Notonectidae - Backswimmers

Introduction

[under construction] Nine species in two genera (Buenoa and Notonecta) have been positively recorded in Michigan, and it is unlikely that others will be recorded. This group has been thoroughly reviewed by Hungerford (1933, 1945) and Truxal (1953). Hilsenhoff (1984) has provided a useful key and notes on the species occuring in the Great Lakes region.

Notonectids are commonly found in ponds, ditches, lake margins with emergent vegetation and, less commonly, larger streams and rivers particularly among macrophytes, overhanging riparian vegetation and undercut banks. They stay at the water surface, ventral side facing up, with their abdomen holding an air bubble with which to breath, then dive down to grab their prey. Notonectids are fierce predators, and can deliver a painful bite.

Notonectids are fierce predators that attack just about any prey that they can overpower, including smaller individuals of their own species. The common name "backswimmer" is derived from habitus: they row with the ventral side up, as do the Pleidae (pygmy backswimmers). They have long, slender to thick-bodied bodies, are convex dorsally, and are somewhat triangular in cross-section (Sanderson 1982). The adult length of our species of species of Notonecta are between 8.5-16mm, and our species of Buenoa between 5.5-8.25mm, with males typically smaller than females. They have large, narrowly-separated eyes, no ocelli, and partly concealed 3-4 segmented antennae. The 4-segmented rostrum is short and stout. The front and middle legs are raptorial, ideally modified for grasping prey and clinging to vegetation, and with conspicuous tarsal claws; the hind legs are oar-like and fringed with long swimming hairs, with inconspicuous claws. The abdomen has a midventral keel with long, inward-directed hairs on the sides of the venter, which close over two troughs that form an air chamber.

These are generally lentic organisms, frequently found in vegetated wetlands, ponds and lakes, although individuals can be found in vegetated backwater or undercut banks of larger streams and rivers. Individuals arise to the water's surface to replenish their air supply by breaking the surface film with the tip of the abdomen. Air then contacts the ventral spiracles and diffuses forward to the thoracic spiracles and to the subelytral air spaces (Sanderson 1982). Buenoa and Notonecta have hemoglobin (Hungerford 1922) in special large hemoglobin or tracheal cells in Ab3-7 (Bare 1928). Hemoglobin is also known from a number of chironomid taxa and a genus of bot flies (Gasterophilus), but the hemoglobin in notonectids has a considerably lower affinity for oxygen than these other taxa (Chapman 1998). This presumably aids in respiration and/or hydrostatic equilibrium. The later was hypothesized by Miller (1966, in Chapman 1998), who observed for a species of Anisops that O2 is released by the hemoglobin cell until the insect's density equals that of water, so that the animal can float in midwater. With deoxygenation, the insect sinks and it must motor upwards to the surface to replenish its bubble.

Most of our species of Notonecta overwinter as adults, either actively or in a state of hibernation. Hungerford (1933) and Lauck (1959) report seeing adults swimming beneath ice. Eggs are laid in spring and summer, and there appear to be overlapping broods (Rice 1954, Lauck 1959). Rice noted that N. borealis overwinters in egg stage, as do perhaps two species of Buenoa. Eggs are laid in plant tissue, or on rock/plant surfaces. Rice (1954) found Plea, Buenoa and 2 Notonecta species at Douglas Lake inserted eggs in stems and those of three other Notonecta speceis were not inserted; Lauck (1959) suspected Buenoa oviposits throughout summer. Eggs glued to surface of stems had characteristic hexagonal reticulations, and inserted eggs had characteristic nobudles in addition to reticulations. Buenoa eggs have distinct anterodorsal cap, which Notonecta and Plea do not have. Egg development for several Buenoa species varies from 42-77 days Bare (1926), whereas Clark and Hersh (1939) report Notonecta egg development varying from 3 days to 2 weeks, depending on temperature. Lauck (1959) suggested bivoltinism.

Adults fly readily, disperse over considerable distances; Hungerford (1933) reports attracted to light. Stridulation for Buenoa has been observed (Hungerford 1924), accomplished by use of a stridulatory comb at base of male tibia, rostral prong with filelike teeth that stands in opposition when the fore limbs are brought up to the head. Stridulatory ridges on the fore femora of Buenoa can be seen only if the front legs are bent outward, or detached, and mounted for examination (Truxel 1953).

Adults (adapted from Hungerford 1933, Hilsenhoff 1984, Truxal 1953, and Polhemus 1996)

