Here is a series of close up images of a green Blow Fly that came to rest on a Prickly Pear Cactus, Opuntia engelmannii
. She is most likely a female Lucilia mexicana
Full Size Image: Click Here
Keying to species in the genus Lucilia: (Whitworth 2006)(1)
Some parts of the key are not visible in these images and I'm skipping some of that here in these notes; though I did look for or actually saw most of it anyway.
Also, most of the other look-alike species choices of the first part of the key can safely be ruled out, due to the range being outside of my south-western location.
Sex - Female (wide eye-gap & angled or kinked inner eye-lines = ♀)
1. - Ocellar triangle of female small, not reaching halfway to lunule = 2
2. - Length of head at level of lunule less than half head height; third abdominal tergite with marginal setae not especially strong or erect = 5
5. - Two postsutural acrostichal setae; abdomen usually uniformly metallic or microtomentose = 6
6. - Basicosta usually black or dark brown, sometimes lighter ground color, but always with dusky shading (mostly southern distribution) = 7
7. - Two or more complete rows of postocular setae; genal dilation and parafacial mostly black, with black vestiture; female frons broader, avg. = 0.28/6 head width, at narrowest (primarily south-western U.S. into Mexico) = species mexicana Macquart 1843