 (1): 12–20.

[PY01] Philips, T. K., & D. K. Young. 2001. Micromalthidae Barber, 1913. In: Arnett, R. H., Jr & M. C. Thomas (eds) American Beetles vol. 1. Archostemata, Myxophaga, Adephaga, Polyphaga: Staphyliniformia pp. 22–23. CRC Press: Boca Raton.

[PN02] Pollock, D. A., & B. B. Normark. 2002. The life cycle of Micromalthus debilis LeConte (1878) (Coleoptera: Archostemata: Micromalthidae): historical review and evolutionary perspective. Journal of Zoological Systematics and Evolutionary Research 40 (2): 105–112.

[P02] Ponomarenko, A. G. 2002. Superorder Scarabaeidea Laicharting, 1781. Order Coleoptera Linné, 1758. The beetles. In: Rasnitsyn, A. P., & D. L. J. Quicke (eds) History of Insects pp. 164–176. Kluwer Academic Publishers: Dordrecht.

Ponomarenko, A. G. 2006. On the types of Mesozoic archostematan beetles (Insecta, Coleoptera, Archostemata) in the Natural History Museum, London. Paleontologicheskii Zhurnal 2006 (1): 86–94 (transl. Paleontological Journal 40 (1): 90–99).

[RJ93] Ross, A. J., & E. A. Jarzembowski. 1993. Arthropoda (Hexapoda; Insecta). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 363–426. Chapman & Hall: London.

[W01] Waterhouse, C. 1901. Two new genera of Coleoptera belonging to the Cupesidae and Prionidae. Annals and Magazine of Natural History, series 7, 7: 520–523.

[Y01] Young, D. K. 2001. Cupedidae Laporte, 1836. In: Arnett, R. H., Jr & M. C. Thomas (eds) American Beetles vol. 1. Archostemata, Myxophaga, Adephaga, Polyphaga: Staphyliniformia pp. 19–21. CRC Press: Boca Raton.

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