## 1 Introduction  
Insects make up a large proportion of the environment; therefore, it is not surprising that insects would be present in crime scenes. Insects play an important role in decomposition, therefore they become forensically useful in estimating the postmortem interval [1]. Insects recovered from the body can be used as a \"clock\" to determine the minimum postmortem interval (PMI\({}_{\text{MIN}}\)), or the time since the body has been available to the insects, as most species in North America will only lay eggs or larvae on a resource if it is dead [2; 3]. The most common fly that will be encountered in casework is the blowfly (Diptera: Calliphoridae). The species of the collected specimens is determined, and some measure of their age--usually the length of the larvae or the life cycle stage of the insects' development--is used to extrapolate PMI\({}_{\text{MIN}}\), referencing published developmental data (e.g., [4; 5; 6; 7; 8; 9]). To estimate the PMI\({}_{\text{MIN}}\), investigators should attempt to collect the oldest insect specimens present at the crime scene, and in some cases, these are pupae. Due to their lack of morphological characters, it is often difficult, if not impossible, to identify the species. Therefore, pupae are typically placed in an environmental chamber at known temperature and allowed to complete their development. Once they are adults, the species can be determined using published morphological keys [10]. Two shortcomings of this technique are the additional time needed for the pupae to complete development, as well as the assumption that the pupae are still viable. Any damaged or dead pupae will never be identified by this method, and few crime laboratories have the resources available to rear flies. An alternative is to extract the DNA from the pupae and determine the species typically by sequencing a mitochondrial DNA locus [11; 12; 13; 14; 15; 16; 17], however, expertise is necessary for the analysis of the DNA sequence, and a thorough representation of all possible species is required [18]. Further complicating the use of blow fly pupae for forensic postmortem interval estimations is the process of estimating the age of the pupae. Various methods have been proposed, which include an analysis of the differential expression of genes [19; 20]. However, this requires a well-preserved, high-quality sample with intact mRNA. Other proposed methods include the analysis of the internal morphology of the pupa [21; 22; 23], however, this requires a great deal of expertise, and has only been done on a single species to date. Considering these issues, experts in the field of forensic entomology have suggested chemical analyses to complement, or substitute for, traditional morphological examinations [2] and/or DNA-based methods. Such chemical analyses would be easily conducted on instruments common to all forensic laboratories, alleviating the burden on smaller labs. They would also be more objective, subject to empirical scrutiny, and able to report on their own propensity for error. Several possible methods of analysis have been proposed in recent years, but the idea that has shown the most promise relies upon the analysis of the organic compounds that can be extracted from the insect cuticle [2]. Associated with the cuticle are a multitude of chemicals--hydrocarbons, free fatty acids, alcohols, aldehydes, wax esters, and fatty acid methyl esters [24]--which serve a variety of purposes with respect to the insect's physiology and life cycle. All insects have cuticular hydrocarbons, those hydrocarbons can often be individualized to species--or, even further, to the sex of a species, to a part of the life cycle, or to a caste [25]. The evolution of such compounds, which include long-chain hydrocarbons and hydrophobic moieties, appears to have originated from the need, by insects, to retain water within their bodies and prevent desiccation [26].