Posterior Spiracles
Definition
The paired breathing openings at the hind end of a blow fly larva. Their slit count and sclerotisation state are the primary morphological characters used to assign instar: 1 or 2 slits in L1, 2 slits in L2, 3 slits with complete peritreme in L3.
- Location
- Hind end of a blow fly larva
- Diagnostic features
- Slit count and sclerotisation state
- L1 instar
- One or two slits
- L3 instar
- Three slits with complete peritreme
Common questions
Why do entomologists rely on the spiracles rather than larval body length to determine instar?+
Body length varies with feeding, temperature, and species, making it an unreliable stage indicator on its own, whereas the spiracle slit count changes at fixed moults regardless of size, giving a more consistent morphological marker for instar assignment.
What is the practical value of correctly identifying instar in a death investigation?+
Instar stage anchors the minimum time since colonisation when combined with temperature data and species-specific development rates, which is one of the main inputs used to estimate a post-mortem interval from insect evidence.
Related terms
- Accumulated Degree Hours (ADH)
- A temperature-corrected measure of developmental time. Calculated by summing (hourly ambient temperature minus the species-specific base temperature threshold) across the period of...
- Base Temperature
- The species-specific temperature below which development effectively ceases. For most forensically relevant blow fly species this is in the range of 2-10...
- Instar
- A developmental stage between moults (ecdysis). Blow flies have three larval instars (L1, L2, L3), each ending when the larva sheds its...
- Ptilinum
- An inflatable frontal sac on the newly eclosed adult blow fly, used to burst through the puparial operculum and push through soil....
- Pupariation
- The process by which the third-instar larva contracts and hardens its own cuticle to form the puparial case (puparium) that encloses the...