This document provides information about various aquatic insect species found in Iowa's Driftless Area streams and rivers. It describes the lifecycles and habitats of mayfly, caddisfly, stonefly, dragonfly and damselfly nymphs. Specific insects covered include Blue-winged Olives, March Browns, Miniature Eastern Sulfurs, Little Black Sedges, Grannoms, and Green Rock Worms. Photos show the insects at different life stages and provide tips on imitating their appearances with fly patterns. The document emphasizes the importance of aquatic insects as a food source for trout.

1. Aquatic Insects in Iowa’s Driftless Area

2. Aquatic insects are abundant in Iowa’s Driftless area streams and
rivers. Some species hide under rocks, some burrow in the mud and
others seek shelter among water weeds. They are a critical food
source for most fish. Let’s start by talking about these lovely Mayflies.

3. Mayflies are
included in the
Insect Order
Ephemeroptera
Adult Mayflies
truly are
ephemeral, living
for only a single
day or so after
emerging.
Unlike most adult
insects, Mayflies
transform from
young Duns to
sexual Spinners
before dying

4. Trout feeding season for Mayflies opens in early spring when some
similar pint-sized species descriptively referred to as Blue-winged
Olives (BWOs) appear along area cold-water streams

5. Blue-winged Olive nymphs (Baetis sp.) are classified as swimmers.
Commonly called Small Minnow Mayflies, they’re streamlined and
quick and dart around like mini-minnows.
Like most Mayfly larvae, they graze on algae and rock slime.

6. Here’s a good-looking buggy Baetis nymph for trout fishing

7. Blue-winged Olive nymphs like to live among the dense vegetation in
shallow weedy runs. PT4

8. Blue-winged Olive duns, a buggy red-eyed male here, emerge early in
the spring, April in Iowa, often on dreary cold damp 400 days. Look for
another flush in autumn.

9. Blue-winged Olive dry fly pattern. As the old saying goes “Don’t leave
home without them”

10. March Brown (Maccaffertium vicarium) nymphs are classified as
clingers. Commonly called Flathead Mayflies. They lie flush on the
undersides of rocks in running water.

11. March Brown nymphs are synonymous with Pheasant-tail nymphs

12. March Brown
nymphs inhabit
fast rocky runs.
Drift your
Pheasant-tail
nymph
downstream as
shown. PT4

13. March Brown duns have
mottled brown bodies.
They hatch out in May
during midday.
Noted May 26

14. March Brown dry fly. A
good pattern to have
on hand in the spring.

15. Miniature Eastern
Sulfur (Ephemerella
dorothea) nymphs
are classified as
crawlers, just like
their slightly larger
Great Eastern Sulfur
cousins. Commonly
called Spiny Crawlers.

16. Tie your favorite Pheasant-tail type fly a little smaller for the Miniature
Eastern Sulfur.

17. Miniature Eastern Sulfur nymphs live in the leafy debris beside deep
quiet pools. Work them slow. PT1

18. Miniature
Eastern Sulfur
duns pop up in
pools during
May & June.
They are
fragile
mayflies with
creamy yellow
bodies and
three trailing
tails.
Females drop
their eggs like
little bombs
over water

19. Miniature Eastern Sulfur dry fly pattern.
Also called the Little Sulfur Dun or Pale Evening Dun.

20. This monster Giant Michigan Mayfly (Hexagenia limbata) larvae is
nearly two inches long. Those toothbrush-like bristles are breathing
gills.

21. Giant Michigan Mayfly nymphs are classified as common burrowers.
Note the elephant-like tusks to help them tunnel down.

22. Burrowing mayfly nymphs dig out hollow U-shaped
burrows in the substrate of their choice, where they
can sit in safety and filter microscopic food from the
water passing by.

23. Giant Michigan Mayfly
nymphs prefer to
burrow into marl, a
harder silty clay
substrate sometimes
found in slow-flowing
pools. PT4

24. This dried-out stream bottom is made of marl. Notice how the holes
don’t cave in.

25. Here’s a Giant Michigan Mayfly nymph that highlights legs and gills

26. Giant Michigan Mayfly duns are huge. Mating swarms arrive in June.

27. The Giant Michigan Mayfly hatch is legendary on the Au Sable & Pere
Marquette rivers in that state. Around here they’re called Fishflies.

28. This “snowstorm” around the streetlight is a swarming Michigan Hex
hatch along the Mississippi River.

29. Among aquatic insects,
Caddisflies are included
in the Order Tricoptera.
Many Caddis larvae hide
in homemade cases.

30. Little Black
Sedge
(Chimarra)
larvae are
pumpkin-
colored.
They are
classified as
Finger-net
makers,
trapping tiny
food particles in
a very finely-
woven mesh net
Look for them
to be full size by
March.

