Cochliomyia hominivorax.  

Female, dorsal view. 
The New World screwworm fly, Cochliomyia hominivorax, is an obligate ectoparasite and will infest almost all warm-blooded livestock, wildlife and humans; it is unable to breed in carrion. These bluish to bluish-green flies have three prominent longitudinal black stripes on the thorax.
Female, lateral view. 
The face and the eyes are orange-brown. The palps are short and thread-like (filiform) and the antennae are feathered to their tips.
zzz22.jpg (1311 bytes) Female, frontal view.
Adult screwworm fly egg-laying while feeding. 
Individual females lay batches of 200 to 300 eggs in compact masses on the skin around fresh wounds. A cut, abrasion, or other insult to the skin is generally required for the larvae to invade the host tissue except in the case of body orificies; however, even in the latter case it is believed that the mucous membranes must be broken or abraded.
Batch of eggs laid at the adult feeding site. 
The eggs of Cochliomyia hominivorax are laid in batches of up to 500 eggs, with a mean of 200 per batch, at the edges of 2- to 10-day-old open wounds or in body orifices. They hatch in 14-18h.
Wound myiasis may attract carrion breeders flies. 
Within 24 h of hatching, the maggots start to feed, burrowing into the living tissue. Wounds infested by screwworm larvae become increasingly attractive to flies species which larvae are usually deposited on dead organic material or even on wounds of a live host.
Developmental larvae. 
Once hatched the larvae bunch together to feed with their posterior spiracles exposed. 
Larvae dropping from the wound. 
The larvae are fully developed  in 5 to 9 days, leave the host and pupariate in the surface layers of the soil. Wounds infested by screwworm larvae become extensive and attractive to gravid females; consequently, the syndrome is self-perpetuating in endemic areas and the usual result is death of the struck animal.
musc23.jpg (3735 bytes) Larvae dropping from the wound.

Alveolar myiasis of sheep. 
Even wounds the size of a tick bite are reported to be sufficient to attract oviposition. The precise semiochemical and chemotactile cues causing attraction and leading to oviposition are unknown, but wound fluids and blood are known to be attractive. Bacterial metabolites also may increase the attractiveness of screwworm infested wounds as oviposition sites.

Posterior spiracles of screwworm larva. 
In the screwworm third-instar larva the posterior spiracles are large, each with a prominent dark-pigmented peritreme which apparently does not completely surround the three straight slitlike apertures.