Identifying Arthropods of Medical Importance

Arthropods can be identified into class by following a simple classification scheme.

Number of Legs Present	Group
3 pairs, with antennae	class Insecta
4 pairs, without antennae	class Arachnida
5-9 pairs (legs or swimmerets)	class Crustacea
10 or more pairs	class Chilopoda or Diplopoda

Class Insecta (insects)

Insects Group 1

(1) abdomen with 3 long terminal tails: order Thysanura (silverfish or firebrats)

(2) abdomen with narrow waist: order Hymenoptera (ants)

(3) abdomen with prominent pair of cerci or forceps: order Dermaptera (earwigs)

(4) body flattened laterally; antennae small and fit into grooves in side of head: order Siphonaptera (fleas)

(5) body flattened dorsoventrally; antennae project from side of head and do not fit into grooves: Group 1b

Insects Group 1b

Antennae	Finding	Additional	Order
9 or more segments	pronotum covering head		Dictyoptera (cockroaches)
	pronotum not covering head		Isoptera (termites)
3 - 5 segments	mouthparts consist of tubular jointed beak	3 to 5 segmented tarsi	Hemiptera (bedbugs)
	mouthparts retracted into head, adapted for sucking blood	1 or 2 segmented tarsi	Anopleura (sucking lice)
	mouthparts of the chewing type	1 or 2 segmented tarsi	Mallophaga (chewing lice)

Insects Group 2 - order Diptera:

(1) wings with scales: mosquitoes

(2) wings without scales: other flies

Insects Group 3

Mouthparts	Wings	Other Feature	Order
adapted for sucking, with elongate proboscis	densely covered with scales	proboscis coiled	Lepidoptera (butterflies and moths)
	not covered with scales	proboscis not coiled but directed backwards	Hemiptera (bedbugs and kissing bugs)
adapted for chewing, without elongate proboscis	both pair of wings membranous and similar in structure but may vary in size	both pairs of wings similar in size	Isoptera (termites)
		hind wings smaller than front wings	Hymenoptera (wasps, hornets, bees)
	front wings leathery or horny, serving as covers for second pair, without distinct veins, meeting in a straight line down the middle	abdomen with prominent cerci or forceps; wings shorter than abdomen	Dermaptera (earwigs)
		abdomen without prominent cerci or forceps; wings cover the abdomen	Coleoptera (beetles)
	front wings leathery or paperlike with distinct veins, serving as covers for second pair	wings usually overlap in the middle	Dictyoptera (cockroaches)

Class Arachnida (spiders, ticks, mites, scorpions)

Body Segments	Additional Features	Subclass
single saclike oval body		Acari (ticks and mites)
two distinct regions (cephalothorax and abdomen), joined by a slender waist	abdomen with segmentation indistinct or absent; stinger absent	Araneae (spiders)
two distinct regions (cephalothorax and abdomen), broadly joined	abdomen distinctly segmented, ending in a stinger	Scorpions

Class Crustacea (copepods, crabs, crayfish):

(1) one or two pairs of antennae

(2) principally aquatic organisms

Class Chilopoda (centipedes):

(1) one pair of legs per body segment

(2) swimmerets absent

(3) one pair of antennae

(4) terrestial organisms

Class Diplopoda (millipedes):

(1) two pairs of legs per body segment

(2) swimmerets absent

(3) one pair of antennae

(4) terrestrial organisms

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