American carrion beetle
The American carrion beetle is a North American beetle of the family Silphidae. It lays its eggs in, and its larvae consume, raw flesh and fungi. The larvae and adults also consume fly larvae and the larvae of other carrion beetles that compete for the same food sources as their larvae. They prefer to live in marshy and woody habitats. Necrophila americana emerge from their larval state in the early summer. A cuckoo bumble bee, Bombus ashtoni, displays close mimicry with the American carrion beetle. They are important in forensic studies because of their tendency to thrive on large carcasses.
Appearance
Necrophila americana are distinctive in that they have a relatively large, broadly rounded, and flattened body with a yellow pronotum. It is the only North American silphid with a mostly yellow pronotum, and it is the only species of Necrophila found in North America, as all other species are found in Asia. There is sexual dimorphism, with the males having rounded elytral apices and females having their apices more prolonged. They typically range from 12 to 22 millimeters long. The larvae can be characterized by their black color, two-segmented urogomphi, and the presence of numerous plates on the sensory area of the second antennal segment.Range
The beetle lives in North America, east of the Rocky Mountains. Its southern boundary is from eastern Texas to Florida and the northern boundary is from Minnesota to southeastern Canada, including New Brunswick and Maine.Habitat
Dense populations of N. americana have been found in eastern parts of Kansas. Over half of the 140 specimens collected were reported to be found in wooded areas, while 25% were found in marginal areas of woods. They are often found in open fields, while being rarely found in open meadows. However, it has also been shown that in habitats with trees present, the beetles prefer habitats that are humid and overgrown with thick trees, as opposed to the meadows or fields.Other studies have shown that the American carrion beetle also reside in Typha marsh habitats, with lots of water accumulation and heavy rainfall. They prefer marshes over woody areas such as forests, but still prefer both of these habitats over open meadows, which they are rarely found in.
Food resources
As adults, N. americana can be captured using carcass bait in a pitfall trap. Their appearance has also been documented in a studied site in Indiana multiple times, and they have appeared consistently as their abundance has aligned.They can also be baited using pitfall traps containing isopropanol. While the traps attract other insects, it is possible that these traps simply attract carrion beetles by the volatiles produced by carcasses of other insects. However, this is an unlikely explanation. Rather, isopropanol likely acts as a kairomone since there is no differentiation between the preference from females and males. It is possible that isopropanol is perceived as a chemical produced by decomposing bodies during the bloating stage, which would explain why N. americana is attracted to them.
Resource partitioning
N. americana also practices resource partitioning with other silphids such as Oiceoptoma noveboracense and Thanatophilus lapponicus. These three species are reproductively active at different periods of the year. O. noveboracense is active in the early spring, while T. lapponicus is active late in the summer. This resource partitioning behavior has likely evolved due to limitations in resource availability and must be adapted for rapid reproduction.Life cycle
The American carrion beetle has three instar stages, with differing body measurements at each stage. The first stage has a body length ranging from about 10 to 15 millimeters in length, the second stage ranging from 13 to 21 millimeters in length, and the final stage averaging 23 millimeters in length. The life cycle of the beetle is thought to take around 10 to 12 weeks.From spring through fall, during daylight, a few hours after flies begin arriving at a carcass, the adult beetles will arrive as well. They immediately begin eating the already hatching fly larvae, mating, and laying their own eggs. As long as the carcass lasts, the adults will remain eating competitors to give their own larvae a chance to eat and grow. Upon hatching from the eggs, the larvae will eat both the carcass and other larvae that are within it. The fly larvae digest part of the carcass, and the beetle larvae will consume the parts that the fly larvae did not, which typically consists of flesh left on the bones and on the moist inside of the face. Eventually the larvae will fall to the ground, dig into the dirt, and pupate. Adults practice overwintering.
The beetle is a ground-dwelling silphid that emerges from its larval state to feed on carcasses in early summer, as its greatest abundance is from June through October. They arrive early during the decomposition process compared to other beetles, which are most prominent during late August. While they do feed on carcasses, they don’t exhibit any signs of parental care.
Protective coloration and behavior
Mimicry
The American carrion beetle displays close mimicry with Psithyrus ashtoni, a cuckoo bumble bee. Both species have a black head, yellow thoracic disk, and a dark pile on their abdomens. The differentiation is that P. ashtoni have less pile on their abdomens. These two species have been confused for each other, with one instance where N. americana was collected instead of P. ashtoni by mistake. There are also other species of carrion beetle that mimic other species of bumble bee. Necrophorus investigator is a mimic of Bombus terrestris and B. lucorum in not only visual characteristics, but also sound.N. americana’s mimicry is not only limited to morphology; both species also emerge early in the year to begin flight, and they have similar flight patterns. N. americana has a zigzag flight pattern when searching for food, while P. ashtoni has a circular flight pattern which is typically slow and leisurely. Both species are also diurnal, flying highest during the day and lower during the night. The most likely explanations for the evolution of this mimicry are that they can avoid the same predators by sharing their pattern of coloration, and that they can use aposematic coloration to warn predators not to consume them.