Background
Insects have played vital roles in providing evidence in legal cases.2 Entomologists use standard insects with well-documented life cycles in relation to the decomposition process. If only one type of insect existed, then entomologists would be unable to use characteristic behaviors to calculate the post-mortem interval (time of death). As such, insect speciosity is essential to crime scene investigations. For this, a general understanding of the factors that have led to multitudes of species is beneficial. This content overview presents four sub-sections: evolutionary biology, insect diversity, introductory classification, and the roles of insects in criminal investigations.
Evolutionary Biology
The primary emphasis of evolutionary biology is to examine the processes that contribute to the diversity of species from a common origin through descent. Even in the early history of insects, diverse species existed, filling ecological niches from areas heavy in vegetation to watersheds3. Each of these insects is generally suited to its environment, with features that increase the likelihood for survival. These features are known as adaptations. Discourse on specific details of adaptation is widely available, and many classic curricular units related to those topics exist, hence will not be addressed herein.
Besides adaptations that sometimes can be used as characteristic features of an organism, all organisms including insects express at least minimal degrees of variation, spanning across qualitative differences such as color range and color patterns. Quantitative differences may include overall size, body mass, and resistance to stimuli. It is well-established that individuals within a population can vary. Whether it was in Charles Darwin’s original collections during his voyages as a naturalist with Captain FitzRoy on the HMS Beagle or in the color patterns of urban insects, visually observable differences exist. This understanding that all organisms exhibit some variation is among the four tenets of natural selection as popularized by Darwin. Even the variations in features invisible to the naked eye, such as human blood type, provide examples of natural variation. Being observed and documented, natural variation was accepted even before the notion and discovery of DNA and genes, although these minor differences originate on a genetic level. Each individual has slight differences in the genetic code (genotype). These slight differences may be expressed in the physical characteristics (phenotype) of an organism, such as wing length or keenness of eyesight. As part of the genetic code, these minor differences are heritable in a population, meaning that it can be passed down from parents to offspring.
Physical traits that confer a survival, and hence reproductive, advantage allow the genotypes that coded those traits to be passed down to subsequent generations. The ability to survive and reproduce (described using the biological term fitness) differs for each member of that species. As a result, the survival and reproduction rates become non-random, and some traits are perpetuated in the population. Through this selection process across very many generations, some descended forms will no longer resemble those of the original ancestral genotype or species due to various selection pressures. Natural selection describes the process for changes within a population, ultimately being one of the mechanisms that can lead to speciation.
Species themselves are defined as distinct once they are no longer able to interbreed to produce fertile offspring. The process of speciation requires selective pressures that create a genetic isolation that has resulted from range expansion and physical boundaries or limitations4. Whether this occurs through natural selection, sexual selection, artificial selection, genetic drift, bottleneck effects, or other evolutionary forces, some groups of organisms exhibit more speciosity than others.
Speciosity, sometimes referred to as species richness, describes the propensity for some groups of organisms to be more speciose (having more species represented in the group) than others. Plants, for instance, can be subdivided into cone-bearing plants and flowering plants, of which there are many more varieties of the latter. The factor that is attributed to flowering plants being more speciose is the flower itself, as a structure that can easily change through natural selection to match its environment and ultimately lead to new species formation. In animals, the arthropod limb is similarly considered easily molded by selection, thus contributing to speciosity in insects and other arthropods.5
Evolutionary biology includes a distinction between proximate and ultimate variables. As indicated by the term, proximate variables address actions or features that involve an organism’s immediate lifetime, such as reproductive rate. Ultimate variables delve into the evolutionary history of an organism and hence providing reasons that support the proximate variables. The difference between proximate and ultimate variables is thus marked by the relative time that is being considered. Questions derived from proximate variables regarding insects would be: How do insects follow scents? Or: What types of insects are attracted to corpses? An ultimate question would be: Why are there so many different types of insects?
