ICHNEUMON-FLIES, PART 21 EPHIALTINAE 415 



4. Genus Megarhyssa 



Figure 304,a 



Thalessa Holmgren, 1859, Ofvers. Svenska Vetensk.-Akad. Forh., vol. 16, p. 122. 



Name preoccupied. (Ichneumon clavator Fabricius) = histrio (Christ) ; 



designated by Ashmead, 1900. 

 Megarhyssa Ashmead, 1900, Canadian Ent., vol. 32, p. 368. New name. 

 Megalorhyssa Schulz, 1906, Spolia hymenopterologica, p. 115. Emendation. 

 Eurhyssa Derksen, 1941, Zeitschr. Morph. . . ., vol. 37, p. 721. Type: Ichneumon 



superbus Schrank; designated by Townes, 1951. 



Front wing 10 to 30 mm. long; clypeus small, transversely rec- 

 tangular, its apicolateral corner produced as a small blunt point, 

 sometimes also a small blunt point at middle of its apical margin; 

 junction of occipital and hypostomal carinae distant from base of 

 mandible by about 0.4 the basal width of mandible; mandibular teeth 

 of equal length, the lower tooth pointed at apex, the upper tooth more 

 or less chisel-shaped; second trochanter of middle leg with a sharp 

 ventral longitudinal ridge; stigma about 5.5 as long as wide; areolet 

 present except in occasional dwarf males; first tergite immovably 

 attached to its sternite, without a glymma; tergites 3-6 of male 

 strongly depressed, polished, with scattered small punctures, with a 

 median longitudinal apical or subapical membranous area, their 

 apical margin with a broad, deep, V-shaped notch; tergites 3-6 of 

 female usually smooth, sometimes with large aciculate areas but 

 never aciculate all over, with rather numerous setiferous punctures; 

 sternites 3-6 of female with a pair of tubercles near the base; last 

 tergite of female with an apical hornlike process which is truncate at 

 the tip; male clasper lanceolate, strongly depressed, with a sharp 

 setiferous groove paralleling most of its inner lower margin and a 

 short setiferous groove on its outer edge near the apex. 



In dwarf males of all the Nearctic species and of at least some of 

 the Old World species the abdominal and genitalic specializations are 

 lost to some degree. In these the tergites tend to be less depressed, 

 more densely hairy, less deeply notched at apex, and lack the median 

 submembranous area. The clasper tends to be shorter, less depressed, 

 and the grooves as described above tend to be broad, short, and shal- 

 low, so as to be indistinct or in extreme cases to be lacking entirely. 



This genus is Holarctic and Oriental. It contains some very large 

 species. Large females of the four North American species are our 

 largest ichneumonids, rivaled only by some species of Conocalama. 

 The facts that they are very large, have a spectacularly long ovi- 

 positor, and are moderately common make our Megarhyssa species 

 the best known of the ichneumon flies, and in fact the only ichneumon 

 flies familiar to a large number of people. They have been figured 

 and discussed in textbooks, encyclopaedias, dictionaries, magazine 



