Stomosis arachnophila Brake
Field observations by M. von Tschirnhaus for sample X651: "16 milichiids are flying in front of a huge spider (Araneidae), which sits in the middle of her web. Finally the flies alight on a winged ant, which is caught in the web and is still alive, though hardly moves. There are several dead ants of the same species in the web as well as a number of silvery Therediidae. The female spider (deposited in UBI) stays for a long time in her place even when provoked. There are up to three milichiids on one prey, that has to be freshly dead. The two flies in sample X649, which were sitting on a Phonognatha (Araneidae) case (made from dead eucalyptus leaves) probably belong to the same species. 20 more cases were searched unsuccessfully. One milichiid alights on spider leg and runs up and down the leg dabbing, unheeded by the spider. One milichiid on ant with regurgitation droplet."
As described in the field notes, adults of the new species, Stomosis arachnophila, are kleptoparasitic on spiders.
Differing from congeners in the combination of yellow anterior margin of frons, completely brown basoflagellomere, completely brown thorax and shape of male terminalia.
Coloration and vestiture: Head brown posteriorly, yellow anteriorly, slightly microtomentose except for frons. Frons usually brown on posterior 2/3 and yellow on anterior third of frons, but in a few specimens the frons is completely yellow except for a brown ocellar triangle and orbital plates; lunule yellow, face light brown; gena yellow except for posterior margin; antenna brown; palpus yellow with narrow brown tip and proboscis light brown. Thorax completely brown, slightly microtomentose except for shiny area posterior to base of fore coxa, wing and calypter hyaline, veins brown, halter white, legs black except for lighter ventral side of fore coxa and yellow narrow bases of all tibiae as well as distal tip of fore tibia, and all basitarsi. Abdomen with all tergites and sternites brown, slightly microtomentose except for anterior half of synsternite 7/8 and of epandrium.
Head: Frons with 3 orbital setae, posterior seta lateroreclinate, the medial seta lateroclinate, anterior seta lateroproclinate, and 2 medioclinate frontal setae, postocellar setae medioclinate (cruciate). Arista about 2.5x as long as basoflagellomere width, pubescence on arista very short. Tip of ocellar triangle nearly reaching anterior third of frons. Gena height 0.13-0.15x eye height (Brake 2000, Pl. 1D).
Thorax: 2 dc, 1 prsc, 1 pprn, 1 prs, 1 sa, 1 pa, 1 keps setae, posterior prsc about a third as long as posterior dc, seta between posterior prsc and posterior dc absent.
Wing (Brake 2000, Pl. 10B): with veins R4+5 and M parallel; M-ratio is 2.9; length: 1.7-2.1 mm.
Male abdomen: Sternite 5 divided into a narrow anterior and a narrow posterior part. The anterior part bent behind sternite 4 and covered with setulae, which may be part of a gland (Brake 2000, Pl. 12A). Male genitalia as in Fig. 1-3; distiphallus membranous and tubular, about as wide and 6-7x as long as epandrium length, with many setae at apex.
Australia (Western Australia)