The Torsalo (or "human bot fly") is a member of the family Oestridae. It is native to the New World tropics where it is regarded as a pest because its larvae burrow under skin and live as internal parasites of warm-blooded animals. Torsalos are the only species of bot fly that manage to regularly parasitize humans.
An adult torsalo is a rather large insect (10-15 mm) with a bluish-black body, brown wings, and yellow markings on the face and legs. It is a loud, buzzing flier that tends to attract attention to itself -- unusual behavior for a parasite! Ordinarily, such a large, obnoxious insect would have difficulty approaching a large animal to lay eggs. But female torsalos overcome this problem by catching smaller flies (like mosquitoes) and gluing eggs to the underside of the captive's body. The captive is released unharmed, carrying the torsalo's eggs until it lands on a warm-blooded host. Body heat from the host triggers rapid hatching of the torsalo's eggs. The tiny maggots burrow quickly into the skin (even through clothing) and begin development as internal parasites.
Larvae develop over a period of 5-10 weeks, forming a painful boil or cyst under the skin. When mature, they emerge from the host, fall to the ground, and pupate. The adult fly emerges several weeks later.
The relationship between the torsalo and the fly that delivers its eggs is commensal (the torsalo benefits; the fly is unaffected). This type of symbiosis, in which one species uses another only as means of transportation, is also known as phoresy.