31. Identifying aquatic
insects gets nit-
picky. Caddis
larvae in the
Genus Chimarra
have a little notch
cut into their
forehead between
the pincers.
Even though they
look like
dangerous
predators, Little
Black Sedge larvae
are collectors and
chew up leaf bits
and such that get
caught in their
nets.

32. Little Black Sedge
larvae are on the
small size, as seen
on this dime.

33. Little Black Sedge larvae are
fairly finicky and live in cold
spring- fed streams, weaving
their finger-nets under the front
sides of sizable rocks in fast
riffles. PT3

34. Little Black Sedge
larvae net open
for business.

35. Little Black Sedges emerge during daylight hours at Hendrickson mayfly hatch time, later April
extending even into June. They lay their eggs on the water surface while sitting still.

36. The Leadwing Coachman is a good dry fly to try for the Little Black Sedge.

37. The Green Sedge or Green Rock Worm (Rhycophilia) is classified as a free-living larvae, which
means it doesn’t manufacture a safe shelter like most Caddisflies. They do spin a safety rope
that keeps them from drifting away from bigger rocks where they hunt for smaller bugs to eat.
Noted 2-19-13 Bohemian Creek.

38. Green Rock Worms live
in rocky riffles given
good current flow. PT1

39. The Green Rock Worm pupates and emerges as the Green Sedge, even though the wings are
grey. Adults show up along the stream in May, extending into June. Afternoon hatches are
common. Females dive to the bottom where they lay their eggs on rocks.

40. The Grannom
(Brachycentrus) larvae
is classified as a
Humpless Case-maker.
Grannom larvae
construct ornamental
four-sided cylinders for
their mobile homes.

41. Grannom larvae fasten their cases to rocks so they aren’t swept away. They can rappel from
rock to rock on silk ropes they spin. Grannom can be quite abundant in Driftless streams

42. Grannom larvae thrive in fertile streams which are full of their favorite food – algae. PT1

43. Grannom adults are dark brown. Look for them to on the water in May and June, emerging
during the day and then laying their eggs on the surface before expiring.

44. Little Tan Short-horn Sedge (Glossosoma) larvae are classified as Saddle Case-makers. They
construct little turtle-like shells for their portable shelters that they carry on their backs while
scraping detritus off rock surfaces.

45. Little Tan Short-horn Sedge
larvae often leave their cases
and drift free in the current.
If not eaten by a trout,
they’ll build a bigger home
somewhere downstream.

46. Little Tan Short-horn Sedge larvae live in the gravelly sections of refrigerated trout waters,
blending right into the small colorful stones. PT0

47. Little Tan Short-horn Sedge adults start emerging in late April and continue into June.
Caddisflies (and Stoneflies) are colloquially called “Downwings” by flyfishers.

48. White Miller (Nectopsyche) larvae are classified as Longhorned Case-makers, some species
making stone homes and other out of woody plant matter.

49. White Miller larvae shred plants
apart for their food.

50. White Miller larvae
live in slower-flowing
weedy streams. PT4

51. White Miller adults are unmistakable, looking like white moths since they only emerge after
dark. Look for them to be bouncing around on the water surface on midsummer nights.

52. Stoneflies are in the aquatic insect Order
called Plecoptera. As a rule, Stonefly
nymphs require cool clear pollution-free
streams to survive. Look for them to be
crawling around under the bottom debris
eating up dead leaves

53. Little Black Stone
(Allocapnia) larvae
mature in late
winter. Even then,
they are easy to
overlook. In
contrast to
Mayflies which
have typically have
three tails as
immature nymphs,
Stonefly nymphs
only have two.

54. This woodland trout stream has a good supply of drowned
leaves for hungry Little Black Stone larvae to eat. PT1

55. The legendary Prince Nymph is a dead ringer for most Stonefly larvae.

56. Little Black Stoneflies are tough critters, coming out of the water in March when snow still
often covers the stream bank.

57. Little Black Stoneflies are aptly named. Notice how the wings are folded flat over the back,
rather than tent-shaped like a Caddisfly.

58. Golden Stonefly (Perlesta) larvae are a cluster of several related colorful forms. Some species
look and act like little aquatic tigers.

