Introduction to the Identification of Adult Insects

and Related Arthropods - 2010

P. M. Choate

"Much of our usual appreciation of an animal - in any condition -

depends on our ability to identify and name it..."
R. M. Knutson (1987), "Flattened Fauna."
2 Identifying Insects and Related Arthropods

Identification Key to the Classes of Adult Arthropoda

Insects represent one Class of animals within the Phylum Arthropoda. If you do not immediately
recognize an insect you may need to identify some arthropods to first determine if they are in fact insects
before proceeding further.
Biologists have adopted the use of dichotomous keys to identify organisms. Starting at couplet 1, decide
which of the first 2 choices best fits the organism you are trying to identify. Proceed by going to the couplet
indicated at the end of your choice. By process of elimination you will arrive at an identification. Compare
your results with pictures and notes in this handout and in your books to see if you have arrived at a likely
identification. If you are satisfied with your result, proceed to the next key that you wish to use and follow
the same process. As you move from Class to Order to Family and perhaps to Genus and Species you
will notice that choices may become more difficult. This is due to the details necessary to separate these
categories. Since this key is designed to help you recognize insects, and to also recognize Arthropods that
might be confused with insects, we will start with an obvious and surefire couplet, #1. There are many
insects which do not appear to have wings or actually lack wings. However, many have easily observable
and functional wings which immediately identify the creature as an insect.

1. With 1-2 pair of obvious, visible, usually transparent

wings (Fig. 1). (If it's not a bird or a bat then it
is an insect)............................................. Insecta
Antenna

Wings

‑ Without obvious wings (front wings may be modified

in the form of hardened covers for flight wings;
elytra, tegmina, hemelytra)............................... 2

2. With 1 or 2 pairs of antennae ("segmented feelers")

Pedipalps, not of various shapes (see Figs. 1, 3-5) inserted on
antennae front of head, usually between the eyes. Anten-
nae may be inconspicuous, hidden beneath head
when viewed from above, or small and more
bristle-like than typical segmented structure.
Note: some non-insects carry their front legs
or modified mouthparts (pedipalps, Fig. 2) in a
manner resembling antennae ........................... 3
‑ Lacking segmented antennae and always lacking
Fig. 2. Spider any suggestion of wings........8
3. With 2 pairs of antennae (one pair may be smaller
than the other; 2nd pair is vestigial in terrestrial
Isopoda (pillbugs, sowbugs); body usually with
2 distinct regions (Figs. 3-4), cephalothorax
Insect Identification - Part I - (Classes of Arthropoda, Orders of Insecta) 3

and abdomen; variable number of legs on cepha-

lothorax, abdomen with or without appendages
Fig. 3. which when present are not leg-like; (amphipods,
sowbug sowbugs, lobsters, crayfish)............... Crustacea
‑ Always with only 1 pair of antennae; body regions
and numbers of legs variable; appendages not

Fig. 4. Crayfish

biramous.............................................................. 4

4. With only 3 pairs of legs and often with 1-2 pairs of

wings; 3 body regions (Fig. 5 ). Abdomen without
segmented legs but may have appendages; body
shape variable......................................... Insecta
‑ With 9 or more pairs of legs (Figs. 6-7 ) which are on
most segments posterior to head; head distinct;
wings absent; body elongate and wormlike....... 5

5. Legs evenly spaced along body, usually 1 pair of legs

per segment......................................................... 6
‑ Legs arranged in pairs, 2 pair per segment (Fig. 6)(mil-
lipedes)...............................................Diplopoda
Fig. 5. Grasshopper
6. Body flattened, with 15 or more pairs of legs; size vari-
able but usually longer than 25mm.(Fig. 7a)
(centipedes ).......................................Chilopoda
‑ Body cylindrical; minute forms with 9-12 pairs of legs...
............................................................................. 7
Fig. 6. 7. Antennae branched (Fig. 7b); 9 pairs of legs ...............
.......................................................... Pauropoda
Millipede ‑ Antennae not branched; 10-12 pairs of legs (Fig. 7c)....
.............................................................Symphyla

