Case #451 - September, 2017

Two maggots were retrieved from a wound in an elderly woman living in rural Manitoba. Images were taken (Figures AC) and sent to the DPDx Team for diagnostic assistance. What is your diagnosis? Based on what morphologic features?

This case and images was kindly provided by the Cadham Provincial Public Health Laboratory, Winnipeg, MB

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Figure A

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Figure B

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Figure C

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Figure D

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Figure E

This was a case of facultatitive myiasis caused by either Phormia sp. or Lucilia sp. Morphologic features include:

  • Presence of a weak and incomplete peritreme (Figures D and E).
  • Spiracular slits that point towards the opening of the peritreme (Figures D and E). Sarcophaga slits are angled somewhat away from the opening.
  • In at least one image the maggot appears to be ready to molt: it is possible to see 3 slits, indicating a 3rd instar, below two slits (2nd instar) of the spiracular plate (best shown in Figure E).
  • Since this specimen is in-between 2nd and 3rd instar development, the peritreme may not be fully developed and the presence of the button not clearly evident. The morphology of these two features as well as the morphology of the mandible are critical to determining if the maggot is that of Phormia sp. or Lucilia sp.

Both Phormia sp and Lucilia sp are species that colonize wounds or diseased tissue and feed only on dead tissue. These types of flies have forensic importance in computing the time of death of a victim and medical importance in maggot therapy.

Please note that most identification materials, including the CDC guide to identifying flies of medical importance, only address adult flies and 3rd instar larvae. Accordingly, it is more difficult to identify 2nd instar larvae and identification is best left to a trained entomologist.

More on myiasis:

Images presented in the monthly case studies are from specimens submitted for diagnosis or archiving. On rare occasions, clinical histories given may be partly fictitious.

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Page last reviewed: December 5, 2017