Insect Speciosity
The fossil record indicates that insects preceded humans on this planet.6 Hexapods that included early proto-insects are in the fossil record as early as 411 – 407 million years ago, and likely descendants of these non-true insects are silverfish. Despite representing over half of the species that have been catalogued, explaining the richness of insect diversity posed problems. Peter Mayhew tabulates eighteen contributing hypotheses to explain why are there so many insects, of which four will be described in this background information. Ultimate factors pertinent to the morphology and ecology of hexapods fuel the proximate factors: clade age, high speciation rate, low extinction rate, and carrying capacity. Together, these generate the pattern of insect speciosity.7
Insects are members of a group of organisms known as arthropods, which includes commonly recognized animals such as spiders, centipedes, crustaceans, and insects. The term arthropod derives from Greek origins meaning “jointed feet”. Each of its sub-categories is distinct. Spiders, for instance, have eight legs and a different body scheme from insects. Crustaceans not only include crabs and shrimp, but also some land creatures such as the pill bug, which all also differ from insects in various ways. Each of the aforementioned animals has different appendages with specialized features, whether in the form of legs, pedipalps, feelers, or wings. Research suggests that through diverse adaptive radiations, appendages are easily modifiable into a variety of forms.8
Insects of related categories tend to have similar shapes and structures on their bodies; however, some of these features are noticeable only during embryonic development. The labrum, for instance, is a structure associated with an embryonic fusion process that may have actually been remnants of an appendage.9 Although that particular piece no longer exists as an appendage in most insects, there is still residual evidence for it.
In general, the small size of insects increases their ability to inhabit myriad fine-grained (small-sized) niches. Speciation rates can be driven by the opportunities to fill niches on the macro-evolutionary level.10 Anecdotally, a colleague who breeds Madagascar hissing roaches (Gromphadorhina portentosa) once noted that he found young insects everywhere from his kitchen drawers to his bedroom, even though he housed his adult breeder beetles in terrariums with lids. In this case, if somehow these roaches became completely separated from others for sufficient generations, the isolated population could perhaps undergo its own selection pressures that ultimately give rise to new adaptations. This scenario is plausible because the environment provides extensive habitats and ecological niches relative to the body size of most insects. Conversely for other arthropods such as coconut crabs and giant isopods, members of those groups exceed several centimeters in size, and neither crustaceans nor isopods are nearly as speciose as insects.
Besides small body size, the relatively short generation cycles of many insects increase opportunities to recover from disturbances such as environmental changes.11 When genes are shuffled during mating this creates variation; hence, species that have rapid generations can be favored in rapidly changing environments because abundant variants are available for the evolutionary process of natural selection to keep pace with changing habitats. However, survival of many offspring in each generation might consequently drive organisms to undergo dispersal to reduce local competition for resources. Slightly different from the idea of dispersal, range expansion is a component of speciation because it increases the likelihood that populations will become sufficiently isolated to prevent gene flow and interbreeding.
Taxonomy
With so many documented insect species, specialists must be able to recognize their features, ecology, and behavior. Whether seeking insects to perform decomposition functions or tracing evidence of illegal poaching and distribution, the geographic ranges and species diversity thus become cornerstones of forensic entomology.
Taxonomy refers to the classification of organisms. Carl Linnaeus developed what is now known as the Linnean system. Although genetically “truer” phylogenetic relationship systems now exist12, as well as the higher classification of Domain, only the conventions of the traditional Linnean system will be used for this curricular unit. In the Linnean system, organisms are sorted into the categories Kingdom, Phylum, Class, Order, Family, Genus, and Species, in which each descending group is a smaller subset of the previous. For instance, a Kingdom is a group of closely related Phyla (the plural form of Phylum). A Phylum is a group of closely related classes, and so on. Following that, Class will tend to be what a layperson uses in common reference, as in the class Insecta, simply as insects. A Linnean order subdivides Classes into the household names that a person may recognize, such as “beetles” for Coleoptera or “dragonflies” for Odonata. Those who recognize specific beetles, such as a ladybug or a firefly (for which, incidentally, ladybugs are not true bugs and a firefly is not a fly), are using Family, Genus, or Species names. Species refers to the most closely related organisms that can still interbreed to produce fertile offspring. Categorical charts that list each of the insect orders and representative members are easily available from textbooks and online resources, and will not be listed within this overview since the study of any particular insect should vary by location.
Taxonomical knowledge within the context of entomology is also important since not all species are located in all parts of the world. Knowing similarities enables comparisons to be made using regional species for assessment purposes. For instance, the Calliphoridae family of insects (the blowflies) are used for determining time of death. In Köln, Germany, the blowfly species Lucilia ampullaceal is well-documented, whereas in California (United States) Lucilia sericata is standard. Each of these species is found in particular climate conditions and function analogously to a calibrated piece of equipment when its life cycle is measured in conjunction with weather data.
