1968] 
Kukalovd — Mayfly Nymphs 
325 
it is much more conspicuous in the nymphs. In Recent adults and 
nymphs, it is very small in even the most primitive families. Its 
significance has long been a puzzle. However, in view of the structure 
of the Permian nymphal wings, a possible function for the brace in 
the ny?nphs needs to be considered ; it may well have been a strength- 
ening structure for the nymphal wings, which in the protereismatids 
extended outward from the body without much other support and 
which might have been carried over in vestigial form to the adults. 
In the more highly evolved nymphs, as the wing pads developed along 
the body, the support of the brace may have no longer been needed. 
Still another notable feature of these Permian nymphs is the nature 
of the legs, especially the tarsal segmentation. In the adults of 
Protereismatidae, so fas as known, the legs were exceptionally long 
and slender, possessed five tarsomeres, and a pair of long pretarsal 
claws. They are not very different, except for their length, from 
those of living adults, in which the number of tarsomeres ranges 
from 5 to 1. The legs of the protereismatid nymphs, although shorter 
than those of the adults are basically similar to them in structure, 
with 5 tarsomeres and two slightly curved pretarsal claws. 5 In con- 
trast, the legs of Recent mayfly nymphs are modified for swimming 
and have only partially divided tarsi; so in nearly all species there 
are only one or two tarsomeres and there is only one claw. The 
cursorial legs of the protereismatid nymphs clearly show that they 
were not active swimmers and suggest that they walked actively 
along the bottom of ponds or streams. 
In contrast to the legs, the tracheal system of the protereismatid 
nymphs was highly modified for an aquatic environment. Especially 
surprising is the presence of tracheal gills along the first to the ninth 
abdominal segments ; in living nymphs the gills do not extend beyond 
the seventh segment, at most. It is interesting to note, also, that the 
few Permian nymphs known show considerable variation in the form 
of the gills. In the specimen from Oklahoma (nymph no. 1) the 
first two pair are much enlarged and resemble somewhat the opercula 
that occur in some Recent nymphs; in those from Moravia, the gills 
on any one nymph appear very much alike. In one of the Moravian 
nymphs (no. 6) they are much longer and more slender, without 
differentiation of the anterior pairs. It is possible that some of these 
5 The five tarsomeres do not appear to be adult structures visible through 
the nymphal cuticle. As noted above, these fossil nymphs almost certainly 
consist of cast cuticles; furthermore, the tarsomeres are present in some of 
the young nymphs from Moravia. For the same reasons, the pretarsal claws 
are not regarded as adult structures seen through the nymphal cuticle. 
