Metrius contractus on a leaf feeding. Image taken by and used with permission from Sarah Crews.


Before extrapolating on the details of classification, it should first be establPhylogenetic tree displaying where Metrius contractus fits into the largest groupings of life, domain and major clade. Image created by Sarah Lloyd. Information was obtained from lectures of  Dr. Perez (2013).ished what the implications of these main categorizations are. It should also be noted that traits associated with groupings were all at some point in the ancestry present in the organism. The domain Eukarya consists of a multitude of major clades including all kinds of life forms. Some unifying traits of this large group are cytoskeletons, nuclei in cells, and organelles such as mitochondria which evolved from an endosymbios with bacteria (Tree of life 2009). The Tree of Life (2009) also discusses the kingdom of Metrius contractus. It is in the Animal Kingdom meaning it is part of the major Eukaryotic clade of Opisthokonta, has some type of posterior flagellated cell, and is multicellular.

Phylogenetic tree displaying where Metrius contractus fits into the immense diversity of life. Image created by Sarah Lloyd. Information was obtained from lectures of  Dr. Perez (2013).Getting a bit more specific, this organism is part of the Arthropoda Phylum. The Bug Guide (2013) discusses that organisms in this phylum have jointed pairs of legs, bodies divided into segments, and exoskeletons commonly made of chitin. The evolution of the exoskeleton in this group of organisms was really quite critical in their ability to develop well-suited mouth parts, limbs, outstanding agility, and other special adaptations (Eisner and Meinwald 1995). Metrius contractus is in the Insecta Class (Tree of life 2009). These organisms have bodies and legs in parts of three, and most also have compound eyes and wings (Bug Guide 2013). The next level, order, is specifically for beetles which have a leathery exterior, reduced wing structures, and a larval stage (Bug Guide 2013). This order is called Coleoptera (Bug Guide 2013).

Example of a type of bombardier beetle different from Metrius contractus spraying its jets. Image published by Berenbaum (2011).Following order is the family which is Carabidae. This family is made up of all bombardier beetles (Eisner et al. 1977). What is a bombardier beetle? Bombardier beetles make up the family Carabidae but also the Brachinini family (Eisner et al. 1977). These beetles all excrete an irritating quinone solution from powerful chemical jets in their posterior abdomen regions (Eisner et al. 2000). These tiny jets essentially comprise little bombs that are always ready for the defense of the beetle (Schwarcz 2010). This is where the name bombardier beetle came from (Schwarcz 2010).

Now getting back on topic and more specifically into the focus organism, it is in the Metrius Genus. All of the beetles in this genus are black and emit a clouded spray of defense when threatened (Bug Guide 2013). Finally, the species of Metrius contractus is reached! . Its tiny size is probably where it got the name Metrius contractus. The Latin root meta means a change in form, and contractus means narrow or restricted (Learn That Word 2005). Often this spectacular organism is lump summed into the common and larger grouping of bombardier beetle, but such a general grouping doesn’t quite do it justice. To dive into more details on the species, explore habitat, geography, & interactions, reproduction, or even some crazy chemistry!

After understanding the basics, it is time to dig into the more extensive classifying divisions of some of these large groups. As mentioned earlier, the two families in the generalized bombardier beetle group are Carabidae aFamily tree with the Metrius contractus groups in bold text. Image created by Sarah Lloyd. Information was obtained from Eisner et al. (2000).nd Brachinini (Eisner et al. 2000). Inside of the Carabidae family are the subfamilies Carabinae and Paussinae (Eisner et al. 2000). There are also three main tribes that are included in the Paussinae subfamily called Metriini, Ozanini, and Paussini (Eisner et al. 1977). This being said, Metrius contractus is part of the Carabidae family, the Paussinae subfamily, and in the Metriini tribe. Given that there are only three species known in the Metriini tribe including Metrius contractus, there is still much information to be uncovered about it and its evolutionary connectedness to other organisms (Eisner et al. 2000). The most prevalent and accepted hypothesis is that the Metrius Genus is representative of more primitive beetles. The true debate in the phylogeny of bombardier beetles lies in whether or not their ability to chemically bomb predators is a homologous trait or the result of convergent evolution (Eisner et al. 1977). As research advances, however, it is becoming quite clear that there was likely a single common ancestor that gave rise to this trait in the beetles (Eisner et al. 2000).

 Phylogenetic classification of Metrius contractus. Click for further details in the tree of life! Image created by Sarah Lloyd (2013). Information collected from The Bug Guide (2013) and Tree of Life (2009).
Use this figure to help you summarize some of those complicated classifications or click on any of the phyogenetic trees to explore the Tree of Life a bit more!

Move on to Habitat, Geography, and Interaction!