Bot Fly Larvae, as a group, are not one of the more well-loved insects. However, they are quite interesting and wide-spread, and parasitic in many species of mammals as well as humans.
After mating, Bot Fly females will lay 1200-4000 eggs, using chemical cues to locate sites where a future host will visit often, such as a small mammal burrow or runway. In 5-7 days, the larvae will hatch in response to sudden changes in temperature or CO2 concentration, likely heralding the presence of a host. However, all of the eggs in the clutch will not hatch at the same time, probably to reduce mortality if the host turns out to be unsuitable. The larvae can live only several hours outside of a host. The larvae, when they first emerge from the 
eggs, are elongated, small, and thin. If they are successful, they will enter the host through the eyes or nose, or through a fresh wound. They are positively thermotactic, always moving towards heat. Within two or three days, the larvae have moved to the nasal passages or the esophagus. From there, they penetrate their host. Within 3-6 days after entering, the larvae are in position just below the skin. Depending on the species, they tend to occupy different positions on the host’s body.
During the following 25-30 days, the small larvae will enter their second instar, during which they become fairly robust and have well-developed spines covering their body. They will feed on the sera and white blood cells of their host, allowing them to grow quickly. They will continue eating that as they enter their third and final instar, readying them for emergence. They will then be large, robust, and cylindrical. Third-instar larvae are generally dark in coloration and covered with small spines. Some species are huge, up to 42 mm in length at emergence. In fact, is has been estimated that the mature third instar larvae are 100,000-fold larger than the small, first instar larvae.
The larvae emerge through the skin of their host, and immediately burrow into the ground and pupate within two days. They may pupate for from only 28 days up to 11 months, depending on the temperature. Pupae in temperate areas can have diapause (a pause in development) that allows them to overwinter. They emerge as the adult fly.
Bot Flies are large, robust flies with prominent eyes. They are often sexually dimorphic, with distinctly different coloration on males and females. They possess mouthparts, but they are vestigial, as the adult fly never feeds. Indeed, the entire life span of an adult fly rarely exceeds two weeks. The males gather on terminal stems of vegetation to await females. As receptive females pass, the male will take flight and mate in the air. Several days later, the female will lay her eggs in a spot frequented by a host mammal.
These larvae are not very harmful to their host, although they can be a threat to young hosts. This species typically attacks the White-footed Mouse, although members of its genus parasitize anything from rabbits to chipmunks. Humans are victims to a different genus.
This Bot Fly Larvae was found by Courtney, emerging from a captive mouse. It will probably pupate as soon as it is given the chance.
Article by Hazel Galloway
View images of adult Rodent Bot Flies here, courtesy of Bugguide.net.
Sources:
- www.michigan.gov/dnr/0,1607,7-153-10370_12150_12220-26354--,00.html
- Samuel, William, Margo Pybus, and A. Kocan. Parasitic Diseases of Wild Mammals. Iowa State University Press, 2001.