This is a dichotomous key to identify microarthropods to small groupings of similar-looking organisms. In most cases these groupings have a narrow taxonomic range. However, there are a few cases where our groupings may include species from a few taxonomic groups. This artificial clustering is particularly an issue for the very small mites, which we refer to as mini mites. These mites often lack features that are useful in identification, either because they are undeveloped (i.e. larval forms) or because their features are so small that they are difficult to resolve with a stereo microscope. The goal of our lab group is typically to distinguish that we have two different species, rather than to determine the precise identity of the species. Consult the resources listed in the links page for guidance in performing more specific identifications.

This key was developed using keys in Krantz and Walter's Acarology (2009) and Moldenke and Fichter's Invertebrates of the H.J. Andrews Experimental Forest, Western Cascade Mountains, Oregon: IV. The Oribatid Mites (Acari: Cryptostigmata) (1988) as a point of departure. Couplets were modified to be accessible to non-experts and to facilitate identification under a dissection microscope.

1aMore than 4 pairs of legsLots of Legs
1b4 pairs of legs or fewer2
2aBody is long and thin (vermiform), often with many setae, dorsal plates, small legs, and mouthparts at the anterior end that are darker in coloration than the rest of the body (Fig. 1)Insect Larvae
2bBody is not vermiform in shape3

Fig. 1. Insect larvae with a vermiform body shape

3aAntennae extending from the head (Fig. 2) and 3 pairs of legs. Eyes often present21 (Adult Insects and Collembola)
3bNo antennae. Typically 4 pairs of legs but some may have 3 pairs of legs or the number of legs may be difficult to distinguish4 (Arachnids)

Fig. 2. Antennae on an insect (left) and a collembolan (right)

4aPalpi are claw shaped (Fig. 3) such that the specimen appears like a scorpion with no tail. Typically 4 pairs of legsPseudoscorpions
4bNo claw like appendages present5

Fig. 3. Pseudoscorpion body form with detail on chelate palpi

5aTwo distinct body segments (the cephalothorax and the abdomen) separated by a narrow pedicel. 1 to 4 pairs of spinnerets located on the ventral posterior (Fig. 4). 4 pairs of long legs30 (Spiders)
5bDistinction between body segments is not obvious. Spinnerets absent6

Fig. 4. Spider body form with detail on spinnerets

6a4 pairs of legs that are very long and thin compared to the size of the body. The second pair of legs is longer than the other pairs. Eyes are located in the mid dorsal portion of the head (Fig. 5). Specimen is relatively large (>1000 μm)Harvestmen
6bLegs are not extremely long in comparison to body or if they are, no eyes are present on the specimen. Specimen is relatively small (100-1000 μm)7 (Mites)

Fig. 5. Harvestman body which appears as a single segment and long legs, with the 2nd pair typically longer than the others

7aDark in color (yellow-orange to reddish brown). Epimeral (between the legs), genital, and anal plates visible ventrally, and trichobothria visible antero-dorsally (Fig. 6). Palps are not easily distinguished as they are small and often concealed by the body armor. Similarly, the 4 pairs of legs may be visible but may be difficult to see if they are tucked into the body armor. Specimen may be shiny or coated in a waxy substance that can collect debris32 (Adult Oribatids)
7bPalps are pronounced and/or the specimen is pale without distinct genital and anal plates8

Fig. 6. Oribatid body form showing ventral plating and trichobothria

8aFront legs are extremely long and/or thin as compared to the body size (Fig. 7)Peculiar Predators
8bFront legs are not extraordinarily long relative the other legs9

Fig. 7. Peculiar predatory mites with unusual long and thin front legs

9aChelicerae come to a point, such that the specimen appears to have a pointy nose. Palps are typically elbowed such that they are directed sideways and then forwards, ending with long setae (Fig. 8)Bdellids
9bPalps are not elbowed and specimen may or may not appear to have a pointy nose10

Fig. 8. Bdellid body form with pointed gnathosoma and elbowed palps terminating in long setae

10aChelicerae and palps come to a point, palps typically terminate in claw-like structure. Body is diamond in shape with four long setae extending from the mid dorsum. Specimen is relatively smallCunaxids
10bChelicerae and palps are not pointed and four long setae do not extend from a diamond shape body11

Fig. 9. Cunaxid body form with palps and chelicerae coming to a point and rhombus shaped body