59. This Golden Stone
(Agnetina) larvae is
boldly decorated in
bumble-bee colors

60. Golden Stonefly larvae are lightning quick predators and terrorize Caddisfly larvae in swifter
stream sections under rocks. PT1

61. Golden Stonefly adults are aptly named.

62. What are called Salmonfly (Pteronarcys) larvae out West look like some science fiction movie
monsters here in the Midwest. Notice those fluffy gills sticking out around their legs.

63. Pteronarcys nymphs are
slow and lazy and like to
eat dead leaves in
rubble-bottomed rivers.
PT0

64. Iowa’s adult Salmonflies look very similar to their famous western cousins. Too bad they don’t
hatch out in the huge numbers seen on Rocky Mountain freestone streams.

65. Alderflies and Dobsonflies are included in the aquatic insect Order Megaloptera. Sometimes
referred to as “Fishflies”, these bugs are big but not very common and as such the fish seldom
see them. We seldom see them either as they emerge along the shore after dark.

66. Alderfly (Sialis sp.)
larvae look similar
to Stonefly larvae,
but see how the
gills stick out like
little spears all
along their
abdomen.
Alderfly larvae are
predators on
smaller Mayfly
and Caddisfly
nymphs.
When full-grown,
they crawl out on
shore and pupate
in a cocoon over
winter before
coming out after
dark next June.

67. Look for Alderfly larvae to be
prowling around in slower stream
sections where there’s lots of
bottom debris. PT4

68. Alderfly adults superficially resemble Caddisflies with the tent-type wings, but they’re a much
more laid-back bug. They seldom fly, preferring to crawl on the ground. Search for them about
summer solstice time.

69. Dobsonfly (Corydalus cornutus) larvae are the original “ugly bug”. Often called Hellgrammites
or Go-Devils, they look rather daunting when encountered. It goes without saying that they
are predators on just about any other insect in the stream.

70. Dobsonfly larvae are big, up to 3” long.
Compare with Golden Stonefly larvae.
They crawl to shore and pupate
underground over winter.

71. Dobsonfly larvae
terrorize their
prey among
sizable stones in
streams with
good current
flow. Mayflies
and Stoneflies
beware. PT0

72. Look for Dobsonfly adults to hatch
out at the end of June. They’re
about as big as a small bird with a
5” wingspan.

73. Adult Dobsonflies can barely fly and only live for a couple days. Pick one up if you dare.

74. Damselflies and Dragonflies are included in the Order Odonata. While most species are more
likely to be found flying around lakes or muddy marshes, a few seek shelter around our
streams. This Green Darner dragonfly has been called the Devil’s Darning Needle and was said
to sew the ears of bad boys shut in an earlier era. Green Darners are a calm water species.

75. Damselflies are the dainty
members of the Odonata
order. Around here, the
Broad-winged
Damselflies(Calopterygidae)
are commonly found in
moving waters. They have
skinny bodies that look like
walking sticks under water.

76. This Ebony Jewelwing
(Calopteryx maculata)
larvae lives among the
water weeds where it
hunts for tiny meaty
Mayflies.

77. Ebony Jewelwing larvae as well as adults like
quiet shady stream sections. PT5

78. Ebony Jewelwing adults may be one of the easiest aquatic insects to identify. Note the black
wings and iridescent blue-green body.

79. Female Ebony Jewelwings lay their eggs under water on aquatic vegetation. See the white dots
on their wings, which the males lack. That flying guy is guarding his wives from possible attack,
at least from above.

80. Dragonfly nymphs are more robust than Damselflies and have flattened fat abdomens. These
voracious creatures can even catch and devour creek minnows.

81. That lower lip, called a labium, can shoot out like frog’s tongue to snare passing prey.
Dragonfly nymphs are often referred to as naiads.

82. When mature, after
spending up to two
years underwater,
Dragonfly naiads
crawl out of the
water and up on a
rock or weed stem,
where they change
into the flying form.
Around here, most
stream Dragonflies
belong to the
Gomphidae Family.

83. Stream Dragonfly (Gomphidae) larvae and adults are likely to be collected or spotted along
creeks and rivers with scattered rocks and shallow debris-covered bottoms. PT1

84. This Midland Clubtail (Gomphus fraternus) feeds on Deerflies, Midges and Mosquitoes.
Notice the swollen tail-end.

85. Midland Clubtails like to bask in the sun
on big rocks along the creek. Look for
them to be buzzing about in June.

86. True flies are found in the insect Order Diptera, meaning they have only two wings. Several
families of True Flies are found in and around water, and at least one is a major player for fly
fishers. This Midge looks a lot like a Mosquito but lacks the bloodsucking sticker.

