Micromalthus debilis, LeConte, 1878

Hörnschemeyer, Thomas, Wedmann, Sonja & Poinar, George, 2010, How long can insect species exist? Evidence from extant and fossil Micromalthus beetles (Insecta: Coleoptera), Zoological Journal of the Linnean Society 158 (2), pp. 300-311 : 307-309

MICROMALTHUS DEBILIS

In the case of M. debilis , the critical environmental conditions might have been constant for an extended period. This beetle occupies a very special ecological niche. The peculiar and complex life cycle of M. debilis occurs in dead wood of a wide range of trees decayed by red-rot fungi ( Pollock & Normark, 2002; Hörnschemeyer, 2005). The mature timber habitat is an important component of forest ecosystems and offers ecological niches for a wide range of species (e.g. Grove, 2002). It can be assumed that this habitat remained more or less unchanged since the existence of deciduous and coniferous forests; i.e. since the middle Cretaceous for deciduous trees and significantly earlier for conifers (e.g. Nabors, 2007). In the life cycle of M. debilis ( Fig. 5), there are two different types of parthenogenesis (‘virgin birth’), both of which have their origin in the so-called paedogenetic larva. In one pathway, the larva lays one egg, which then develops into a male beetle. In the other cycle a paedogenetic larva gives birth to about ten small larvae of the triungulin type, which are very agile and can disperse easily in the decaying wood. The triungulin larva passes through an unknown number of moults to become the cerambycoid larva, which may develop into a female beetle. But more often, this cerambycoid larva moults into a paedogenetic larva, and the cycle of larval reproduction starts anew.

To date it is not known for certain whether the adult beetles can reproduce successfully. It may be that reproduction and also dispersal of M. debilis is completely dependent on the larval stages. In this case, the very small triungulin larva could cling to other larger flying insects as a means of transport to another decaying log, a behaviour known as phoresy ( Wheeler, 1919; Clausen, 1976; Hörnschemeyer, 2005). Confirmation that this type of development (*also mentioned in Grimaldi & Engel, 2005: 642).

is ancient is shown by Micromalthus larvae from Miocene Mexican amber ( Rozen, 1971) and Eocene Baltic amber ( Lawrence & Newton, 1995). The larvae most likely feed on fungi in the rotten wood. This remarkable life cycle entails a very efficient usage of resources and the species is well protected against potential predators. Obviously, M. debilis was able to survive with this life style for millions of years.