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Introduction

The class Insecta contains a richness of species which is unrivaled in the living world. Insects have evolved to occupy nearly every terrestrial niche and exploit every possible lifestyle. Insects are also fantastic opportunists and are often the first organisms on the scene of a resource windfall. A perfect example of this are the insects which colonize and decompose carrion. From nearly the moment a body hits the ground insects begin taking up residence, feeding and laying their eggs on it. The faunal succession which follows is regular enough to be used to later estimate the time of death and predictable enough to be admissible as evidence in a court of law (Berenbaum, 1999).

Dating as far back as thirteenth century China, forensic entomology is the use of insect evidence in criminal and civil investigations. Modern use of forensic entomology often focuses on the estimation of the postmortem interval (PMI) using knowledge of insect fauanal successions and the developmental rates of carrion-associated insects (Berenbaum, 1999). The two groups which define the faunal succession arriving at carrion are the Diptera (flies) and the Coleoptera (beetles) (Wolff et al., 2001). The order Coleoptera contains a number of forensically significant families, namely Staphylinidae, Scarabaeidae, Carabidae, Silphidae, Dermestidae, and Histeridae (Kulshrestha and Satpathy, 2001). Many of the beetles found on remains are not there to consume the remains, but, rather, to prey on other insects which come to them. One such group are the hister beetles (family Histeridae) whose larvae and adults feed primarily on maggots developing in the remains (Crowson, 1981). The family Histeridae is of particular interest because of its effect on decomposing flies, which can lead to a false estimation of the PMI. With approximately 4,000 described species worldwide (Caterino, 2002), histerids demonstrate evolutionary adaptations to a wide variety of fascinating lifestyles. The unique biology of this often overlooked insect family provides an excellent example of insect opportunism at its finest.

Figure 1: Histerid beetle (Block carving: S. A. Stephens).

Discussion

Morphology and Physiology
The family Histeridae is a distinctive group of shinny, hard-bodied, small to medium sized beetles (Figure 1). They are characterized by geniculate antennae comprised of eleven segments with a three segmented club and shortened elytra which leave one or two abdominal tergites exposed. Their heads are usually deflexed with grasping mandibles that are visible from above. They are usually black or brownish-black with a few bronze and metallic green species. Histerids have two general body forms, cylindrical and flat, which correspond to the habitats they occupy. Cylindrical genera are found in the wood-galleries of the bark beetles they prey on. Flattened histerids tend to live under the bark of trees and include the genera associated with carrion (Arnett, 1968). Their very hard and smooth exoskeletons make histerids unlikely prey for other beetles, while fused segments and reduced sutures make them difficult to parasitize. Histerids are also capable of tightly retracting their head, legs and antennae into grooves of their integument, rendering their bodies impenetrable and compact. As an additional defensive mechanism, disturbed members of the genus Hister will fall on their backs and exude droplets of an unpleasant smelling liquid from pores on the ventral surface of their thorax and abdomen (Crowson, 1981).

Development
Perhaps the most remarkable adaptation of the hister beetles is their shortened developmental time. Unlike most beetles, histerids undergo only two larval instars before pupation into the adult stage (Kovarik, 1995). The developmental time of histerids from oviposition to adult ranges from one to two months during summer temperatures of 20-25 degrees Celsius (Crowson, 1981). Adult histerids of carrion-associated species produce large eggs which hatch into precocious larvae that are capable of capturing prey as soon as they emerge. Large larvae at the time of eclosion and a rapid developmental rate are believed to both be adaptations to the exploitation of ephemeral systems such as carrion and animal dung (Kovarik, 1995). In these ways, histerids have evolved to be adept exploiters of the short-term abundance of maggots on carrion (Crowson, 1981).

Ecology and Habitats
Members of the family Histeridae occupy many unusual ecological niches and are adapted to a variety of peculiar lifestyles. Species of several tropical genera are myrmecophiles (living in ant nests) and some are termitophiles (living in termite nests). These histerids take on a wide range of relationships with their host ants. Some are scavengers or predators of the ants and are treated with hostility by their hosts, while others feed on surplus food and have developed a more mutual relationship with the ants (Arnett, 1968). Some histerids are even known to produce appeasement substances to gain acceptance by their host ants (Gillot, 1995) .

The unifying characteristic of the remaining lifestyles is the predation of other arthropods. Many species of these histerids are found on animal dung, carrion, decomposing plant material, and other rotting organic matter. Some histerids find their prey items by living in the nests of pocket gophers, birds, foxes and other vertebrates. Others are found on sap oozing from trees to which potential prey insects are attracted (Arnett, 1968). Multiple species of Histeridae prey on destructive wood-boring pests (Bostrichoidea, etc.), but it has not been established that they act as significant control agents of these populations (Crowson, 1983). Other histerids have been demonstrated to diminish dung-breeding fly populations and have been suggested as agents for the control of these pests (Davis, 1994).

Carrion-associated Histeridae
The histerids of the greatest significance to forensic entomology are those associated with carrion. On carrion, adult and larval histerid beetles will feed primarily on dipterous maggots (Crowson, 1981). In a 1994 study of Afrotropical Coleoptera associations with decaying matter, six species belonging to three subfamilies of Histeridae (Abraeinae, Histerinae, and Saprinae) were lured to traps baited with a mixture of course-chopped sheep’s offal and rancid beef fat. In this study, it was found that although the greatest number of beetles came to traps baited with animal dung, over 10% of the histerids counted were trapped using decaying animal matter. In particular, the species Saprinus splendens was caught almost exclusively using animal matter (Davis, 1994). Histerid adults arrive at carrion slightly behind the colonization of prey items like flies. Although this reduces the histerid larval foraging window, a lag of several days is believed to increase the likelihood that larvae will encounter plentiful prey (Kovarik, 1995).

