Technical note: A rapid, non-invasive method for measuring live or preserved insect specimens using digital image analysis

Abstract

The measurement of insects is an important component of many entomological applications, including forensic evidence, where larvae size is used as a proxy for developmental stage, and hence time since colonization/death. Current methods for measuring insects are confounded by varying preservation techniques, biased and non-standardized measurements, and often a lack of sample size given practical constraints. Towards enhanced accuracy and precision in measuring live insects to help avoid these variables, and that allows for different measurements to be analyzed, we developed a non-invasive, digital method using widely available free analytical software to measure live blow fly larvae. Using crime scene photographic equipment currently standard in investigation protocols, we measured the live length of 282 Phormia regina larvae. Repeated measurements of maggots, for all instars, were performed for several orientations and images. Most accurate measurements were obtained when maggots were oriented in their natural full extension. Killed specimens resulted in greater length measurements (Mean 1.79 ± 1.11 mm) when compared to live length. Herein, we report a technically simple, fast, and accurate measurement technique adapted for field and lab-based measurements, as well as, a simple linear equation for conversion of live length to standard killed length measurements. We propose this method be utilized for the standardization of forensic entomological evidence collection and development model creation. Keywords: Forensic entomology, Digital image measurement, Morphometric, Insect size, Phormia regina, Forensic photography