Orders of Aquatic and Semiaquatic Insects

Below is a provisional key for insects that one generally encounters in freshwater aquatic and semiaquatic environments (from Daly 1996). Sometimes during sampling terrestrial organisms are taken, and may not key correctly here. In this case the reader is referred to the work by Borrer et al. 1989.

1a	a. Wings or wing pads present, fore wings sometimes hard and shell-like, concealing hind wings	2
	b. Legs present
1b	a. Wings or wing pads entirely absent	27
	b. Legs present or absent
2a(1a)	Wings fully developed, usually conspicuous and movable (adults)	3
2b	Wings developing in fixed wing pads (larvae and pupae)	15
3a(2a)	Forewings leathery or hard, at least in the basal half	4
3b	Wings entirely membranous	6
4a(3a)	Sucking mouthparts united in a jointed beak, mandibles concealed	Heteroptera (Hemiptera), adults
4b	Chewing mouthparts, mandibles visible	5
5a(4b)	Forewings leathery, veins distinct; hind femora enlarged for jumping	Orthoptera, adults
5b	Forewings sclerotized, hard (elytra), veins indistinct; hind legs not modified for jumping, but suited for walking or swimming	Coleoptera, adults
6a(3b)	One pair of wings	7
6b	Two pair of wings	8
7a(6a)	a. Abdomen ending in 2-3 long filaments	Ephemeroptera, adults
	b. Mouthparts inconspicuous
	c. Metathorax without club-shaped structure
7b	a. Abdomen without conspicuous filaments	Diptera, adults
	b. Mouthparts well-developed, forming a proboscis
	c. Metathorax with club-shaped halteres
8a(6b)	Wings covered with scales or hairs, obscuring venation	9
8b	Wings bare or with only minute hairs, wing venation clearly visible	10
9a(8a)	a. Wings with scales	Lepidoptera, adults
	b. Mouthparts usually fitted with a coiled sucking tube
9b	a. Wings with hairs	Trichoptera, adults
	b. Mouthparts without a coiled sucking tube, rather mandibulate
10a(8b)	Antennae short, bristlelike, and inconspicuous	11
10b	Antennae of various shapes, conspicuous, not bristlelike	12
11a(10a)	a. Abdomen ending in 2-3 long filaments	Ephemeroptera, adults
	b. Wings about equal in size
11b	a. Abdomen without long filaments	Odonata, adults
	b. Wings about equal in size
12a(10b)	a. Tarsi 2- or 3- segmented	Plecoptera, adults
	b. Abdomen ending with 2 conspicuos cerci (reduced in some adult Nemouridae)
12b	a. Tarsi 5-segmented (except 3-segmented in some Hymenoptera)	13
	b. Abdomen without conspicuous appendages
13a(12b)	a. Abdomen with narrow constriction at junction with the thorax	Hymenoptera, adults
	b. Marginal veins in basal half of forewing parallel to leading edge
	c. Wings with fewer than 20 closed cells
	also: Aquatic forms very small, usually < 3 mm in length
13b	a. Abdomen broadly joined to thorax	14
	b. Front margin of forewing in basal half with many small veins perpendicular to edge
	c. Wings with more than 20 closed cells
14a(13b)	Hindwings folded or pleated lengthwise	Megaloptera, adults
14b	Hindwings not folded	Neuroptera (Sisyridae), adults
15a(2b)	a. Active insects with legs freely movable	nymphs/larvae, 16
	b. Not in cocoons or capsule-like cases
15b	a. Usually inactive insects, "mummy-like" with appendages drawn up and free or fused to body	pupae, 20
	b. Sometimes in cocoons or sealed in capsule-like cases or puparia
16a(15a)	Sucking mouthparts united in a jointed beak with mandibles concealed	Heteroptera (Hemiptera), nymphs
16b	Chewing mouthparts, with mandibles distinct	17
17a(16b)	a. Hindlegs suited for jumping, hind femora greatly enlarged	Orthoptera, nymphs
	b. Abdomen without long cerci
	c. Found in moint polaces and only temporarily in water
17b	a. Hindlegs suited for crawling, hind femora not greatly enlarged, approximately the smae size as front and middle femora	18
	b. Abdomen with or without conspicuous terminal appendages
	c. Usually submerged, truly aquatic
18a(17b)	Labium (lower lip) masklike, extendable into a graspable, often scoop-like structure longer than the head	Odonata, nymphs
18b	Labium normal, smaller than head, not large and masklike	19
19a(18b)	a. Tarsi with one claw (sometimes foretarsi bifurcated, but eminate from one claw base)	Ephemeroptera, nymphs
	b. Abdomen terminating with 3 (sometimes only 2) long filaments
	c. Gills and or lamellae located on sides of abdomen, may be platelike, filamentous, or feathery
19b	a. Tarsi with two clearly differentiated claws on all legs	Plecoptera, nymphs
	b. Abdomen terminating only with 2 long filaments
	c. Gills absent or present, fingerlike or filamentous, located at base of mouthparts (inconspicuous), head, coxae, or on thorax or abdomen segments 1-2 (in Michigan)
20a(15b)	Appendages free, distinct, not fused to body (exarate)	pupae, 21
20b	Appendages fused to body (obtect), or concealed in hardened capsule (coarctate)	pupae, 26
21a(20b)	Abdomen with constriction where joined to thorax	Hymenoptera, pupae
21b	Abdomen broadly joined to thorax	22
22a(21b)	One pair of wingpads	Diptera, pupae
22b	Two pair of wingpads	23
23a(22b)	a. Pads of forewings thickened	Coleoptera, pupae
