Wading for Bugs: Exploring Streams with the Experts

11. A Criminal "Case" Made with Caddisflies

61 11 A Criminal “Case” Made with Caddisflies John R. Wallace and Richard W. Merritt Approximately twenty years ago, I was crawling around a small stream on South Mountain near Shippensburg University in south central Pennsylvania with my master’s advisor Fred Howard, studying the life history of caddisflies. Little did I know that what I was learning would come in handy during a murder investigation in south central Michigan. I became interested in how aquatic insects could be used in forensic investigations while working on my PhD at Michigan State University with my major professor, Rich Merritt. My interests have pursued the unusual and somewhat bizarre direction of forensic entomology ever since. The arrival time, development rates, and succession of terrestrial insects on many food resources is fairly predictable. Because of what we knowabouttheirlifehistories,physiology, and behavior, experts can analyze the colonization of wood consumed by termites, foodstuffs invaded by beetles or moth larvae, and dead bodies that attract a host of other insects. This makes insects very useful to forensic studies in estimating the interval of time since colonization. However, aquatic insects have not truly evolved to feed on carrion. For the most part, they utilize corpses as extensions of stream substrates. In fact, their presence on a corpse may, in many cases, be happenstance. On land, terrestrial insect colonization of human remains has been intensely studied, but using aquatic insects in these types of investigations is severely limited. We 62 Sleuthing for Caddis know that the key for precisely estimating the time from when a body was submerged in water to the time of discovery (which we call the Postmortem Submersion Interval Estimate, or PMSI) is to understand events in the life history of the colonizing insect. I became especially interested in the phenology, or timing, of specific life history events for aquatic insects found on human remains in aqueous environments. Our case in point: around the second week of June 2005, the unidentified remains of a male victim, partially buried in cement and placed in a duck decoy bag, were recovered from the Red Cedar River, a river we fished and canoed during my PhD program. When several large case-building caddisflies were collected at autopsy, Rich Merritt called me because of my expertise with this group of aquatic insects. My task was to determine how long the caddis might have been on the body. The victim was reported missing on March 21st, and the remains were discovered on June 9th; however, it could not be determined when he died or when he was placed in the water. Without considering the aquatic insects on the remains, the forensic team estimated that his remains might have entered the stream any time from when he was last seen to when his body was discovered. I identified the caddisflies found on the remains to be Pycnopsyche guttifer and Pycnopsyche lepida, belonging to the family Limnephilidae. Pycnopsyche hatch within a gelatinous substance that is deposited by adult females along stream banks close to the water’s edge. This jelly-like matrix oozes, with the first instar hatchlings in tow, into the stream. During the first instar, larvae feed on the bacterial/fungal layer that forms on leaf surfaces. Hatchling larvae construct cases out of bits of debris, but they gradually incorporate small leaf discs cut out during the second through fourth instars. As they grow, their diets incorporate entire leaves that are skeletonized by their mandibles. Sometimes the cases are flat; others are triangular in cross section. Like their close terrestrial relatives in the order Lepidoptera (butterflies and moths), caddisflies produce silk that has many functions during their life spans. Interestingly, Pycnopsyche larvae switch their case materials and microhabitat A Criminal “Case” Made with Caddisflies 63 as they enter the fifth instar. In this last stage, they either construct cases of sticks for slow-water habitats or build cases with stones to move towards fast water. These cases serve two functions: firstly, they act as ballast in the stream current to maintain the larvae’s position in the stream, and secondly, they function as camouflage, blending with other woody debris or small stones in the stream. Thus the larvae are protected against both aquatic (fish) and terrestrial (avian) predators. I learned during my master’s degree research that Pycnopsyche in Pennsylvania switch their case materials in early- to mid-March and continue to feed through May. By June...