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Photo#672571
TX - Fly - Physoconops excisus - female

TX - Fly - Physoconops excisus - Female
Liberty County, Texas, USA

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TX - Fly - Physoconops excisus - female TX - Fly - Physoconops excisus - female TX - Fly - Physoconops excisus - female TX - Fly - Physoconops excisus - female TX - Fly - Physoconops excisus - female

Good illustration of the very large theca of female P. excisus
The "theca" is a structure associated with female conopids of the subfamilies Conopinae and Myopinae. In the genus Physoconops it is typically a forward-pointing finger-like protuberance attached to the bottom of the 5th sternite...with most of its bulk appearing under the 4th (and sometimes 3rd and even 2nd) sternites. As can be seen here, the theca of females of P. excisus are among the largest in the subfamily.

Note that in our other conopine genus, Physocephala, the female theca is typically a much smaller "pad-like bump", attached to the 5th sternite but usually bent forward and pressed against the bottom of the 4th sternite:

               

The theca, together with (or "in opposition to") the downwardly-hooked terminal segment of the female's abdomen, is used to help pry open the abdominal tergites of hymenopteran hosts when the female conopine attempts to insert its eggs therein. This is typically done quickly, while the host is in flight or visiting a flower.

In recent years the term "(female) genital plate" has sometimes been used in place of the term "theca". This seems to me a misnomer, as the theca's function appears to be related solely to oviposition within the abdomen of a hymenopteran host. The theca is not part of the reproductive apparatus...nor known to be involved in copulation, during which it is usually held sticking straight-out from the 5th sternite and far from the contact zone of the true (external) female genitalia, as shown in the images below:

               

So the term "genital plate" seems somewhat misleading, as the theca is not part of the genitalia as generally understood or defined.

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