|Megarhyssa macrurus female|
It is a parasitoid, notable for its extremely long ovipositor which it uses to deposit an egg into a tunnel in dead wood bored by its host, the larva of a similarly large species of horntail. Another of its common names is stump stabber referring to this behaviour.
Megarhyssa macrurus has a reddish-brown body approximately 2 inches (51 mm) long. It has black and yellow-orange stripes. Its wings are transparent and the body elongated. The body and ovipositor together can be more than 5 inches (130 mm) long. Males are smaller (and have no ovipositor).
The ovipositor appears as a single filament, but it comprises three filaments. The middle filament is the actual ovipositor which is capable of drilling into wood. This central filament also appears to be a single filament, but is made of two parts. These parts have a cutting edge at the tip. They interlock and slide against each other.
Although very thin, the ovipositor is a tube and the egg moves down a minute channel in its center during egg laying. The outer two filaments are the sheaths which protect the ovipositor. They arc out to the sides during egg laying.
M. macrurus is harmless to humans; they are parasitoids on the larvae of the pigeon horntail (Tremex columba, Symphyta), which bore tunnels in decaying wood. Female Megarhyssa macrurus are able to detect these larvae through the bark, and lay their eggs on them; within a couple of weeks, the Megarhyssa larvae will have consumed their host and pupate. They will emerge as an adult the coming summer.
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