87. Midge (Chironomidae) larvae are called bloodworms because many are bright red. They only
measure a few millimeters in length. They live in the mud on the stream bottom.

88. Bloodworm close-up showing the oxygen-carrying
hemoglobin coursing through the larvae

89. Midge larvae live by the millions in the soft mud along shallow weedy stream runs. Midge
larvae are one of the brave few that emerge all winter long on relatively warm days. PT6

90. Midge larvae are real easy to imitate. Just wrap a little red or black or green thread around a
tiny hook and go to town.

91. Most adult Midges are either green or black if seen up close, though they’re no bigger and
often mistaken for mosquitoes. They whine but they don’t bite.

92. Midges can be
mesmerizing
when we see
their swarms
dancing up
and down
along the
stream bank in
the evening
light.

93. That little Griffith’s Gnat can be deadly when a midge hatch is on, as the bitty fly still looks like
a whole cluster of floating Midges to greedy trout.

94. This innocent-looking little bowling-pin shaped larvae will one day turn into the dreaded
Blackfly (Simuliidae). They attach to rock surfaces with that sticky suction cup at their butt.

95. Blackfly larvae can be
incredibly abundant in
streams given good oxygen
content. As filter feeders,
they glean algae and
diatoms from the stream.

96. Blackfly larvae love to live in fast broken water. PT6

97. Adult Blackflies are sometimes called Buffalo Midges because of that shoulder hump. They can
bleed you dry in June and July.

98. There’s a couple troublesome true flies in the Family Tabanidae. This little white larvae,
pointed at both ends, is an immature Deer Fly (Chrysops sp.)

99. Deer Fly larvae live in quieter stream segments where they hunt for midges and such in the
bottom muck. They pupate in the damp mud beside the stream in autumn & overwinter there.
PT6

100. Deer Flies are about the
size of a standard House
Fly, but they have black
wings broken by
transparent bars, giving
them a striped look
when they land on your
skin to inflict a painful
bite.
Deer Flies have a habit of
landing on high points,
like our heads, so
wrapping some sticky
double-sided tape to
your hat band will simply
stick it to those
persistent pests.
June and July are the
peak swarming months.

101. This tubular white larvae is a bit bigger than a Deer Fly larvae, but belongs to another
troublesome cousin called the Horse Fly (Tabanus sp.).

102. Horse Fly larvae live in shallow muddy stream bottoms where they eat small snails & other
animal matter amongst the debris. They take two years underwater to mature. Pupate on land
PT6

103. Adult Horseflies are big
biters with bright green
eyes. Once bit, you never
forget.

104. This juicy-looking ringed grub will one day turn into a Crane Fly (Family Tipulidae).

105. Crane Fly larvae live in the
mud along stream shores,
either in or out of the water,
where they feed on fungi
and plant debris.
Crane Fly larvae crawl out of
the water and pupate in the
soft mud on the stream
bank in autumn, emerging
the following spring or
summer. PT3

106. Crane Fly larvae are called Leatherjackets by some fishers since their tough skin helps keep
them on the hook. Crane fly larvae are big enough to interest hungry Trout or Smallmouth
Bass in mid-summer. Noted 2-19-13 Bohemian Creek.

107. Crane Fly adults
often creep people
out since they look
like super-duper
mosquitoes.
Crane Flies have
very long fragile
legs that often
break off if caught.
Crane flies do not
bite. In fact, they
don’t even eat
anything.
Watch for
different species
all summer long.

108. Though Crane Flies are most often
found hiding in the dense
vegetation along stream banks,
they sometimes show up on our
window screens back home.

109. Aquatic Beetle larvae,
belonging to the Insect
Order Coleoptera, can be
confused with Caddisfly
larvae.
This Riffle Beetle (Family
Elmidae) larvae lives
among bottom debris
under rocks and feeds on
organic detritus, meaning
any disgusting junk on the
stream floor.

110. Riffle Beetle larvae
require lots of
dissolved oxygen in
the water to survive,
and concentrate
around rapids and
riffles where that
requirement has the
best chance of being
met. PT4

111. Riffle Beetles have
long legs for a beetle
and can resemble a
slow-moving spider
crawling over a rock
in the creek.
Riffle Beetles eat
diatoms and algae
on rock surfaces.

112. Riffle Beetles are not
very big, smaller
than a pea.
Compare with Caddis
& Mayfly larvae
Riffle BeetleCaddis
Mayfly

113. This scary little larvae will one day turn into a Whirligig Beetle (Family Gyrinidae). They swim
snake-like through the water and prey on water mites and other slower bugs.

