Peratrimera mexicana, Hauser & Irwin, 2005

Hauser, Martin & Irwin, Michael E., 2005, A new remarkable Xestomyzinae (Insecta, Diptera, Therevidae) genus from Mexican Amber, Zootaxa 1008 (1008), pp. 39-45 : 40-44

Peratrimera mexicana n. sp. ( Figs. 1A–B; 2A–B)

Diagnosis. Same as the genus. The important species specific characters (i. e., pubescence on the frons, pleuron and mesonotum, as well as the color pattern of the hind leg) are not discernable.

Etymology. Named after Mexico, the country where the fossil was found.

Description of the female holotype (MEI # 164794):

The holotype specimen is deposited in The Natural History Museum London (In. 2158(1)).

Body length: 10 mm, wing length: 7 mm.

The specimen is not in perfect condition. It decomposed somewhat after being engulfed by the resin. The body is translucent and slightly laterally compressed ( Fig. 1A), therefore no pubescence is visible. Also the matrix above the tip of the abdomen is damaged so some structures of the last segments are not discernable. Both hind tarsi and the left middle tarsus are missing; the left middle leg and the left front leg are not discernable.

HEAD: The antennae are slightly longer than the head. The scape is more than four times as long as wide (because of the slightly decomposed condition of the insect and the distortion of the amber, the measurements are not as precise as they would be in a fresh specimen) and is covered with short, semi­appressed setae ( Fig. 2A). A strong, dark, dorso­apical setae exists on the left scape. The first flagellomere is nearly nine times as long as wide and covered with many small, appressed setae. The second flagellomere is twice as long as wide; the third flagellomere is nearly three times as long as wide and possesses a long style. Eyes separated by the width of the ocellar triangle. Some short setae are visible on the frons along the inner margins of the eyes down to the base of the antenna. THORAX: The mesonotum is covered with short, semi­appressed setae. Two notopleural setae (np), one supraalar seta (sa), one postalar seta (pa), no dorsocentral setae and a pair of scutellar setae (sc) are discernable ( Fig. 1B). The legs are long and slender; hind coxa with hind coxal knob. Lateral macrosetae on hind coxa absent. Wing cells m 3 and cu­p are closed and neither of them reaches the wing margin. The veins M 1 and M 2 are parallel and reach the wing margin ( Fig. 2B). The coastal vein ends at M 2. The length of the wing vein R 1 is covered with setae, and a few setae are discernable on R 4+5 (inset of Fig. 2B). ABDOMEN: The abdomen is long, slender, and covered with very short setae. The 8th sternite has many long setae which are slightly up­curved ( Fig. 1B). Amongst the mainly thin setae are some thicker ones of the same length.

Placement of the genus. The elongate gestalt is typical for several genera of Xestomyzinae . The cell m 3 is closed, which is the case in all Phycinae and Xestomyzinae , but also many other Therevidae genera have this cell closed. The ending of the costal vein at M 2 is found in all Xestomyzinae , while all Agapophytinae and Therevinae have the costal vein completely around the wing. In the Phycinae , the vein stops in most genera at CuA 2 ( Fig. 2C), but in some genera at M 1 or M 2. The up­curved setae on the 8th sternite of the female place this species clearly within the subfamily Xestomyzinae . The females of Agapophytinae and Therevinae have acanthophorites at the last tergites and no up­curved setae on 8th sternite. In the Phycinae the setae of the female are reduced, but the 8th sternite never has thick up­curved setae between filiform setae. The setae on R 1 and R 4+5 are found in all species of Henicomyia ( Lyneborg (2002: 104) stated that Henicomyia has no setae on these wing veins, but this is not the case. We examined all species known to Lyneborg and several undescribed species, and the setae were always present). These setae are always present in the Phycinae (exception: Schlingeria Irwin ) and in very few Therevinae (e.g. Protothereva Malloch ) but never in Xestomyzinae other than Henicomyia .

From all the Phycinae , the fossil resembles the South American genus Ataenogera Kröber the most. The three segmented, laterally compressed antenna of the fossil with its apical style is very similar to the antennae of present day species of Ataenogera . While the flagellum of Henicomyia is always two segmented, the first flagellomere is cylindrical, and the style is inserted inside an invagination of the second flagellomere, the flagellum of Ataenogera is three segmented with an apical style. Despite the Ataenogera­ like antenna, there are several characters that place the fossil close to the genus Henicomyia . In Henicomyia and Peratrimera , R 4 and R 5 are only slightly divergent ( Fig. 2B). In all Ataenogera R 4 and R 5 diverge strongly after half of their length ( Fig. 2C). Another important characteristic is the costal vein, which ends in all Xestomyzinae , including Henicomyia and Peratrimera at M 2, while it ends at CuA 2 in Ataenogera . Also does the vein M 3 never reaches the wing margin in Henicomyia and Peratrimera , while in Ataenogera it always merges with C at the wing margin. The alula is much reduced in Henicomyia , while it is relatively broad in Ataenogera . This character is not clearly visible in Peratrimera , but it seems that the alula is reduced.

From all Xestomyzinae only Henicomyia and Peratrimera share the absence of a lateral macrosetae on the hind coxa. The three segmented flagellum separate Peratrimera from all the known recent (described and undescribed) species of Henicomyia , which possesses only a two segmented flagellum. The characteristics of the antenna are the main reason for justifying the new genus.

Remarks. This fossil indicates that more than 20 million years ago, members of the Xestomyzinae occurred on the North American continent. This raises doubts that the Xestomyzinae are, as was previously thought, of Gondwanan origin, which dispersed from South America to North America over the past 4 million years, once the Mesoamerican land bridge was formed.