Answer to Parasite Case of the Week 591: L3 larva of Phormia species
As Blaine mentioned, this is a case of facultative myiasis involving an existing wound. The wound was caused by removal of the eye due to a large cancerous growth. What I didn't share is that the growth had been neglected, and the wound may have been as well. Several of you mentioned that this is a third stage (L3) larva, which indicates that the larva was allowed to mature for sometime within the wound.
Although we can't see the tracheal trunks to say if they are pigmented or not, the posterior spiracular plate is clearly visualized and shows an incomplete peritreme, relatively-straight slits without lateral swellings, and slits are directed towards the region where the peritreme is incomplete, consistent with Phormia sp.
Some of you mentioned Lucilia sp. in the differential, although in my interpretation, Lucilia has a complete peritreme with distinct button, unlike what we are seeing here.
The CDC has freely-available pictorial keys online which are a useful starting place when examining fly larvae. Blaine and I also published a guide for Laboratory Identification of Arthropod Ectoparasites. If you don't have access to manuscript, send me an email and I can send you a copy for your own personal use.