	b. Antennae usually with 11 or fewer segments
23b	a. Pads of forewings not thickened	24
	b. Antennae 12 or more segments
24a(23b)	a. Mandibles curved, projecting forward and usually crossing each other	Trichoptera, pupae
	b. Pupae usually submerged in water (maybe in damp areas of overhanging stream banks)
	also: Always in cases
24b	a. Mandibles not as above, rather stout and not crossing each other	25
	b. Pupae terrestrial (near water's edge), not normally submerged
25a(24b)	a. Smaller, body length < 10 mm	Neuroptera (Sisyridae), pupae
	b. Pupae in double-walled, meshlike cocoons in sheltered places
25b	a. Larger, body length > 12 mm	Megaloptera, pupae
	b. Pupae in chanbers in soil or rotten wood, without cocoons
26a(20b)	a. Appendages visible on pupal surface	Lepidoptera, pupae
	b. Without obvious breating tubes or gills
	c. Two pairs of wingpads present (hindwings mostly concealed beneath forewings)
26b	a. Appendages visible or entirely concealed in barrel-shpaed capsule	Diptera, pupae
	b. If appendages visible, then usually with projecting respiratory organs, or paired, dorsal, prothoracic breating tubes, sometimes with gills at the abdominal tip
	c. One pair of wingpads
27a(1b)	a. Abdomen with 6 or fewer segments, with a ventral tube (collaphore)	Collembola, immatures and adults
	b. Minute, body length < 5 mm
27b	a. Abdomen with more than 6 segments, without a ventral tube	28
	b. Body length longer, usually > 5 mm
28a(27b)	Three pairs of jointed legs present on thorax	29
28b	True legs absent; fleshy, leglike protuberances or prolegs may be present on thorax, but fewer than three pairs and not jointed	34
29a(28a)	a. Middle- and hindlegs long and slender, extending considerably beyond the abdomen	Heteroptera (Hemiptera), nymphs and adults
	b. Compound eyes present (nymph, wingless adult Gerridae)
29b	a. Legs not longer than the abdomen	30
	b. Compound eyes absent
30a(29b)	Abdomen with at least two pairs of ventral, fleshy, leglike protuberances tipped with tiny hooks (prolegs with crochets)	Lepidoptera, larvae
30b	Abdomen without leglike protuberances, or, if present, not tipped with tiny hooks	31
31a(30b)	a. Last abdominal segment with lateral appendages bearing hooks (anal hooks)	Trichoptera, larvae
	b. Antennae 1-segmented, inconspicuous
	c. Gills, if present, seldom confined to lateral margins of body
	also: Larvae free-living or in cases made of stone or sand grains and/or bits of plant matter
31b	a. Last abdominal segment without anal hooks	32
	b.Or, if anal hooks present, antennae of more than 1 segment
	c. Gill insertions lateral
32a(31b)	a. Mouthparts formed with mandibles and maxillae united at each side to form long, straight or slightly recurved thread-like suctorial tubes	Neuroptera (Sisyridae), larvae
	b. Laterally inserted, segmented gills folded beneath abdomen
	c. Body length small, < 10 mm
	d. Found in or on freshwater sponges
32b	a. Mounthparts with mandibles not united with maxillae, if mandibles suctorial, then strongly curved	33
	b. Gills seldom segmented and not folded beneath abdomen
	c. Body length variable
	d. Not directly associated with sponges
33a(32b)	a. Abdomen with 7 or 8 pairs of lateral filaments or gills, arranged 1 pair on each segment	Megaloptera, larvae
	b. Ab9 with hooked lateral appendages (anal hooks), or a single, medial, caudal filament
33b	a. Abdomen usually without lateral gills, or, if present, then 1) anal hooks absent, or 2) Ab10 with 4 gills, or 3) caudal appendage paired or absent, never single	Coleoptera, larvae
34a(28b)	Head capsule distinct, partly or entirely hardened, and usually pigmented, but may be deeply withdrawn in prothorax	35
34b	Head capsule absent, not distinct, hardened, or pigmented, often consisting of a few pale rods	36
35a(34a)	Posterior end of body with at least one or a combination of gills, hair brushes, a sucker, or breathing tube	Diptera (in part), larvae
35b	Posterior end of body simple, or with small processes or isolated hairs, but without gills, brushes, suckers, or breathing tubes	Coleoptera (Curculionidae), larvae
36a(34b)	a. Body usually > 5 mm, elongate, somewhat cylindrical, spindle-shaped or maggotlike	Diptera (in part), larvae
	b. Mouthparts may be reduced to a pair of retractile mouth hooks that move vertically
36b	a. Body usually < 5 mm	Hymenoptera, larvae
	b. Mouthparts may be reduced to a pair of opposable, acute mandibles that move horizontally; parasitoids on or inside insect hosts

References

Borrer DJ, Triplehorn CA, Johnson NF. 1989. An Introduction to the Study of Insects (6th Edition). Saunders College Publications: Philadelphia, Pennsylvania. 875 p.
Daly HV. 1996. General classification and key to the orders of aquatic and semiaquatic insects, pp. 108-112 in Merritt RW, Cummins KW, An Introduction to the Aquatic Insects of North America, 3rd Edition. Kendall/Hunt Publishing Co.: Dubuque, Iowa. xiii + 862 p.

Page created: May 19, 2003 (EB)
Page last reviewed: December 17, 2003 (EB)