114. Whirligig Beetle larvae
cruise around in quieter
river stretches with lots of
rich organic matter around.
Whirligig Beetle larvae
pupate in cocoons attached
to submerged plant stems
near shore. PT5

115. Adult Whirligig Beetles look like shiny little black stones floating on the water surface. They
are also predaceous, and feast on snails, Midge larvae or other soft-bodied invertebrates.

116. Whirligig Beetles look like little motorboats tooling around in calm shallow spots along the
stream. They can dive underwater for extended periods to find food or hide from danger,
carrying along an oxygen tank in the form of an air bubble attached to their body.

117. Whirligig Beetles will often congregate in
flotillas that spin wildly about like bumper
cars

118. True Bugs, such as
this menacing
Giant Water Bug
(Family
Belostomatidae),
belong to the
Insect Order
Hemiptera.
Nymphs look just
like the adults,
only smaller as
they molt into
older forms.
Giant Water Bugs
are affectionately
known as Eastern
Toe-biters.
Small fish are
included in their
varied animal diet

119. Giant Water Bugs
swim or crawl
around in quieter
waters while they
hunt for food. PT6

120. Adult Giant Water Bugs can fly and sometimes show up around porch lights at night, hence
another common given name “Electric Light Bug”. They can live for years and prey on tadpoles,
small Frogs and even Salamanders. They fit comfortably in your hand.

121. Another big mean bug out there in the slower moving river waters is the stick-like Water
Scorpion (Family Nepidae). Water Scorpions sit still in the water weeds and wait for Stoneflies
or other aquatic insects to wander within range of those specialized front legs that work like
long pincers.

122. Water Scorpions can row themselves through the water in slow-motion but they’re more likely
to be found hiding in flooded grass close to shore. PT5
Water Scorpions can grow to over two inches long and live at least a year.

123. Water Striders (Family Gerridae) are
pretty cool True Bugs that are often
confused as being water Spiders.
But they only have six legs

124. Water Striders are also
called Pond Skaters or
my personal favorite –
Jesus Bugs – since they
can walk on water.
Notice how their legs
never break through
the fluid surface film.

125. Water Striders glide over still pools where
they catch and devour mosquito larvae
suspended under that busy surface film. PT6

126. True Bugs can be told from Beetles by looking at their backs. This Water Boatmen (Family
Corixidae) has leathery wing covers rather than a hard shell. Notice how their paddle-like
middle legs oar them through the water

127. Water Boatmen swim around where water weeds give them cover as they catch and eat algae.
PT9

128. Look close and you can see that this supposed Water Boatmen is actually a Common
Backswimmer (Family Notonectidae). They strangely swim around upside down and look like
little torpedoes under water, but instead of rocketing around they move in herky-jerky
motions.

129. Common Backswimmers have a lot of white on their backs, which would be seen from the
stream bottom, so they blend right in with the bright surrounding sky. They eat animal matter.
PT8

130. These last two aquatic
invertebrates are not
insects but they look like
bugs and trout definitely
relish them.
Freshwater Shrimp
(Gammarus)
are Arthropods
In the Order
Amphipoda.
Freshwater
Shrimp are
commonly
known as
Scuds
or
Sideswimmers,
since they have
a different way
of swimming
too

131. Freshwater Shrimp often swim on their sides. Most species are orange though some are pink
and others are green. Trout that feed heavily on Gammarus have bright orange flesh, thanks to
all that carotene contained in the scud’s carapace. Active all year.

132. Scuds require lots of water weeds to hide in and hunt for food. Being omnivorous, they munch
on those water plants as well as smaller Mayfly larvae. Scuds can be super-abundant in such
situations, and trout gorge on them, but good luck getting a lure in there. PT6

133. Scud patterns should be thought of as a must-have fly, tied up in orange and green for sure.

134. Aquatic Sowbugs (Asellus) are Arthropods in
the Order Isopoda. They feed on algae, plant
parts and even other immature invertebrate.
Aquatic Sowbugs are commonly called
Cressbugs.

135. Aquatic Sowbugs build up their biggest numbers in
quieter waters with heavy weed growth. PT8

136. Aquatic Sowbugs are
cousins to the terrestrial
Cellar Sowbugs we all
know as Pillbugs or Roly-
polys.
Aquatic Sowbugs are
seldom seen in the same
numbers as Freshwater
Shrimp, and they crawl
around on the bottom or
rocks rather than swim,
so they’re not quite such
a big target for feeding
trout to hit.
Active all seasons.