Forensic Significance
Estimation of the PMI is critical in any forensic investigation of death. Establishing how long a victim has been deceased is the first step in solving a crime. Modern forensic entomologist have learned to use the predictable nature of insect faunal successions on decomposing remains to estimate the PMI and shed light on criminal investigations. For this purpose, four categories of carrion-associated insects have been recognized: necrophagous insects, predators and parasites, omnivores, and incidental species (Wolff et al., 2001). Belonging to the predator and parasite group, Histeridae is one of several beetle families that prey on the necrophagous and omnivorous insects that are attracted to decomposing animals.

There is some disagreement as to how histerids fit into the insect faunal succession on carrion. A 2001 study of forensic entomology in Medellin, Colombia observed histerid adults at the 7-12 day stage of decay and noted no larvae until the 52-207 day dry stage. The larval faunal succession created from this study shows histerid larvae being present from 77-118 days (Wolff et al., 2001). In contrast, Kovarik (1995) observed histerids arriving at remains shortly after they were colonized by flies. Differences in the placement and timing of Histeridae in faunal successions may simply reflect their roles in various biogeographic regions.

The interaction of Histeridae with other carrion-associated insects can have a significant effect on the faunal succession and, consequently, the estimation of the PMI. A significant decrease in the number of maggots due to predation slows decay, making the body appear to have been deceased for a shorter period of time. Similarly, the presence of ants at a carcass has been shown to effect the maggot population and related insect faunal succession. Being predators of maggots, histerids are particularly effected by the presence of maggot-raiding ants (Stoker et al., 1995).

Most histerids, and other beetles associated with ephemeral systems, such as dung or decaying animals, feed on the soft-bodied larvae of insects. However, in 1996 researchers first observed histerid beetles of the species Euspilotus bisignatus feeding on adult flies. These dung-dwelling beetles were seen capturing adult flies and then consuming only the contents of their head capsules (Carlton et al., 1996). This case clearly demonstrates the need for further research and exploration into the ecology and biology of the Histeridae. As with many of the carrion-associated beetle families, histerids are a largely ignored and poorly known group. A better understanding of their life-histories is in great need and would be of significant use to the field of forensic entomology.

Conclusion

In modern forensic entomology, beetles have been demonstrated to provide significant entomological evidence. Their predictable roles in the faunal succession of insect communities on decomposing remains aid investigators in their estimation of the PMI. As a family, Histeridae are remarkably adapted to exploit ephemeral resources such as decomposing remains. Their distinctive morphology of compactable limbs, fused exoskeletal segments, and extra hard cuticle make them highly adept to the harsh and competitive environment on remains. The evolution of large eggs, precocious larvae, and rapid developmental rates in this group have suited them for the inhabitation of ephemeral resources like carrion. As forensically significant organisms, they are capable of altering decomposition rates with their predation on dipterous maggots. Knowledge of their role in colonization of remains and the resulting insect faunal succession is key to the practical use of forensic entomology. As is the case with many of the smaller coleopteran families, Histeridae is relatively unknown and often neglected by entomologists. The study of their ecological roles and biological adaptations is crucial to the advancement of forensic entomology as a reliable technique for estimating the time of death. Further understanding of these remarkable beetles will surely yield even more fascinating examples of insect opportunism at its finest.

Literature Cited

Arnett, R. H. 1968. Histeridae. In: The Beetles of the United States (A manual for identification). The American Entomological Institute, Ann Arbor, xii + 1112 pp.

Berenbaum, M. 1999. Of Maggots and Murderers. In: Hoyt, e. and T. Schultz. Insect Lives: Stories of Mystery and Romance from a Hidden World. John Wiley & Sons, Inc., New York, viii + 360 pp.

Carlton, C. E., R. A. B. Leschen and P. W. Kovarik. 1996. Scientific Note: Predation on Adult Blow Flies by a Chilean Hister Beetle, Euspilotus bisignatus (Erichson) (Coleoptera: Histeridae). The Coleopterists Bulletin. 50(2):154.

Caterino, M. 2002. Santa Barbara Museum of Natural History: Entomology Research: Systematics of Histeridae (Coleoptera). http://www.sbnature.org/collections/invert/entom/histeridae.htm.

Crowson, R. A. 1981. The Biology of the Coleoptera. Academic Press, London, xii + 802pp.

Davis, A. L. V. 1994. Associations of Afrotropical Coleoptera (Scarabaeidae: Aphodiidae: Staphylinidae: Hydrophilidae: Histeridae) with dung and decaying matter: implications for selection of fly-control agents for Australia. Journal of Natural History. 28: 383-399.

Gillott, C. 1995. The Remaining Endopterygote Orders. In: Entomology. Plenum Press, New York, xviii + 798 pp.

Kovarik, P. W. 1995. Development of Epierus divisus Marseul (Coleoptera: Histeridae). The Coleopterists Bulletin. 49(3):253-260.

Kulshrestha, P. and D. K. Satpathy. 2001. Use of beetles in forensic entomology. Forensic Science International. 120: 15-17.

Stoker, R. L., W. E. Grant and S. B. Vinson. 1995. Solenopsis invicta (Hymenoptera: Formicidae) Effect on Invertebrate Decomposers of Carrion in Central Texas. Entomological Society of America. 24(4):817-822.

Wolff, M., A. Uribe, A. Ortiz and P. Duque. 2001. A preliminary study of forensic entomology in Medellin, Colombia. Forensic Science International. 120:53-59.

©2003 Stephanie A Stephens