146 Ittinois NaturaAt History Survey BULLETIN 
among the various genera in this family. 
All known nymphs have each three rela- 
tively short caudal filaments, with each outer 
filament bearing a dense fringe of setae on 
the mesal margin only. 
KEY TO GENERA 
ADULTS 
1. Median caudal filament well developed, 
almost as long as cerci........-.-.+-: 
pee Seger ste ere ee 36. Ametropus 
Median caudal filament vestigial or 
represented by only a one- to four- 
segmented stub............-. sai aio rh 2 
2. One pair of cubital intercalary veins pres- 
ent in fore wing..... 38. Metretopus 
Two pairs of cubital intercalary veins 
present in each fore wing........... 
3. First segment of fore tarsus of male three- 
fourths as long as second segment; 
ninth abdominal sternite of female ex- 
tended caudad and with a pronounced 
median notch on posterior margin, fig. 
| PMs ai Sai aah pean 39. Pseudiron 
First segment of fore tarsus of male 
slightly longer than second segment; 
ninth abdominal sternite of female not 
produced caudad and without a median 
notch on posterior margin, fig. 306.... 
40. Siphloplecton 
Marure NympuHs 
1. Eyes directed anteriorly; each fore coxa 
with a large, lobelike, median append- 
PTR iy Fee ae 36. Ametropus 
Eyes directed laterally or dorsally..... 2 
2. A median, ventral, hooklike spur present 
on each thoracic segment and a median, 
dorsal, hooklike spur present on each 
abdominal segment, fig. 312.......... 
satire et coe tener (oe Se Cree 37. Metreturus 
Thoracic and abdominal segments with- 
out median, dorsal, or ventral, hooklike 
SOMES ae eae ah eats ae 3 
3. Head flattened dorsally, prognathous; 
eyes dorsal; claw of fore tarsus single. . 
SP ee Ewe eee one 39. Pseudiron 
Head not flattened dorsally, hypogna- 
thous; eyes lateral; claw of fore tarsus 
donble, he. 30S. etre forts pees hae 4 
4. Maxillary palp two-segmented......... 
eRe eR an ot th pais 38. Metretopus 
Maxillary palp three-segmented........ 
yg ao eR a Bate 40. Siphloplecton 
36. AMETROPUS Albarda 
Ametropus Albarda (1878:129). 
In the genus 4 metropus, the wing vena- 
tion differs only slightly from that of the 
typical heptageniid form, the fore wing hav- 
ing two pairs of cubital intercalary veins. 
The hind wing has an acute costal angula- 
tion, vein M is forked at a point halfway 
Vol. 26, Art. 1 
from its base to the apex, and veins R, and 
R, are fused throughout their length. The 
male penis lobes are fused to form a conical 
structure with a narrow, V-shaped apical 
cleft, somewhat as in Baetisca and some 
species of Ephemerella. The apical abdom- 
inal sternite of the female has a median 
caudal cleft. The median caudal filament is 
long, nearly as well developed as are the 
cerci. 
The nymphs are fishlike in general body 
form, with the eyes directed anteriorly. The 
head is small, with the frontal margin cut 
away almost to the antennal sockets, nearly 
completely exposing the mouth-parts. The 
pronotum is wider than the head, and a pro- 
jecting, membranous flap is borne by the 
prosternum. Each fore coxa has a fleshy, 
lobelike mesal projection. All the tarsal 
claws are single, slender, and pointed, and 
are much longer than the tibiae. The ab- 
dominal gills are single and platelike. 
Only two Nearctic species are known in 
the genus Ametropus. One of these, neavei 
McDunnough (1928a:9), was described 
from Alberta. The other, albrighti Traver 
(1935a:431), described from the nymph 
only, was first collected in New Mexico 
and has subsequently been found in Utah. 
37. METRETURUS new genus 
The genus Metreturus is here erected for 
a new species, known only in the nymphal 
stage, fig. 312, which is radically different 
from that of all other North American may- 
flies. It is referred to the family Ametro- 
pidae because of the very short tibiae, the 
long, slender tarsal claws, and the three 
short caudal filaments, each of the outer 
ones of which bears a dense fringe of setae 
on the mesal margin only. The wing vena- 
tion, visible in the nymphal wingpads, can 
be seen with sufficient clarity to show that 
the adult of this form could be referred to 
the Ametropidae, as there are two pairs of 
parallel and basally detached cubital inter- 
calary veins. 
The nymph of Metreturus has the head 
small and hypognathous, with the eyes lat- 
eral; there is a pair of short, submedian 
horns near the antennal bases, on the face 
above the clypeus; there is a median, acute 
projection on the margin of the clypeus, and 
there is an oblique, laterally projecting lobe 
arising at the lower margin of each com- 
