4 Intinors NaruraAt History Survey BULLETIN 
flies had evolved over a long period in 
streams which did not contain game fishes. 
As a result, the nymphs of such forms as 
Oniscigaster wakefieldi McLachlan, instead 
Fig. 3.—Adult male of Hexagenia limbata. 
Vol. 26, Art. 1 
of remaining concealed beneath rocks and 
debris as our American mayflies do, swam 
freely in the open water. Shortly after the 
turn of the century, brown trout and rain- 
bow trout were introduced into the New 
Zealand streams to provide game fishing. 
The trout ate the mayfly nymphs in such 
numbers that within about 20 years the 
once extremely abundant mayflies had be- 
come almost extinct. Tillyard (1926: 64) 
states, “The introduced trout have greatly 
reduced this once abundant fauna [of may- 
flies] and some species are now extinct, or 
nearly so The mayfly fauna of 
Australia and New Zealand is not specialized 
to hold its own against the introduced brown 
and rainbow trout and is rapidly being re- 
duced to a minimum.” 
Habitat Preference 
Some species of mayflies may develop in 
a variety of situations, but most species are 
restricted to definite types of aquatic hab- 
itats. The various types of mayfly habitats 
found in Illinois and the species which have 
been observed to be characteristic of them 
are as follows: 
1. Large, relatively slow rivers, such as 
the Mississippi, Ohio, and Illinois: Hexa- 
genia bilineata and limbata, Pentagenia vit- 
tigera, Tortopus primus, Potamanthus myops 
and verticis, and Ephoron leukon and album. 
2. Moderate-sized, fairly rapid rivers, 
such as the Kankakee, fig. 4, and Rock: 
Hexagenia rigida and limbata, Potamanthus 
myops and verticis, Ephoron leukon and 
album, Ephemerella dorothea, invaria, and 
simplex, Baetisca bajkovi, Pseudiron cen- 
tralis, Siphloplecton interlineatum, Steno- 
nema spp., and Heptagenia spp. 
3. Small rivers or creeks with fairly 
rapid flow, such as the Salt Fork River and 
Lusk Creek: Ephemera simulans, Hexagenia 
atrocaudata, Ephemerella frisoni and need- 
hami, Baetisca laurentina, obesa, and baj- 
kovi, Oligoneuria ammophila, Paralepto- 
phlebia praepedita and ontario, Ameletus 
lineatus, Isonychia sicca, bicolor, and rufa, 
Centroptilum walshi and rufostrigatum, 
Baetis spinosus, propinquus, harti, pygmaeus, 
frondalis, pallidulus, and intercalaris, Pseu- 
docloeon punctiventris, dubium, parvulum, 
and veteris, Stenonema spp., and Heptagenia 
spp. 
4. Sluggish creeks or small rivers with a 
great deal of silt, constituting the majority 
