Download Arthropod key

Arthropod Key
Adapted from Dr. Kelly Kissane, Blinn College
Your first task in this lab will be to classify each of the arthropods you sampled in your traps. For
this purpose you will use the simple key in this document. To use the key successfully, make sure to
read the Arthropod Anatomy section before coming to lab.
Arthropod Anatomy
Insects always have three body parts, and six legs. The three body parts are the head, thorax,
and abdomen (other arthropods have more or fewer body parts, as you will see in the key).
The
head has one segment, the thorax has three segments, and the abdomen can have many segments
depending on the insect species.
The six legs are always attached to the thorax, one pair of legs per segment. Each leg has a
femur (thigh), a tibia (shin), and finally the tarsus. Tarsus means toe, so the tarsus is the toe of the
insect.
If the insect has wings, the wings will also be attached to the thorax. Most insects have two pair of
wings, the forewings and the hind wings. They are always attached to the 2nd and 3rd segment. But
flies have only one pair of wings – the forewings. The hindwings are replaced with tiny knobs called
halteres. These halteres help a fly do somersaults in the air, something that other insects cannot do.
The insect in this picture is in the order Orthoptera. Orthopterans include grasshoppers, crickets,
and katydids. Remember, if it does not have three body parts (head, thorax and abdomen) and six
legs, it is not an insect!
An important body part to know is the tarsus (tarsi) of the insect. The tarsus can have one, two,
three, four or five segments. You count the number of segments after the tibia, but you do not count
the claws. See the picture below to learn how to count the segments of the insect’s toes.
This picture of an insect’s leg shows that the tarsus has four segments.
Sometimes the segments of the tarsus look different from each other, like the legs of a honeybee.
Look at the picture of a honeybee’s leg on the next page. This insect has a tarsus with five
segments. Can you count them all?
Remember, you do not count the claws as one of the tarsal segments. The first tarsal segment of
bees is enlarged because honeybees use it to gather pollen from flowers.
Wings are also important. Most insects have two pairs of wings. But Diptera (true flies) only have
one pair of wings. They also have two knobs or club-like structures where the hind pair of wings
normally are on insects. The picture below shows you what some of the halteres look like. Some
flies, however, have small, fan-shaped halteres instead of knobs. If you are in doubt, remember that
only flies have one pair of wings. All others have two pairs of wings.
All juvenile insects are wingless or have very tiny wings (wing buds). Most adult insects have
large, fully developed wings. But some insects are wingless even as adults. In most cases, an adult
will have a hard exoskeleton (the hard shell covering the soft internal organs of an insect). Juveniles
will have softer bodies. This will help you determine if the insect is a wingless adult, or a juvenile.
The last body part you should know well is the cerci. Many, though not all, insects have thread
like or curved appendages on their abdomens. These appendages are cerci. The picture below will
show you what curved cerci look like. Some insects, like Dermaptera (Earwigs), always have curved
cerci.
Other cerci are threadlike. This picture shows an Ephemeroptera (mayfly). Mayflies usually have
three threadlike cerci.
Arthropod key
Keys are commonly used to identify animals. You start with question one, choose the sentence
that matches your insect, and then you follow the directions listed afterwards. You continue to follow
the directions until you are given the name of the arthropod. For example, in this key, Question one
asks you if the animal has legs or no legs. If it has legs, you go to question 2. If it does not have
legs, then you have either a worm or a juvenile fly. If you become stuck on a particular question, go
back to the previous question. Sometimes you may need to start over when you first begin to use a
key. With practice, you will be able to recognize arthropods very quickly.
1. Organism has no legs…………...……….….…Worms or Juvenile flies.
1.” Organism has legs………………………...…..….(arthropods) goto 2.
2. Organism has more than 6 legs……………..……..……….…….goto 3.
2.” Organism has 6 legs…………………………………..….………..goto 7.
2.”’ Organism has fewer than 6 legs…………………...….juvenile insects.
3. Organism has more than 3 body segments…………...………...goto 4.
3.” Organism has 3 body segments or less…………………..…..…goto 5.