Forensic Entomology
Post Mortem Interval
Pairing the notion of insects and crime conjures the image of maggots (fly larvae) wriggling in a corpse. This image is not entirely untrue, but it neglects the eventual presence of beetles and moths, where together, these three orders provide key evidence for determining the post-mortem interval. The decaying process emits different chemicals at specific points in time, hence has time-markers for the attraction of certain insect clientele13. The combined information of the insects present provides clues regarding time of death (or child neglect, etc.) and discovery by officials.
The green bottle fly (Lucilia sericata), a common blowfly, provides a well-documented and consistent life cycle of value to forensic entomologists. Blowflies (Calliphoridae family) are fairly ubiquitous14. When used in conjunction with daily weather data, the life cycle of this species enables entomologists to calculate a reasonably precise date on which a corpse was first exposed to the environment. This initial exposure is known as the minimum post mortem interval and contributes to clues that piece together an investigation. The life cycle spans approximately three weeks; the presence of only eggs indicates an exposure of potentially only hours. The mass and feature development of the maggots correlates to the number of days during which the maggots have been able to feast upon the body. Moreover, disturbances to the body, such as its movement from a different location, would consequently be reflected in disruptions to the blowflies’ development.
Fly evidence can also be used to exclude scenarios of some crime scenes that involve blood spatter evidence that was initially flagged as violent crime15. Blowflies consume blood but they also regurgitate it as part of their feeding habit. This behavior can produce patterns that appear similar to high-velocity blood spatter, characterized by small circular droplets, typically less than 1 mm in diameter. If these spatters are present, a cursory crime scene investigation may suggest there was slinging around of blood or a possible gunshot wound. If the bloodstain pattern is ultimately attributable to blowflies, then it could negate the latter possibilities. The minute differentiation of the blood spatter would be determined by analysis of whether most blood spatter tails point in one direction or whether the tails point toward multiple directions. Since fly meal timing can be estimated or calculated, the presence of regurgitated blood provides additional clues regarding the time of death.
Beetles, scientifically known as the order Coleoptera, generally arrive at a corpse after decomposition has progressed.16 Unlike flies, beetles have a mandible that has the ability to cut flesh that has softened sufficiently. Additionally, some beetles may be attracted by the presence of fly maggots upon which they feed. The last beetles to arrive, if regionally present, will be those of the Dermistid family. Dermestid beetles consume dried skin and animal hide, and some species within this family have enzymes that break down keratin, the key component of hair and fingernails. The range of characteristics from mandible shape to enzymes confers survival advantages to beetles since they consequently do not compete for the same resources.
Moths and butterflies, Lepidoptera, are associated with advanced stages of decay.17 Moths and butterflies are differentiated by foraging habits and body structures. Nocturnal foraging is more common for moths, whereas butterflies tend to be more active in the day. Moths have feathery antennae, and butterflies have club-ended antennae. Wing frenulums, a small connective tissue between the top and bottom wing, are more characteristic of moths rather than butterflies. Naturally, exceptions to each trend listed above exist for some species within each family. In the larval stages, several Lepidoptera consume mammalian hair or the mites associated with the hair. Eggs are often laid on exposed bone, especially when near foliage.
Poaching
Crime scenes do not necessarily involve violent crime and homicide and it is important to emphasize the range of crimes that actually exist. The term poaching often conjures images of large, charismatic animals such as lions, tigers, and bears (oh my), but insects are often captured and killed illegally. Common targets are butterflies and beetles with lustrous wing coverings. The Lacey Act18 bans “interstate and international transport of endangered or protected species that have been illegally captured”, but enforcement is not strict for insects. Full enforcement raises the concern that even innocuous collectors, museums, and researchers could be prosecuted if their specimens were collected illegally. Entomologically, it is important to be able to identify various species, sometimes a daunting task because some males and females of the same species are vastly different in size and color.
Collectors of rare insects may jeopardize the success of wild populations. Although the typical large population sizes of insects cause a tendency for them to be less threatened by extinction relative to quadrupeds, ecological concern should still be exercised.19
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