1a	a. Large insects, >8.5 mm long	Notonecta, 2
	b. Hemelytral commissure without a definite hair-lined pit at the anterior end
	c. Antennae 4-segmented
1b	a. Small insects, <8.5 mm long	Buenoa, 6
	b. Hemelytral commissure with a definite hair-lined pit at the anterior end
	c. Antennae 3-segmented
2a(1a)	a. Hemelytra with an irregular pattern of black and orange marks	Notonecta irrorata Uhler
	b. Length 13.1-14.5mm
2b	a. Hemelytra with large white areas, never with orange	3
	b. Length variable
3a(2b)	a. Mesotrochanter acutely angulate posteriorly	Notonecta lunata Hungerford
	b. Small, length 9.0-10.2mm
3b	a. Mesotrochanter rounded posteriorly	4
	b. Larger, length 10.5mm or more
4a(3b)	a. Abdominal sternal keel completely covered with long setae	Notonecta undulata Say
	b. Smaller, length 10.9-12.3mm
4b	a. Abdominal sternal keel bare of abdominal sternum 4	5
	b. Larger, length 12.4mm or more
5a	a. Scutellum and most of the hemelytra white, or pale yellow	Notonecta borealis Bueno and Hussey
	b. Length 12.9-14.8mm
5b	a. Scutellum black, and with dark markings on hemelytra	Notonecta insulata Kirby
	b. Length 14.0-15.0mm
6a(1b)	a. Terminal segment of rostrum as long as penultimate (second to most distal) segment	Buenoa margaritacea Torre-Bueno
	b. Wings without markings
	c. Pronotum broadest at base, where it is distinctly wider than head
	d. Male profemur distinctly narrowed toward apex
6b	a. Terminal segment of rostrum distinctly shorter than penultimate segment	7
	b. Wings almost always with dark marks, especially near the humeral angle
	c. Pronotum narrower than head, except for the inflated pronotum of the male Buenoa limnocastoris
	d. Male profemur almost as broad at apex as at base
7a(6b)	a. Infuscations on wings, if present, limited to lateral edge anteriorly	Buenoa macrotibialis Hungerford
	b. No infuscations on dorsum
	c. Male protibia almost as wide at base as profemur
	d. Metatarsal setae infuscate dorsally, but never black
	e. Interocular space wide, approximately 0.5x or more the anterior width of the vertex
7b	a. Lateral infuscations on wings near the anterolateral angle, and usually also in the apical fourth	8
	b. Infuscations often present on dorsum
	c. Male protibia about 0.5x wide as profemur
	d. Metatarsal setae may be black dorsally
	e. Interocular space narrow, < 0.5x the anterior width of the vertex
8a(7b)	a. A linear infuscation near the anterolateral margin of the wing, the posterior infuscation small or absent	Buenoa confusa Truxal
	b. Metatarsal setae black dorsally, or nearly so
	c. Male prothorax not inflated, distinctly narrower than head
	d. Smaller, length 5.0-6.0mm
8b	a. Infuscation near the anterolateral angle of the wing not linear, but widest at base, the posterior infuscation large, at least 0.5x width of wing	Buenoa limnocastoris Hungerford
	b. Metatarsal setae infuscate dorsally, but not black
	c. Male prothorax inflated, wider than head
	d. Larger, length 5.8-7.5mm

Nymphs (adapted from Usinger 1956)

1a	a. Abdominal spiracles small, round, <0.10x as long as their respective abdominal segments	Notonecta
	b. Hind legs with rows of short, stout, black spines
1b	a. Abdominal spiracles large, oval, about 0.25x long as long as their respective abdominal segments	Buenoa
	b. Hind legs without rows of conspicuous, stout spines

References

Bare CO. 1926. Life histories of some Kansas "backswimmers". Annals of the Entomological Society of America 19:93-101.
Bare CO. 1928. haemoglobin cells and other studies of the genus Buenoa. University of Kansas Science Bulletin 18:265-349, 14 pl.
Chapman RF. 1998. The insects: structure and function. 4th Edition. Cambridge University Press: Cambridge, UK. xvii + 770 p.
Clark LB, Hersh AH. 1939. A study of relative growth in Notonecta undulata. Growth 3:347-372.
Hilsenhoff WL. 1984. Aquatic Hemiptera of Wisconsin. The Great Lakes Entomologist 17(1):29-50.
Hungerford HB. 1922. Oxyhaemoglobin present in backswimmer, Buenoa margaritacea. Canadian Entomologist 54: 263.
Hungerford HB. 1924. Stridulation of Buenoa limnocastoris Hungerford and systematic notes on the Buenoa of the Douglas Lake region of Michigan, with the description of a new form. Annals of the Entomological Society of America 17:223-227.
Hungerford HB. 1933. The genus Notonecta of the world (Notonectidæ-Hemiptera). The University of Kansas Science Bulletin 21(1):5-195.
Lauck DR. 1959. The taxonomy and bionomics of the aquatic Hemiptera of Illinois. Unpublished Masters Thesis, University of Illinois, Urbana, Illinois. 353 pp.
Miller PL. 1966. The function of haemoglobin in relation to the maintenance of neurtral buoyancy in Anisops pellucens (Notonectidae: Hemiptera). Journal of Experimental Biology 44:529-544.
Rice LA. 1954. Observations on the biology of ten notonectoid species found in the Douglas Lake, Michigan, region. American Midland Naturalist 51:105-132, 3 pls.
Sanderson MW. 1982. Aquatic and semiaquatic Heteroptera, pp. 6.1-6.94 in Brigham AR, Brigham WU, and Gnilka A (editors), Aquatic Insects and Oligochaetes of North and South Carolina. Midwest Aquatic Enterprises: Mohomet, Illinois.
Truxal FS. 1953. A revision of the genus Buenoa (Hemiptera: Notonectidae). The University of Kansas Science Bulletin 35, part 2(11):1351-1523.
Usinger RL. 1956. Aquatic Hemiptera, pp. 182-233 in Usinger RL (editor), Aquatic Insects California, with keys to North American Genera and California species. University of California Press: Berkeley, California, USA.

Last updated: November 18, 2006 (EB)