Chapter 13 — Exercises

Work these without looking back at the chapter first; then check yourself. Items marked † have full worked solutions in the answers appendix. There are no answers in this file. Mix of recall, applied reasoning, evidence interpretation, "spot the overstatement," ethics, and a cold-case extension.

A. Recall and definitions

  1. Define forensic entomology in one sentence, and state the single most common question it is used to answer.

  2. Distinguish the postmortem interval (PMI) (Chapter 11) from the minimum PMI that entomology estimates. Why can entomology usually deliver only the second?

  3. † What is accumulated degree days (ADD)? Explain the "casserole" intuition (a fixed total amount of heat delivered fast or slow) in your own words.

  4. Define succession as used in forensic entomology, and explain why it is the better clock for longer postmortem intervals while larval development is better for shorter ones.

  5. Define forensic botany and palynology, and state the one principle from Chapter 3 that both rest on.

  6. Name the family of flies (the metallic green/blue picnic flies) that are usually the first insect colonizers of remains, and state roughly how quickly they can arrive under warm conditions.

  7. What is a developmental threshold temperature, and what happens to a day's degree-day contribution when the average temperature falls below it?

  8. Define entomotoxicology and give the two opposite reasons it matters (one a threat to an estimate, one an opportunity).

B. Applied reasoning and calculation

  1. † The oldest larvae on a body are a species that published data say require about 150 ADD (above a $10^{\circ}\text{C}$ threshold) to reach the observed stage. The scene-corrected average temperature was about $20^{\circ}\text{C}$. Estimate the minimum PMI in days, showing your arithmetic. Then redo it for an average of $13^{\circ}\text{C}$ and comment on what the comparison teaches.

  2. Explain, step by step, how an investigator reconstructs the temperature history at a body that was not discovered until days after death. Name the specific instrument placed at the scene and what it is compared against.

  3. A large maggot mass was present on a body. Did its self-generated heat tend to make the naive (ambient-only) ADD estimate too long or too short? Explain the mechanism.

  4. † A body found in a city alley carries larvae of a species that, in that region, lives only in rural woodland. What is the most parsimonious inference, and what kind of forensic question (from Chapter 2) does it answer? State the honest verb for the conclusion.

  5. Why are live larval specimens collected and reared to adulthood, in addition to specimens killed and preserved at the scene? What error does this guard against?

  6. A pollen profile from a suspect's vehicle matches a region dominated by a single very common tree. A second case has a match driven by a rare, localized plant. Explain why the second is much stronger evidence, by analogy to common vs. rare blood types.

  7. Sequence these insects/stages on exposed remains from earliest to latest, and name the body state each is associated with: dermestid (hide) beetles; third-stage blow-fly larvae; blow-fly eggs in the openings; mites and moths on dry remains.

C. Evidence interpretation

  1. † Re-read Figure 13.2 ("The same maggots, three different stories"). The larvae appear more developed than the weather record predicts. List the three candidate explanations the figure gives, and state the correct methodological response to the discrepancy (what should happen to the estimate?).

  2. An entomologist's report reads: "Based on larval development, the decedent was colonized by insects no fewer than approximately 9–13 days before discovery, assuming the temperature reconstruction and species identification are correct." Identify three separate things this sentence does well (i.e., where it is appropriately honest).

  3. Figure 13.3 ("Pollen on a floor mat") concludes the vehicle is "consistent with" having been at the cabin environment. Write the single sentence an honest expert could say on the stand about this evidence, and the single sentence that would be an overstatement.

  4. A burned body (as in the cold case) yields a minimum-PMI estimate from the few insects present. Explain why this estimate is treated as corroborating, not leading, and state the likely direction of its error (over- or under-estimate of time since death) and why.

D. Spot the overstatement / junk-science alert

  1. † A prosecutor's slide reads: "Entomological analysis establishes that the victim died 9.7 days before discovery." Identify two distinct problems with this statement and rewrite it honestly.

  2. An expert testifies: "The pollen on the defendant's boots proves he was standing in this exact clearing." Name the specific overstatement (what is being claimed that pollen evidence cannot support) and give the defensible version.

  3. A television detective examines a single maggot with a hand lens and announces the hour of death. Using §13.1, explain two reasons this is backward from how the method actually works.

  4. A report computes a precise time of death from insect evidence but never mentions error bars, the species' identity, or the temperature correction. List the missing elements that should make a reader distrust the number.

E. Ethics and reasoning

  1. † An entomologist is told the "expected" date of death by the lead detective before examining the specimens. Using the Cognitive-Bias Watch in §13.2 (and previewing Chapter 31), explain the risk and describe the safeguard. Is an insect estimate computed blind worth more or less than one computed knowing the answer — even if they happen to agree?

  2. Two qualified entomologists examine the same evidence and reach different time-since-death windows that nonetheless overlap. Is this a scandal, or is it consistent with how the method works? Explain, referencing the sources of error in §13.3–13.4.

  3. You are asked to testify that pollen evidence "individualizes" a location the way DNA individualizes a person. Explain why you should decline, and what you can honestly say instead.

  4. Why is the relative scarcity of qualified forensic palynologists, and the thin error-rate literature for forensic botany, itself a fact a careful court should weigh — even when the underlying plant science is sound?

F. Synthesis and validity spectrum

  1. † Place these four methods, justifying each, on the NAS 2009 / PCAST 2016 validity spectrum (strong → discredited): single-source nuclear DNA (Chapter 7); accumulated-degree-day estimation (this chapter); forensic palynology (this chapter); bite-mark comparison (Chapter 16, previewed).

  2. Explain how the autopsy finding of no soot in the airways (Chapter 11) and the entomological caution about fire-delayed colonization (§13.4) are consistent with each other in the cold case.

  3. In one paragraph, explain how this chapter advances at least two of the book's four themes (exclusion over proof; the validity spectrum; cognitive bias; the CSI effect cutting both ways). Name which themes and how.

G. Cold-case extension

  1. † Cold Case. Using only what the pollen-on-the-floor-mat evidence establishes, write the entry you would add to the Mill Creek evidence log (Appendix I). State (a) the defensible inference at its true strength, (b) the honest verb, (c) at least three things this evidence specifically does not establish, and (d) why you decline to attach a person's name to the vehicle at this stage.

  2. Cold Case extension. Suppose investigators later want to strengthen the botanical association of the vehicle with the cabin. Propose two concrete additional steps (think: control samples, distinctiveness, corroboration with other evidence types already in the file) that would do so honestly, and explain what each adds.

  3. Cold Case, integrative. The pollen evidence "places a vehicle at the scene" but not a driver. List two other evidence types from earlier chapters that, combined with the vehicle association, would begin to connect a person to the cabin — and state plainly why the pollen alone cannot make that leap.

H. Short writing

  1. In 150–200 words, explain to a juror why an entomological "minimum time since death" is genuinely useful evidence and why it is never the same thing as "the time of death."

  2. † In 150–200 words, contrast the validity foundations of accumulated-degree-day estimation with those of forensic palynology: what does each rest on, where is each strong, and where is each most easily overstated in court?