Carrion and Burying Beetles

Carrion beetles (Necrophila spp – literally ‘dead-body loving’) is a  primary agent in the dispatch of dead animal carcasses.

Common Names: American Carrion Beetle – Named for its primary carrion food source. American emphasizes that it is endemic to North America east of the Rocky Mountains. Carrion comes from the Latin word caro meaning flesh.
Burying Beetle – A carrion-eating beetle so named because it buries the animal carcass for longer term consumption.

Scientific Names: Necrophila americana – The generic name if from the Greek nekros, meaning “dead body” and philos, meaning “loving” in reference to its primary food source. The species name is a Latinized form to reflect its American provenance.
Nicrophorus carolinus – The burying beetle genus is also a derivative of nekros as above with the addition of the Greek phoros meaning ‘to carry.’ The species name is reference to its first identification in the Carolinas.

Potpourri:  Carrion beetles (Necrophila spp – literally ‘dead-body loving’) and Burying beetles (Nicrophorus spp – literally dead-body carrying’) are two of the primary agents in the dispatch of dead animal carcasses that would otherwise succumb only to the slow but inexorable procession of mephitic bacterial putrefaction. Although they may be followed and dispersed by their larger carrion-eating scavengers like the black and turkey vultures, burying beetles are among the first to arrive at the scene of death, preceded only by the ubiquitous flies. The beetle order Coleoptera (meaning “sheathed wings”) is the largest order in the Animal kingdom (about 300,000 species); it contains about one third of all insects and one quarter of all named species. Carrion and burying beetles are in the family Silphidae, distinguished according to their behavior; both depend on dead animals for their nutrition. S. Marshall in Insects, Their Natural History and Diversity, calls them “nature’s undertakers”. Insect field guides list habitat as “wherever carrion is found.”

All carrion feeders depend on bacteria to initiate the olfactory vectoring process. When an organism dies, its innate defensive chemistry ceases and the resident heterotrophic bacteria begin the process of decomposition, which is essentially a respiratory process in which complex hydrocarbons and amino acid proteins are oxidized to produce carbon dioxide, water and other constituent molecules. The bacteria are responsible (with the fungi) for recycling the complex chemistry of dead living things into the simple constituent atoms that are then available for reuse. Among the products of bacterial decomposition are two very similar toxic diamine (having two NH2 molecules) molecules aptly named cadavarine and putrescine. It is these substances that broadcast the presence of carrion to the flies, beetles and vultures – the putrescent smell of a cadaver.

Carrion beetles at table

Flies and beetles dominate the early stages of nutritive cadaver recycling; the flies have high mobility and high transience, beetles have low mobility and low transience – they are in it for the long haul. Due to their mobility, the flies are the first to arrive; their purpose is to lay eggs. This evolved survival tactic takes advantage of the nutrient rich organic matter that is initially free of predators or competition. The eggs hatch within hours (another adaptation) into white larvae widely known as maggots, the second metamorphic stage of dipterous insects. For a few hours, the maggots and the adult flies (who may also eat some carrion) share the feast only with the bacteria; the fly larvae instinctually burrow into the carcass as a matter of survival. It is fly nirvana until the carrion beetles arrive, ultimately dependent on and drawn to the same carcass for sustaining their own procreative process. Carrion beetles consume maggots. It is a matter of some conjecture whether they eat them for nutrition or whether they actively eliminate them to reduce the offal competition with their own beetle progeny. Following mating, the female lays eggs which hatch into larvae after about a week which will subsist on the carrion in which they are embedded. It takes about a month for carrion beetle larvae to go through three week-long intermediate stages called instars to reach the pupate stage from which they metamorphose into adults. Instar variations are useful in the field of entomological forensics, the determination of the post-mortem interval (PMI) measured by larval evolution – if the third instar is present then death occurred about a month preceding. The book Stiff provides details on an operation in North Carolina where bodies are left to decompose, allowing the staff to validate appropriate PMI indicators by periodic inspections. Now that is a dirty job.

Burying beetles bury their prey to prevent fly infestation

Burying beetles are aptly named; they have a different strategy, laboriously burying relatively small animal carcasses by digging out the soil underneath and covering them over with the excavated material. They are also known as sexton beetles, named for the official responsible for maintaining church property, which traditionally would include digging graves. The purpose of this arduous task is to prevent fly infestation. Burying beetles do not consume nor do they choose to compete with the dipteran larvae; if already infested, they will find another carcass. However, in the ingenious diversity of life – Darwin’s “endless forms most beautiful and wonderful” – there is a third party in the survival story. Several species of arachnid mites of the genus Poecilochirus employ phoresy, a type of mutual relationship in which one organism uses another for transportation (humans riding horses would be an example, I suppose). The transported mites jump off the beetle’s folded dorsal wing covers (elytra) and onto the carrion to eat fly eggs and maggots. It should be noted that this is not altogether a free lunch, as the mites also eat burying beetle eggs and larva, presumably to a lesser extent than the flies or it would be a zero sum game and they would get no free ride to the next meal. The mites sustain the mutualistic association through their own life cycle, they are almost always found on the wings of the burying beetles. Oviposition follows as the female burying beetle lays eggs on the now isolated food source that becomes virtually the sole province of the beetle and the hatched larvae. The female beetle actively rears the young larvae – pre chewing their food to soften it and building a specially chamber in the carcass for them, one of the very few known instances of maternal care in arthropods. Given this virtually avian level of parenting, one would think that the burying beetles would pullulate. However, the opposite is the case; several species of burying beetle are endangered. One of the theories is that the dearth of large predators that is a characteristic of the Anthropocene Era has resulted in an increase in smaller vertebrate scavengers that consume the carrion faster than the burying beetles can dig.