11aLegs are long with many joints; the coxae are free and tarsi on legs II to IV are divided. Legs I are directed forwards. Palps are long and linear. The cheliceral bases are enclosed by a sclerotized ring. Tritosternum is typically present, although difficult to see without slide mounting (Fig.10)35 (Mesostigmatids)
11bCoxae are fused to the ventral side of the body and tarsi are not divided12

Fig. 10. Mesostigmatid body form showing free coxae, cheliceral bases enclosed by a sclerotized ring and the tritosternum

12aBody is large and plump with an orange or yellow color. Short setae cover the entire body like fur. Palps are long and front legs extend forwards (Fig.11) Peculiar Predators
12bBody is not as described above, specimen is typically small and pale in color13

Fig. 11. Peculiar predators with large yellow/orange body covered in short setae

13aPalps and/or chelicerae are small, withdrawn into the camerostome, or poorly developed, usually not extending past the rostrum (Fig. 12)14 (Oribatid-Like)
13bPalps and/or chelicerae are prominent, extending beyond the rostrum as the anteriormost structures (Fig. 12)18

Fig. 12. Oribatid like mite with short chelicerae and palps (left) and Prostigmatid mini mite with prominent palps and chelicerae (right)

14aLegs are distinct15
14bLegs are not distinct17
15aSpecimen typically 300 μm or longer. Long and/or elaborate setation usually present. Body may be coated in wax which can collect debris. Anal opening faintly visible but hardened plates have not formed. Trichobothria located antero-dorsally (Fig. 16)41 (Juvenile Oribatids)
15bSpecimen typically smaller than 300 μm16

Fig. 13. Juvenile oribatids

16aSoft bodied mite with a divided dorsal shield and unsclerotized genital and anal plates that are subequal in size. Looks like a very small pale oribatid (Fig. 14)Brachychthionid
16bGlobose or elongate mites with legs concentrated at the anterior end of the body (Fig. 14)Endeostigmatid

Fig. 14. Body form of Brachychthionid Oribatids (Left) and Endeostigmatid Mites (Center and Right)

17aFlat round mites with minute, poorly developed legs (Fig. 15)Cyst-like Mini Mites
17bVery small mites, typically 100-200 μm in size, with few distinguishing features (Fig. 15)Larval Mini Mites

Fig. 15. Body form of Cyst-like minis and larval mini mites

18aBody is wide, either round or shield shaped with the length roughly equal to width. Legs are fairly equal in length and in most cases short in comparison to the body size (Fig. 16)Wide Mini Mites
18bBody elongate, typically longer than it is wide. Legs are long relative the body size, with legs IV typically extending beyond the posterior of the body19

Fig. 16. Wide mini mites with length roughly equal to width and/or short legs

19aBody size is smaller than 300 μm. In many specimens, the "shoulders" are broad and the body tapers to the posterior. (Fig. 17)Eupodid
19bBody size is larger than 300 μm, can range to 2 mm in length20

Fig. 17. Eupodid-like mini mites with long legs and body that is typically broadest at the "shoulders"

20aBody is long and narrow and narrows at the "waist" between legs III and IV (Fig. 18)Large Rhagidiids
20bBody is rectangular or broadest at the "shoulders"Peculiar Predators

Fig. 18. Large Rhagidiid mite (Left) and peculiar predator (Right)

21aFurcula (springtail) present but may be very small or retracted against the venter (Fig. 19). Body form may be linear or globular. No wings....22 (Collembola)
21bNo furcula. Body has 3 segments (head, thorax, and abdomen). Specimen may or may not have wings25 (Insects)

Fig. 19. Different sizes of furcula

22aCollembola with a linear body plan23
22bCollembola with a globular body plan24
23aAll abdominal segments fairly equal in size (Fig. 20) Collembola with Equal Segments
23bOne large abdominal segment (Fig. 20)Collembola with Large Abdominal Segment

Fig. 20. Collembola with equal sized abdominal segments (Left) and one large abdominal segment (Right)

24aSpecimen is colorful (Fig. 21)Colorful Globular Collembola
24bSpecimen is pale or translucent (Fig. 21)Pale Globular Collembola

Fig. 21. Colorful globular collembola (Left) and pale globular collembola (Right)

25aOne pair of wings and a pair of halteres (reduced wings that look like balls on a stick) (Fig. 22)Flies
25bTwo pairs of wings (when present)26

