Figures: 15, 54, 94-95, 117, 123, 134, 161, 188-190, 204, 232
In addition to the Heptageniinae characteristics, the following combination of characteristics will identify the genus:
Larvae of Maccaffertium are characterized by the combination of having long, narrow lamellae without tracheation or fibrils on gills 7 (Fig. 95) and a submarginal anal rib on gill lamellae 1-6 (Fig. 94). Furthermore, the lamellae of gills 1-6 are truncate apically and the costal rib is absent (Fig. 94).
The 'L' shaped penes (Fig. 161) will separate Maccaffertium from all other Nearctic genera except Stenonema. Males of Stenonema have a median dark spot and a pair of transverse dark lines on the posterior margin of the abdominal terga (Fig. 195), whereas the terga of Maccaffertium are variable, but never like those of Stenonema (Fig. 204). Many species have the crossveins in the first 2-6 interspaces below the bullae crowded (Fig. 189), unlike any other North American genus except Stenonema. The eggs differ from those of all other North American genera except Stenonema by having an elongate sperm guide formed by a break in the adhesive layer (as in Fig. 111).
Nearctic (Fig. 232).