4. Organism has 7 thoracic segments, each with one pair of
legs……..……………………………………...……………....Crustacean.
4.” Organism has more than 7 body segments, each with one pair
of legs………………………………………..…...Chilopoda (centipede).
4.’’’ Organism has more than 7 body segments, each with TWO
pairs of legs……………………...………....…...Diplopoda (millipedes).
5. Organism has one body part and 8 legs…………….…..…...…goto 6.
5.” Organism has two body parts and 8 legs…………….…..….…Spider.
6. Organism is very small, about the size of a pinhead……………...Mite.
6.’ Organism is larger than a pinhead…………….……………………Tick.
6.’’ Organism has very long legs………………………………Harvestman.
7. Organism fully developed wings……...………………..……….goto 33.
7.” Organism has no wings or wing buds (undeveloped)……..….goto 8.
8. Organism has antennae on head…………………..………..…...goto 9.
8.” Organism does not have antennae, lives under rocks………..Protura.
9. Organism sucks blood/fluids from other animals (parasites)…goto 10.
9.” Organism is not a parasite…………………………………….....goto 12.
10. Organism has tarsi with 5 segments………………….. Siphonaptera.
10.” Organism has tarsi with less than 5 segments….…….…..…goto 11.
11. Antennae are longer than the insect’s head…...………….Hemiptera.
11.” Antennae are shorter than insect’s head, tarsi has 1
segment……………..……………………….………..…...Phthiraptera.
12. Mouthparts are straws or beaks, sucking insects…………....goto 13.
12.” Mouthparts have little teeth, chewing insects…………….….goto 15.
13. Tarsi have five segments, wings have scales.………..…Lepidoptera.
13.” Tarsi have 4 or fewer segments…………………………….... goto 14.
14. Mouthparts are like a small cone, long body, small insects
(less than 5 mm).………………………………………...Thysanoptera.
14.” Mouthparts are long, like straws or beaks……………......Hemiptera.
15. Abdomen has a “pinched waist”, ant-like.…..……….…Hymenoptera.
15.”Abdomen is not pinched, antenna not elbowed………..…….goto 16.
16. Abdomen has 3 threads at end, body is scaly….………....…goto 17.
16.”Abdomen has 2 or less threads at end……….………..……..goto 18.
17. Large compound eyes, back has a hump..…………...Microcoryphia.
17.” Small compound eyes, no hump, flat-looking………...….Thysanura.
18. Aquatic insects that live in the water…………………..…..…goto 19.
18.” Terrestrial insects that live on land………….………….……goto 24.
19. The insect has compound eyes ……………………….……..goto 20.
19.” The insect does not have eyes…………………..……………goto 22.
20. Jaw is long, spoonlike, rests under the head………..……..Odonata.
20.” Jaw is not spoonlike…………………….……………....…..goto to 21.
21. Abdomen has small gills on each segment………...Ephemeroptera.
21. Abdomen does not have small gills………………..………Plecoptera.
22. Tarsi have 2 claws, antenna long and slender……....…..Neuroptera.
22. Tarsi have 1 claw, antenna short………………..….…….…..goto 23.
23. Abdomen has 2 hooks at end, insect lives in cases…....Trichoptera.
23.” Abdomen has 4 or zero hooks, insects do not live in cases.
…………………………………………………….……..…...Coleoptera.
24. Abdomen has long threads, small insects less than 7 mm in
length………………………………………………….…….….goto 25.
24.” Abdomen does not have threads…...…………..………..….goto 26.
25. Antenna long, lives under rocks or logs…………………........Diplura.
25.” Antenna short, has a “spring” under abdomen………....Collembola.
26. The insect has compound eyes………..…………………..…goto 27.
26.” The insect does not have eyes…………..………..…Juvenile insect.
27. Tarsi have 5 segments……………………………….…..….….goto 28.
27.” Tarsi with 4 or fewer segments…………………………..…...goto 29.
28. Body slender, insects look like leaves or sticks………..…Phasmida.
28.” Body flattened, insects look like roaches………..….…….…Blattaria.
