Student Handout #2-

Dichotomous Key for Aquatic Insects

1a. More than three pair of legs .............................................................. not an insect
1b. Zero to three pair of legs only ................................................................................ 2

2a. With obvious wings ................................................. adult insect (see adult key)
2b. Without wings or with wing buds only ................................................................. 3

3a. Wing buds present; not worm-like......................................................................... 4

3b. No wing buds; may be worm-like .......................................................................... 8

4a. Long, piercing mouthpart begins at the top of the head; ranges in size from 2 to
60mm (true bugs) ...............................................................Hemiptera (Figure 1)
4b. No piercing mouthpart ............................................................................................ 7

Figure 1.

5a. Underside of mouth with an extension arm that may cover face like a basket, or be
flat against bottom of head, but usually with „teeth‟ (jagged edges).................. 6
5b. Lower mouthparts not obviously enlarged or able to be extended .................. 7

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6a. Tip of Abdomen with three leaf like gills; ranges in size from 13 to 68mm, not
including antennae or tails (damselflies)............................. Odonata (Figure 2)
6b. Tip of Abdomen with terminal triangular-shaped spines; ranges in size from 13 to
68mm, not including antennae or tails (dragonflies) ........ Odonata (Figure 3)

Figure 2. Figure 3.
7a. Abdomen with lateral gills that are „leaf-like‟ or „bushy‟; gills can also look like a
feather duster located all along the sides of the abdomen; also tip of abdomen with
three tails‟ (rarely with only two); ranges in size from 2 to 32mm, not including
antennae or tails (mayflies) .........................................................................................
..................................................................................... Ephemeroptera (Figure 4)
7b. Thorax only with gills, never on sides of abdomen; look for feathery or “leaf-like”
gills under the “armpits”; tip of abdomen with two „tails‟; ranges in size from 5 to
70mm, not including antennae or tails (stoneflies) ....... Plecoptera (Figure 5)

Figure 4.

Figure 5.
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8a. Mummy-like; wings, legs, antennae held tight against body wall, may or may
not have a case or silken cocoon pupa (sing.)pupae (pl., Figure 6)
8b. Body worm-like, maggot-like, or caterpillar-like; zero or three pair of „true‟
legs .............................................................................................................................. 9

Figure 6.

9a. No true legs on the thorax; may have “false legs” on the abdomen ............... 10
9b. Three pair of true legs present on the thorax ...................................................... 12

10a. Worm-like body without a distinct head capsule; ranges in size from 2 to 25mm,
occasionally 100mm as mature larvae (flies) .........................Diptera (Figure 7)
10b. Worm-like body with a distinct head capsule ................................................... 11

Figure 7.

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11a. Worm-like body with a distinct head capsule and no structures on the end of the
abdomen; ranges in size from 2 to 70mm, excluding tails ....................................
.............................................................................................. Coleoptera (Figure 8)
11b. Worm-like body with a distinct head capsule and a breathing tube or other
structure at the end of the abdomen; ranges in size from 2 to 25mm, occasionally
100mm as mature larvae ..........................................................Diptera (Figure 9)

Figure 8. Figure 9.

12a. Large head with strong mandibles; with eight pair of gills, each extending
laterally from an abdominal segment; ranges in size from 10 to 90mm
(hellgrammites)................................................................ Neuroptera (Figure 10)
12b. Worm-like body with or without a case that can be made of sand, pebbles,
or sticks; ranges in size from 2 to 43mm (caddisflies) ...........................................
........................................................................................... Trichoptera (Figure 11)

Figure 10. Figure 11.

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Student Data Sheet

Microhabitat type:

Order
Coleoptera
Diptera
Ephemeroptera
Hemiptera
Neuroptera
Odonata
Plecoptera
Trichoptera
unknown

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 Student Data Sheet

Microhabitat type:

classification
skaters
adapted to skating across the surface of the water with
their tarsi specialized for pushing water to propel them
forward in pools within the stream

divers
adapted for swimming in mountain stream pools by
"rowing" with their hind legs

swimmers
adapted for swimming in pools and runs much like fish
and may cling to submerged objects in between bouts of
swimming

clingers
adapted with long, curved tarsal claws, dorsoventral
flattening, and ventral gills arranged as suckers for
attaching to surfaces in stream riffles or they can have a
fixed retreat

sprawlers
adapted for staying on top of leaves and debris or fine
sediments found in pools within the stream while
keeping their respiratory surfaces free of silt

climbers
adapted for moving up roots, sticks, and branches along
the stream‟s edge (along runs) or stream bank undercuts
(in pool areas)

burrowers
adapted to living in the fine sediments of stream pools,
may even construct burrows of sand grain tubes which
may extend above the surface of the substrate

unknown

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fees for instruction and/or materials, must receive permission from the SC LIFE Project Director. Contact Ginger Foulk, SC
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 References

Borror, Donald J., Charles A. Triplehorn, and Norman F. Johnson. 1989. An

Introduction to the Study of Insects. Harcourt Brace College Publishers,
New York.

Lehmkuhl, Dennis M. How to Know the Aquatic Insects. 1979. WCB/McGraw-

Hill, Boston.

Merritt, Richard.W. and Kenneth W. Cummins. 1978. An Introduction to the

Aquatic Insects of North America. Kendal Hunt Publishing Company,
Dubuque, Iowa.

Images

Images used in this key are either public domain images from the following sources:

Folsom, Justus Watson. 1922. Entomology with Special References to its Ecological
Aspects. 3rd Edition. P. Blakiston‟s Son & Co, Philadelphia.

Miall, L.C. 1895. The Natural History of Aquatic Insects. McMillan and Co, New York.

Osborn, Herbert. 1916. Agricultural Entomology. Lea and Febeger, Philadelphia.

Ross, Herbert H. 1963. How to Collect and Preserve Insects. Circular 39. Natural History
Survey Division, Urbana, Illinois.

Wellhouse, Walter Housley. 1926. How Insects Live: An Elementary Entomology.

McMillan and Co, New York.

SC LIFE materials are available for use only in non-profit educational activities. Any other uses, including activities involving 7
fees for instruction and/or materials, must receive permission from the SC LIFE Project Director. Contact Ginger Foulk, SC
LIFE Project Office, 132 Long Hall, Clemson, SC 29634, 864-656-4224, with questions about any of our SC LIFE materials or
programs. COPYRIGHT SC LIFE.