Fig. 22. Halteres and overall body form of a fly

26aDark-coloured insects with a modified pair of wings forming a hard covering (elytra) over an inner, membranous pair (Fig. 23)Beetles
26bElytra absent27

Fig. 23. Body plan of a beetle and detail of elytra (hard wings) covering membranous wings

27aProminent proboscis (sucking mouthparts) and partial or fully membranous wings (if present) (Fig. 24)True Bugs
27bSucking mouthparts absent28

Fig. 24. Proboscis of a true bug and body plan showing partially membranous wings

28aLarge head and eyes, mandibles and rod-like maxilla, wings may or may not be present (Fig. 25)Booklice
28bRod-like maxilla absent29

Fig. 25. Booklice body plan and detail of mouthparts showing rod like maxilla

29aDeveloped mandibles, two pairs of membranous wings (when present), and tapering in the body that makes the 3 body segments well defined (Fig. 26)Wasps and Ants
29bFlat and narrow-bodied with reduced mandibles and thin feathered wings (when present) (Fig. 26)Thrips

Fig. 26. Winged ant with defined body segments (Left) and thrip with feathered wings (Right)

30aSclerotized epigynum on the ventral abdomen (Fig. 27)Female Spiders
30bSclerotized epigynum absent31

Fig. 27. Female spider with detail on sclerotized epigynum

31aSclerotized swollen pedipalps (Fig. 28)Male Spiders
31bImmature spiders without obvious sclerotization of the exoskeleton or other sex characteristics..Juvenile Spiders

Fig. 28. Sclerotized swollen pedipalps

32aOribatids with large and/or closely spaced genital plates33
32bOribatids with anal and genital plates that are fairly equal in size with a noticeable space between them (at least the size of the genital plate)34
33aPtychoid body form, which allows them to retract into a ball to protect their soft parts from predators (Fig. 29) Box Oribatids
33bDo not have a ptychoid body formOribatids with Close Genital Plates

Fig. 29. Ptychoid oribatid (Left) and oribatid with close genital plates (Right)

34aPteromorphs (projections of the dorsal shield that protect the base of the legs) (Fig. 30)Winged Oribatids
34bOribatids lacking obvious pteromorphsHard-bodied Oribatids

Fig. 30. Oribatid with pteromorphs (Left) and hard-bodied oribatid without pteromorphs (Right)

35aMesostigmatids with distinct ventral and/or dorsal plating and an orange to pale yellow color. Typically greater than 500 μm in body length (Fig. 31)36 (Adult Mesostigmatids)
35bMesostigmatids that are pale in color with indistinct ventral and dorsal plating. Body size ranges from less than 300 to more than 500μm38 (Immature Mesostigmatids)

Fig. 31. Mesostigmatid mites with distinct ventral plating

36aDistinct or prominent setationMesostigmatids with Distinct Setation
36bAdult mesostigmatid without distinct setation37
37aAdult mesostigmatid with a round body shapeRound Mesostigmatids
37bAdult mesostigmatid with an elongate body shapeElongate Mesostigmatids
38a3 pairs of legsJuvenile Mesostigmatids
38b4 pairs of legs39
39aDark (orange or brown) chelicerae (Fig. 32)Mesostigmatids with Orange Chelicerae
39bBody is completely pale, lacking hardened (dark) chelicerae40

Fig. 32. Dark chelicerae

40aImmature mesostigmatid that is 300 μm or smaller in lengthSmall Pale Mesostigmatids
40bImmatue mesostigmatid larger than 300 μm in lengthLarge Pale Mesostigmatids
41aVery long and skinny legs (Fig. 33)Juvenile Oribatids with Long Legs
41bLegs are not extraordinarily long42

Fig. 33. Juvenile oribatids with long legs

42aWrinkles on the exoskeleton (Fig. 34)Plicate Juvenile Oribatids
42bNo wrinkles on the exoskeleton43

Fig. 34. Plicate juvenile oribatids

43aElaborate or distinct setationJuvenile Oribatids with Distinct Setation
43bLacking elaborate or distinct setation44
44aRectangular body shape. These mites also tend to have a waxy appearance (Fig. 35)Rectangular Juvenile Oribatids
44bRound or oval body shape45

Fig. 35. Juvenile oribatids with a rectangular body shape

45aRound body shape and few other obvious features for identificationRound Juvenile Oribatids
45bElongate body and few other obvious features for identificationElongate Juvenile Oribatids

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