The use of wing fray and sex ratios to determine the origin of flies at an indoor crime scene

•Using adult flies collected from indoor crime scene to assist in timeline.•Source of adult flies determined using wing fray and sex ratio.•Creation and use of wing fray measurement technique was efficient and effective.•Emergence flies have less wing fray damage and are male dominant.•Wild flies have more wing fray damage and are female dominant. When adult flies are collected at indoor crime scenes, the question of their origin arises, i.e., whether they originated from the outdoors, coming in to oviposit on the body, or if they developed through the larval and pupal stages from eggs laid on the body. This is particularly important if no empty puparia were collected, potentially because they were overlooked, with a consequent impact on the accuracy of the minimum post-mortem interval estimation. This study used two methods to determine if flies sampled in various experimental conditions and at an actual crime scene were wild flies attracted to the body or had developed on the body, i.e., through the assessment of: (1) wing fray and; (2) sex ratio. A template was created to measure the amount of wing fray damage quantitatively in a simple, rapid and efficient manner. The experiments demonstrated that by combining the two methods it is possible to establish the origin of a population of adult flies at a crime scene.