8. Three pairs of legs; eyes lacking; size ± 1mm................

................................................................ Protura

7b. Pauro- 7c. Symphylan

Fig. 7a.
pod
Centipede
4 Identifying Insects and Related Arthropods

- With more than 3 pairs of appendages; size much

larger................................................................... 9
9. Usually 7 pairs of appendages, 5 pairs of legs; marine
organisms only; abdomen rudimentary...............
.......................................................Pycnogonida
‑ Six (rarely fewer) pairs of appendages, with 4 (-5) pairs
7d. Horseshoe crab of legs; abdomen well developed...................... 10

10. Large marine forms up to 460mm in length; body oval,

covered with hard shell; long spinelike tail pres-
ent. (Horseshoe crabs. Fig. 7d).......Xiphosura
‑ Smaller forms, less than 75mm in length; body without
hard shell and without spinelike tail; See below.
(spiders, ticks, mites, whip scorpions, windscorpi-
ons, scorpions) *immature mites and ticks have
3 pairs of legs, but only 2 defined body regions..
........................................................... Arachnida

Some Arachnids

Mite Crab Spider

Scorpion

Vinegaroon
Pseudoscorpion
Insect Identification - Part I - (Classes of Arthropoda, Orders of Insecta) 5

Dichotomous Key to the Orders of Adult Insects

(Note: Megaloptera is included here in Neuroptera,
Thysanura = Archeognatha; Mantophasmatodea and Grylloblattodea are omitted)
(modified 2010)
Once you have determined that the organism you have before you is an insect you may wish to further
identify it. This means that you may have to use additional keys to determine the Order of that specimen.
Some insects will be immediately recognized as insects but you may not be familiar with the order to which
it belongs. The key that follows will help you determine many of the more commonly encountered insects.
Not all insects will be able to be determined here. If you decide that your specimen may not be included
here, check other reference books. These should permit identification of any specimen you happen upon.
Once you have determined the Order, the next step is to determine the Family within that order to
which the insect belongs. This may mean an increase in complexity for you, and will usually require ad-
ditional knowledge about specific types of structures and the variation that exists within these structures.
Once the family of an insect has been determined you are left to hunt for literature that will permit identi-
fication to genus and species. Not only may this prove difficult, it may prove impossible. Not all insects are
discussed or are identifiable to species. Literature may be scattered, outdated, or non-existent. You may
have to call upon specialists for help. This is a normal part of the identification process. For our purposes
here we will concentrate upon keys that should help you arrive at an Order level identification, and within
a few of these Orders, some of the more commonly encountered families.
The following key to orders begins with a couplet that asks you to determine whether or not the insect
has wings. This may be a confusing beginning for you. Many insects have flight wings which are hidden
beneath another set of modified wings called elytra, tegmina, or hemelytra (see examples on page 10).
Elytra are wings which act as protection and covering for the flight wings of beetles. At first glance there is
little to indicate to you that these insects have wings. To further confuse the issue there are many beetles
that lack flight wings, and whose elytra are fused to form a solid cover. Similar modifications may occur in
such diverse groups as grasshoppers and the true bugs.
A similar point of confusion may be the determination of presence or absence of antennae. Antennae
come in a variety of sizes and shapes (see pages 11-13). Dead insects may have antennae hidden or broken.
If antennae are not apparent check to see if attachment "sockets" are visible on the insect's head in front
of and beneath the eyes.
These two characters (antennae and wings) are mentioned here to emphasize the point that it is almost
impossible to generalize about the characteristics that make up an insect order. There are many excep-
tions. With this in mind, proceed to identify specimens, using your book and examples here to help arrive
at your final determination. When you have finished you should go to a museum or reference collection and
compare your results. With practice you may find that the identification of insects will become a challenging
avocation.

