Necrodes littoralis, also known as the short sexton beetle, is a species of carrion beetle of the genus Necrodes, found in countries across Europe. As a carrion beetle, it feeds on decaying vertebrate remains and maggots. This species' feeding behaviors make it an important asset to forensic entomology.
Description Adults of N. littoralis grow to be 15 to 25 mm (0.59 to 0.98 in) long and have a shiny black body. The beetles can be identified by a characteristic bump about three quarters down the length of their elytra, a hardened forewing that act as armor to protect the beetle from environmental factors and predators. Beetles of the order Coleoptera, like N. littoralis, have truncated elytra. The reason why some beetle species have adapted a shortened protection armor is unclear. The larvae of N. littoralis are campodeiform, meaning they have a flattened body, antennae, and have well-developed legs.
Distribution N. littoralis has a Palearctic distribution, but most observations are reported across Europe. They have been observed in Austria, Hungary, Slovakia, Czech Republic, Belgium, France, and England. Outside of Europe, N. littoralis has been observed in South Korea.
Etymology The Latin prefix necr- denotes corpse, and the Latin word littoralis denotes a coastal environment, which can be misleading given that the beetles are not found solely by the coast. In fact, the beetles are mostly seen in woody areas or fields.
Habitat Adult beetles and larvae are primarily found on large carrion in the late stages of decomposition. Further, the carrion is primarily found outdoors. However, there have been observations of N.littoralis on carrion that are indoors. Researchers hypothesize that the beetles have difficulty accessing decaying bodies indoors as they cannot easily detect openings in buildings. Researchers from Italy reported the presence of N. littoralis on a human corpse in Italy for the first time in 2021. The decomposition conditions of the corpse at the time of discovery align with the consensus that the beetles inhabit corpses at later stages of decomposition. The corpse was found indoors, which the researchers explain that "the access to the building through the open door and the state of total neglect of the area where the corpse was found ... may have favored the indoor colonization by N. littoralis."
Social behavior Studies were conducted to elucidate the benefits of the aggregation behavior adopted by N. littoralis. Data on mortality, rate of growth, size was collected to for adults raised individually and compared to results obtained from beetles reared in aggregations. Scientists discovered that aggregations amongst larvae especially led to decreased development time, reduced mortality, and these beetles even grew to be larger. Temperature was a confounding variable that influenced these results as well. Lower temperatures of approximately 16 °C (61 °F) was the ideal condition to observe the greatest results from aggregative behavior. Larvae adjust for fluctuations in temperature by moving to other locations or raising the temperature within feeding aggregations. Group living has its benefits for these beetles because it makes foraging easier and creates a stronger defense against predators. These behaviors are important to understand because they impact N. littoralis's capacity to survival and develop.
Life cycle Mating of adult beetles typically occurs at night. Following mating, the female lays eggs in the ground near the carrion. Although there is variation in the exact number, females lay between 50 and 70 eggs at a time. Researchers who studied the instar development of N. littorialis explain that the beetles have three larval stages. Another study reveals that the three larval stages are first instar, second instar, and third instar. Post-feeding larva, nymph, and imago are parts of the developmental stage of larvae into adult beetles. The first instar larvae are creamy white when they hatch and shift toward the carrion for food. First instar larvae are the most vulnerable. The second and third instar larvae are also creamy white after ecdysis, which is the process of insects shedding their exoskeleton. As part of the transition from third star larvae to post-feeding larvae, the third star larvae burrow into the ground and form pupal chambers "thrashing the abdomen and thus compacting the soil around them. They go through the prepupal, pupal, and teneral adult stages inside the chambers." The beetles emerge from the pupal chamber after "they become fully sclerotized and colored."
Larvae behavior The larvae of N. littoralis aggregate, as they form large and orderly groups of larvae on the carrion. In these large groups, the larvae feed on the carrion. Researchers who are interested in exploring why the beetle larvae group for feeding performed an experiment to test the "importance of thermal cues and ground-deposited chemical cues for the aggregation behavior..." The experiment involved field data and lab tests. The field data consisted of analyzing the larvae growth results of previous experiments with pig carcasses. The lab tests involved collecting adult beetles, allowing one male to mate with one female, allowing the larvae to grow, and placing them on a sample carcass set-up to observe aggregation behavior. The study found that N. littoralis larvae formed aggregations around a heat source of the carrion, which demonstrates the importance of stable thermal conditions for the larval aggregations. The experiment also found that if the heat source moved, the larvae aggregations followed by disassembling and forming a new aggregation around the new heat source. Stable thermal conditions are important for the development of the larvae into adult beetles. Notably, larvae in the later stages of development, specifically third instar larva, prefer to aggregate in cooler temperatures. The authors hypothesized that this preference allows the larvae to grow larger, though this growth happens more slowly. The third instar larva phase begins the transition to the post-feeding phase, which is when the largest larvae were observed in another study. Additionally, the aggregations form around parts of the carrion that are favorable for feeding. The data from the experiments did not support the ground-deposited chemical cues as an important motivation for aggregation behavior.
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