29. Organism has cerci, tarsi has 3 segments……..……………goto 30.
29.” Organism has no cerci………………………..……………….goto 31.
30. Antenna is very long, half as long as the body……..…...Phasmida.
30.” Antenna is shorter, cerci are very prominent………….Dermaptera.
31. First tarsal segment is enlarged, tarsi has 3 segments
………………………………………………………..…….....Embiidina.
31.” First tarsal segment is NOT enlarged……………….……....goto 32.
32. Organism has large hind legs (3rd pair), jumpers………..Orthoptera.
32.” Organism has long antenna, normal legs, tarsi have 2 or 3
segments…………………………………………..……….Psocoptera.
33. All wings are membranous, soft, no shell…..……....………...goto 34.
33.” Front wings are completely or partially leathery, hard, or
shell like………………………………………………….………goto 47.
34. Organism has two wings (one pair), often have tiny knobs
were the 2nd pair of wings should be………………....………..goto 35.
34.” Organism has four wings (two pair), no knobs…………...….goto 39.
35. Hind legs are large, jumping insects……..………………..Orthoptera.
35.” Hind legs normal, not enlarged………………………...……...goto 36.
36. Abdomen has 1–3 threads at the end, insect has no
mouthparts……………………………………..…..…………….goto 37.
36.” Abdomen has no threads at the end, insect has mouthparts
….……………………………………..……………....………....goto 38.
37. Antenna very long, insects are very tiny (less than 5 mm)
………………………………………………….….......….Hemiptera.
37.” Antenna very short, bristle-like, insects usually over 5 mm in
length……………………………………..…….......Ephemeroptera.
38. Tarsi has 5 segments…………………….………….……...Diptera.
38.” Tarsi has 2 or 3 segments, mouthparts are strawlike.
……………………………………………..……………...Hemiptera.
39. Mouthparts are strawlike, sucking insects……….........Hemiptera.
39.” Mouthparts have little teeth, chewing insects………….…goto 40.
40. Wings have scales; long, coiled mouthparts………...Lepidoptera.
40.” Wings do not have scales…….………..………..………....goto 41.
41. Wings are long and narrow, bordered with very long hairs,
very small insects (less than 5 mm)…….…….…..Thysanoptera.
41.” Wings are wide and rounded, not bordered with very long
hairs. Tarsi have more than 2 segments……………..…goto 42.
42. Wings held straight above body, abdomen has 2 or 3 threads
at the end, antenna short………………….……...Ephemeroptera.
42.” Wings not held straight over body, no abdominal threads
…………………………………………………….…….……goto 43.
43. Tarsi have 5 segments……………………….….……….…goto 44.
43.” Tarsi have 4 or fewer segments………………..……….…goto 48.
44. Wings are hairy, antenna as long as the body.…………Trichoptera.
44.” Wings are not hairy, antenna shorter than body…….…..….goto 45.
45. Abdomen has a pinched waist, hind wings smaller than front
wings…………………………………….………………..Hymenoptera.
45.” Abdomen normal, all four wings same size……………..…....goto 46.
46. Mouthparts form a beak, insect looks like a flying
scorpion……………………………………………………....Mecoptera.
46.” Mouthparts do not form a beak, insect is not scorpion-like
………………………………………………..………….…..Neuroptera.
47. Mouthparts are strawlike, sucking insects………………….Hemiptera.
47.” Mouthparts have tiny teeth, chewing insects………..…..…..goto 48.
48. Abdomen have curved cerci, tarsi has 3 segments, wings
short.…………………………………… …………..……....Dermaptera.
48.” Abdomen has no cerci, wings long……………..…..…..……..goto 49.
49. Front wings very hard, completely shell-like…………...….Coleoptera.
49.”Front wings not shell-like, held rooflike over body….………...goto 50.
50. Tarsi have 4 or fewer segments, large, jumping hind legs
………………………………………..…………..…………..Orthoptera.
50.” Tarsi have 5 segments, normal legs, not jumpers………....goto 51.
51. Front legs are large, made for catching prey……..…..…...Mantodea.
51.” Front legs are normal size, bodies flattened……….………..Blattaria.