1. Wings present ( wings may be hidden under external

elytra (p. 10-11), hemelytra (Fig. 8), or tegmina
(Fig. 9) such that "wings" do not appear to be
present)............................................................. 23
— Wings absent or reduced to small pads; many abdominal
segments visible from above.............................. 2

2. Antennae absent; body slender and whitish in color.

Very small (Fig. 10) (1mm.).................. Protura
— Antennae present (may be difficult to see)................. 3

3. Usually with forked springing mechanism (furcula -

Fig. 11) on abdomen. Size small, 2-4mm. Always
lacking apical abdominal cerci. If furcula absent,
size and body shape are characteristic of order..
Fig. 9. Tegmen .......................................................... Collembola

Fig. 8. Hemelytra
6 Identifying Insects and Related Arthropods

Fig. 10. Protura — Furcula always absent. Body size and shape various..
............................................................................. 4

4. Apex of abdomen with long segmented cerci (Fig. 13),

ventral styliform appendages present; or if ventral
abdominal styliform appendages are lacking, cerci
are short.............................................................. 5
— Cerci short or lacking; abdominal styliform appendages
always absent..................................................... 7
furcula
5. Apex of abdomen with 3 filamentous cerci (silverfish).
Fig. 11. Collembola 6
— Apex of abdomen with 2 cerci, either forceps-like (Fig.
12) or short and segmented....................Diplura

6. Eyes large and contiguous on top of head......................

....................................................Microcoryphia
— Eyes small and separated on top of head......................
...........................................................Thysanura

7. Large unsegmented forceps-like structures at apex of

abdomen(earwigs).......................... Dermaptera
— Cerci (when present) neither forceps-like nor unseg-
Fig. 12. mented................................................................. 8

Fig. 13. Thysa- Diplura 8. Large insects, usually > 25mm in length; antennae
nura frequently very long and slender....................... 9
— Small insects, usually < 12mm in length.................. 10
cornicle
9. 4-segmented tarsi....................................... Orthoptera
— 5-segmented tarsi..................................Phasmatodea
Fig. 14. Homop-
10. Tube-like structures (cornicles) (Fig. 14) protruding pos-
tera teriorly from 4th to last abdominal segment; OR
body covered with waxy filaments or a scale......
basitarsus Hemiptera s.o. Sternorrhyncha
— Cornicles absent AND no scale or waxy filaments cover-
ing body............................................................. 11

Fig. 15. Embiidina 11. Abdomen constricted to narrow waist where it joins
thorax (bees, wasps, ants, sawflies).....................
......................................................Hymenoptera
— Abdomen not constricted into narrow waist............. 12

12. Front legs with enlarged first segment (Fig. 15) (ba-
sitarsus), which is modified for production of silk
Fig. 16. Thysanoptera (webspinners).....................................Embiidina
— First tarsal segment not enlarged............................. 13

13. Mouthparts (rasping-sucking) contained in a short,

cone-like beak; wings often with fringe of hairs
(Fig. 16); size <3mm; abdomen often pointed at
apex (thrips)................................ Thysanoptera
— Mouthparts other than rasping-sucking; may be in
Fig. 17. Siphonaptera form of elongate beak which extends ventrally
and posteriorly beneath head;......................... 14

14. Body flattened laterally, with numerous backward-

Insect Identification - Part I - (Classes of Arthropoda, Orders of Insecta) 7

projecting spines and bristles; legs long, with

greatly enlarged coxae modified for jumping (fleas)
(Fig. 17)........................................Siphonaptera
— Body not flattened laterally; may have hairs or spines
but these are not backwards projecting; if legs
are modified for jumping, femora are enlarged...
........................................................................... 15

15. Mouthparts elongated into piercing-sucking beak......

........................................................................... 16
— Mouthparts not elongated into long piercing beak; head
may be prolonged.............................................. 17

16. Antennae hidden in grooves in head..............Diptera

— Antennae long and easily seen (Fig. 18)... Hemiptera

17. Body covered with dense hair................ Lepidoptera

— Body lacking dense hair............................................. 18

18. Antennae moniliform (segments beadlike); short cerci

present (Fig. 19)(termites)................... Isoptera
— Antennae not moniliform; cerci absent..................... 19

19. Antennae long and slender....................................... 20

— Antennae short........................................................... 21

20. Head prolonged and beak-like (Fig. 20, 34); males of

some species with scorpion-like abdomen (scor-
pionflies)............................................ Mecoptera
— Head not prolonged and beak-like............ Psocoptera

