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It seems the life cycle varies a fair bit within this family regarding where they feed and how they emerge/ where they pupate, and what Mark is referring to is the "Gasterophilus style" life cycle. In G. intestinalis at least, the larvae develop in the stomach (of a horse) then emerge with the faeces when mature and pupate, maybe often in the dung itself. Hence the dung/Oestridae link. I found this: http://www.icb.usp.br/~marcelcp/Gasterophilus.htm This article also mention the hilltopping courtship behaviour of male G. nasailis with males wait/hover on hilltops and chase passing females.