270 
Psyche 
[June-September 
Specimens of the mayfly nymphs are by far the most numerous of 
all the insects in the Midco deposit. Several hundred were collected 
on my 1940 trip, when Dr. Raasch and I made the first exploration 
of the deposit, and as many again were collected on subsequent 
trips. Double that number were simply discarded in the field. 
Because of their number and the nature of their preservation, it is 
virtually certain that these fossils are the cuticular remains shed by 
the nymphs at molting. More than 90% of the specimens consist of 
isolated wing-pads from the nymphs and most of the remainder 
represent a single thoracic segment with two wing-pads attached. 
Only a very few consist of the entire nymph, with all wing-pads and 
many body structures, these being the specimens that I turned over 
to Dr. Kukalova in 1969. Since she has given a detailed account of 
these specimens, I will include here only the salient features, with 
special reference to the venation of the wing-pads. 
The head of the nymphs was slightly narrower than the pro¬ 
thorax, and had well developed, dentate mandibles; the antennae 
were slender; the prothorax about half as long as the mesothorax, 
and the meso- and metathorax nearly equal; the legs were subequal, 
with five tarsal segments; abdominal segments subequal, the cerci 
and caudal filament well-developed; nine pairs of tracheal gills were 
present on the abdomen, the anterior ones somewhat larger than the 
others. 
The wing-pads were well developed but were attached to the 
thorax only along the articular area (of the adult wing), and were 
independent of each other; the pads projected posteriorly at an 
oblique angle to the longitudinal axis of the body. The venation of 
the wing-pads was described by Kukalova, but unfortunately her 
figure (1969, figure 2) and her interpretation of the homologies of 
the veins were incorrect. The most conspicuous feature of the vena- 
tional pattern of the nymphal wings is the difference in the apparent 
degree of development of the convex and concave veins. In the wing 
of an adult mayfly (figure 8), the convex veins include, in addition to 
the main veins R1, M A, and CU A, the intercalary veins of the radial 
sector and of the posterior media; the concave veins include, in 
addition to the main veins RS, MP, and CUP, the intercalary veins 
of the anterior media and the anterior cubitus. In the Midco nymphs 
(figure 9) all of the convex veins are very strong and distinct but all 
of the concave veins are weak and indistinct. Comparison of the 
nymphal wing with the adult wing of Protereisma (figure 8) shows 
