TX - Fly - Physoconops excisus

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 thumb-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. [Note: When counting abdominal segments in conopines (and most flies), the basal (1st) segment if often very short and might easily be overlooked!] As can be seen in the photo above, the theca of females of P. excisus are remarkably large (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 an unwilling hymenopteran host/victim when the female conopine strikes and attempts to insert its eggs therein. This is typically done quickly, while the host is in flight or visiting a flower.

The phrase "(female) genital plate" has sometimes been used in place of the term "theca". This seems to me somewhat of a misnomer, as insect genitalia are typically housed in-and-beyond the 7th abdominal segment...and the theca appears to function solely as a supplemental assist for mechanically separating tergites of a host, with actual oviposition effected by more terminal abdominal segments of the conopine. Thus the theca is not part of the primary reproductive apparatus per se...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 female and male genitalia, as shown in the images below: