I have never even heard of bot fly so I went ahead and googled it. A wikipedia article describes infestation like this...
The larval stages of Cephenemyia are obligate parasites of cervids.[2] Eggs hatch in the uterus of the female. She then flies close to the head of her host species and while hovering ejects her larvae into its nostrils.[3] Larvae migrate to the base of the animal's tongue, where they mature in clusters to a size of 25 to 36 mm. After being ejected by the host, they pupate in soil (2 to 3 weeks) before emerging as a sexually-mature but non-feeding adult, which must quickly find a mate, since its life is short.[3]
Nasty stuff!