21. Tarsi with 4-5 segments.................................Diptera

— Tarsi with 1-3 segments (Phthiraptera)................. 22

22. Chewing mouthparts; head usually broader than long

(Fig. 21a) ...........s.o's Amblycera, Ischnocera
— Piercing-sucking mouthparts retracted into head; head
usually longer than broad; legs greatly enlarged
for grasping (body lice)(Fig. 21b).........................
...................................................... s.o. Anoplura

23. Abdomen with large unsegmented forceps-like cerci

(Fig. 24).......................................... Dermaptera
— Cerci appearing segmented when present, not forceps-
like, or absent .................................................. 24

24. Cerci filamentous, longer than last 3 abdominal seg-

ments combined................................................ 25
— Cerci shorter than last 3 abdominal segments combined,
not filamentous, or totally absent.................... 29

25. Wings folded upright and parallel to body length;

antennae setaceous (Fig. 22. mayflies)................
..................................................Ephemeroptera
— Wings various but not held upright above body; anten-
nae elongate and filiform.................................. 26
 8 Identifying Insects and Related Arthropods

26. Front pair of legs shaped differently than mid and hind
Fig. 25. Odonata wing pair, modified for digging (Fig. 23a) (fossorial) or
grasping (Fig. 23b) (raptorial)..............................
.......................................................... Orthoptera
— Front pair of legs similiar to middle pair.................. 27

27. Hind pairs of legs with femora enlarged for jumping

(Fig. 23c)........................................... Orthoptera
— Hind pair of legs similar to middle pair.................... 28

28. Tarsi 3-segmented; cerci long or short, not forceps-like;

Fig. 26. Aquatic many segmented...............................Plecoptera
— Tarsi variable (4-5 segments). Includes large, bulky
Hemiptera insects, frequently with well developed wings...
32
29. Cerci present, shorter than last 3 abdominal segments
combined........................................................... 30
— Cerci absent (do not be confused by genitalia)......... 34

Fig. 27. Hemiptera 30. Small, delicate insects; wings transparent, uniform
shape and size................................................... 31
— Body shape varied; wings in form of elytra, tegmina,
or hemelytra...................................................... 32

31. Front basitarsi (1st tarsomere) enlarged and dilated

Scutellum to form a webspinning organ............Embiidina
— Front basitarsi not enlarged and dilated, appearing of
normal proportions (termites).............. Isoptera
Hemelytron
32. Tarsi 4-segmented.................................... Orthoptera
— Tarsi 5 segmented...................................................... 33

33. Prothorax much longer than mesothorax; front legs

Fig. 28. Coleoptera modified for grasping..........................Mantodea
— Prothorax not greatly lengthened; front legs not modi-
Elytron fied for grasping.................................. Blattodea

34. Large insects with 2 pairs of wings; wings usually

transparent, each wing with an anterior node
(Fig. 25)or notch (dragonflies, damselflies).........
............................................................... Odonata
— Wings variable but lacking anterior node................. 35

Tegmen 35. One pair of wings; halteres present...............Diptera

— Two pairs of wings; halteres absent.......................... 36

36. Mouthparts in the form of a piercing-sucking, elongate

beak which is mostly held beneath and behind
the head; palpi absent...................................... 37
— Mouthparts other than above; palpi present............ 39

Fig. 29. Orthoptera 37. Hind leg without tarsal claws; adapted for swimming
(Fig. 26)............................................. Hemiptera
— Hind leg with tarsal claws......................................... 38
Siphon
38. Beak arises from anterior part of head; forewings usu-
ally as hemelytra (Fig. 27)............... Hemiptera

Fig. 30. Lepidoptera

Insect Identification - Part I - (Classes of Arthropoda, Orders of Insecta) 9

— Beak appears to originate from between front pair of

legs; forewings of uniform texture.......................
Fig. 31. Neuroptera ................. Hemiptera s.o. Auchenorrhyncha

39. Rasping-sucking mouthparts in form of cone-like beak;

wings fringed with long hairs..............................
crossvein ..................................................... Thysanoptera
— Not as above................................................................ 40

40. Front pair of wings hardened, of different texture than

rear flight wings................................................ 41
— Front wings not thickened or hardened to form cover
for flight wings.................................................. 42

41. Front pair of wings thickened and usually hard, without

crossveins, meeting along midline (meson) of the
body to form elytra (Fig. 28); many forms with
elytra shortened, exposing one or more abdominal
segment from above (beetles); hind legs usually
not modified for jumping......................................
...........................................................Coleoptera
— Front pair of wings with obvious crossveins and veins
(Fig. 29, tegmen), overlapping one another at least
partially; hind legs often enlarged for jumping
(grasshoppers, crickets, Katydids).......................
Fig. 32. Hymenoptera .......................................................... Orthoptera

42. Front basitarsi (1st segment) enlarged to form silk-

producing glands (Fig. 15) (webspinners)...........
............................................................Embiidina
— Front basitarsi not any more enlarged than remaining
segments........................................................... 43

43. All wings equal in size; (termites)............... Isoptera

— Hind wings usually smaller than front pair of wings; .
44

44. Mouthparts in the form of a coiled siphon (Fig. 30);

wings and body usually covered with scales (but-
terflies and moths)......................... Lepidoptera
— Mouthparts not in the form of a coiled siphon; body
scales absent or few in number, restricted to
wings and wing veins....................................... 45

45. Many crossveins in wings (Fig. 31), particularly at

anterior edge; if few crossveins, wings covered
with waxy coating and insect very small............
......................................................... Neuroptera
Fig. 33. Psocoptera — Few crossveins in wings; body and wings lacking waxy
coating............................................................... 46

46. Mouth reduced, vestigial; only palpi obvious; hairs

often present on wings (caddisflies).....................
.........................................................Trichoptera
— Mouthparts not reduced or vestigial; chewing or chewing-
lapping types..................................................... 47
10 Identifying Insects and Related Arthropods

47. Chewing mouthparts elongated into a beaklike struc-

ture. Some males with scorpion-like abdomen
(scorpion flies) Fig. 20, 34................. Mecoptera
— Chewing mouthparts not elongated into beak; or with
chewing-lapping mouthparts........................... 48

48. Tarsi 3, 4- or 5-segmented; wings folded flat over body

(Fig. 32) (bees, wasps, ants, sawflies)..................
......................................................Hymenoptera
— Tarsi 2- or 3-segmented; wings folded roof-like over
body (Fig. 33) (treelice, booklice).........................
.......................................................... Psocoptera

Fig. 34. Mecoptera: Panorpidae; Male, Vermont: Caledonia

Co., West Barnet, Harvey's Lake. Photo P. M. Choate.
Insect Identification - Part I - (Classes of Arthropoda, Orders of Insecta) 11

Elytra covering flight wings

Stag Beetle (Coleoptera: Lucanidae) with elytra Stag Beetle (Coleoptera: Lucanidae) with elytra
closed, appearing to lack "typical" wings. opened, preparing to take flight. Note visible "flight"
wings.

Note "feathery" antennae of this male moth (Lepidoptera).

12 Identifying Insects and Related Arthropods

Beetle

Cicada

Flea

Grasshopper Variations on appearance of insect

antennae.

Long-horned
beetle
Dragonfly
 Insect Identification - Part I - (Classes of Arthropoda, Orders of Insecta) 13

Grasshopper, frontal and lateral view of head.

eye

clypeus

labrum maxillary palp

labial palp

Thorax (wings, legs)

Head Abdomen (cerci, genitalia)
(antennae, eyes, mouthparts) meso- meta-

pro-

spiracle

mesotarsus
protarsus
metatarsus

Body regions of grasshopper

14 Identifying Insects and Related Arthropods

5
3
coxa
2

1
trochanter

femur

tibia 5 segmented tarsus, with segment 4 "hidden"

tarsus
insect leg

tarsal claw

filiform
setaceous

serrate moniliform

pectinate plumose

stylate
aristate

lamellate capitate clavate

A few adult insect antennal types