Small aquatic and ripicolous bugs (Heteroptera: Nepomorpha) as predators and prey: The question of economic importance

Some features of the biology and feeding relationships of small waterbugs (Notonectidae, Corixidae, Pleidae, Helotrephi­ dae, Aphelocheiridae) and ripicolous bugs (Gelastocoridae, Ochteridae) are reviewed. Individual families are discussed as predators or as prey of other animals. Special attention is given to characteristics and relationships that affect human beings directly or indi­ rectly. The most important relationships from the economic point of view are: bugs and blood-sucking Diptera, bugs and fish, bugs and some endangered or protected amphibians and water birds. Of less importance is the occasional use of some aquatic bugs as saprobity bioindicators. It seems that the economic importance of these bugs has been underestimated.


INTRODUCTION
The most comprehensive recent information on the biology and ecology of water bugs, especially of Corixidae and Notonectidae, is given by Jansson (1986), Savage (1989), Williams & Feltmate (1992), and Hutchinson (1993).But these authors do not consider the food relationships from an economic point of view.
Two nepomorphan families comprise ripicolous bugs, the other nine families contain water bugs.Conspicuous giant water bugs (Belostomatidae) (the largest species are 11 cm long), waterscorpions (Nepidae), and creeping water bugs (Naucoridae), are, among others, predators of small aquatic vertebrates.Some belostomatids are used as human food.These families are of some economic impor tance (Keffer, 2000;Sites, 2000;Venkatesan, 2000).The other nepomorphan families are not thought to be eco nomically important.However, perhaps members of these families are indeed of some economic importance; per haps we do not know enough about their biology.We do not know how much they affect, by the means of their food niches, those parts of food webs that are of eco nomic interest to humans or of interest in the nature con servation (Papacek, 2000).Small, approximately 0.1-2.0cm long, predaceous or omnivorous aquatic bugs are often very abundant in various types of water bodies, even in parts of the world like Central Europe, where, for example, giant water bugs do not occur.Under certain conditions at a locality, the importance of these small spe cies can be compared with that of the large predaceous species (Papacek, 2000).The small species of aquatic bugs are probably biologically the most important adult insects in the littoral region of lakes and ponds (Hutchin son, 1993).
The aim of this brief review is to show the role of small nepomorphan bugs* as animals with possible economic importance, or as animals that deserve attention at least from the point of view of anthropic practice.Another goal of this paper is to present a list of the main literature on this topic.

SMALL PELAGIC WATER BUGS
The small swimming pelagic water bugs with feeding relationships with necton, are Notonectidae, Corixidae, Pleidae, and Helotrephidae.They occasionally also attack benthic organisms, and some species feed on a variety of materials on the bottom.Helotrephidae, living also in the sand or roots on the bottom, or under stones (especially species in running waters), have the tightest connection with the benthic way of life among the families men tioned.
Mating behavior was studied by Clark (1928), Larsén (1938) and Lang & Markl (1981) among others.The female is found visually by the male in Notonectinae.Anisopinae stridulate.Acoustic signals may be important for aggregation of both sexes and mating of Anisops and Buenoa spp., as in corixids, but no exact data are avail able (cf.Aiken, 1985;Hutchinson, 1993).Females of some species lay eggs in plant tissues and females of other species on the surface of submerged plants or stones.Fox (1975) measured the reproductive capacity of Notonecta hoffmanni under two different food regimes and found that one female can lay 2.3 ± 1.2 or 0.6 ± 0.2 eggs per day, respectively.He showed that fecundity is strictly influenced by the shortage of food.Embryonic development lasts, according to temperature 2-3 weeks.Postembryonic development of the 5 larval stages can last, with close dependence on the amount and composi tion of food (Gittelman, 1978) and temperature (Hunger ford, 1919;McPherson, 1965;Ellis & Borden, 1969), one to two months.The length of individual preimaginal stages can differ by 14-20 days depending on these fac tors (Waitzbauer, 1976) or on parasitism, for example, by Acarina (Lanciani, 1982;Lanciani & May, 1982).Life cycles of the widely distributed species may be modified by altitude and geographical distribution (cf., e.g., data of Nieser, 1981;Papácek & Soldán, 1987).
Habitats, spatial distribution, and dispersal poten tial.Macropterous backswimmers fly very well, and have, like corixids, an excellent ability to invade most types of waters (e.g., ponds, lakes, rivers, streams, small waters in peatlands, marshes or swamps, and puddles) including artificial water bodies.They are often one of the first successional stages in new waters.Adult macrop terous backswimmers can migrate irregularly in response to favourable or unfavourable conditions of food and oxygen concentration in habitats (e.g., Hoppe, 1912;Clark, 1928;Wesenberg-Lund, 1943), or seasonally, before overwintering, after ice break in temperate regions, as well as before drying of habitats in arid or subarid regions (e.g., Hutchinson, 1929).Sympatric, often closely related species, are vertically and horizontally stratified: they divide the ecological niches from the surface down wards (Gittelman, 1976(Gittelman, , 1977;;Streams & Shubeck, 1982;Cook & Streams, 1984;Streams, 1986).A similar stratifi cation exists among larvae and adults of the same species.This stratification can change with changes in the amount of energy the females need (Cockrell, 1984a), with changes of temperatures and the concentration of dis solved oxygen (Cockrell, 1984b), or with intra-and inter specific predation (Sih, 1982;Streams, 1986Streams, , 1992a, b;, b;Bailey, 1987).
Food relationships with possible economic impor tance.Backswimmers are very aggressive predators, attacking many pelagic and benthic invertebrates, including their own larvae, invertebrates that fall onto the water surface (larvae and adults of various insects, mites, small crustaceans, annelids, and molluscs), and small ver tebrates (fish larvae and amphibian tadpoles) (e.g., Coo per, 1983;Giller, 1986).These polyphagous cannibalistic carnivores detect prey by visual and vibratory signals (Walton, 1943;Schwind, 1980).Notonectids are sizelimited predators (Cronin & Travis, 1986), whose food niches change somewhat with developmental stages.The larger the developmental stage, the larger the prey hunted (Fox, 1975).The largest species o f Notonecta attack prey approximately 2.5-3.5 cm in length (Dahm, 1972;Dubsky, 1991).Swimming with the back down correlates with the strategy of respiration.The ventral body surface air bubble, removable when rising to the water surface, helps to keep these bugs with the venter up.Nevertheless, "back down position" is an important phenomenon playing its strategical role in attacking the potential prey, which they recognize and drive towards the water surface.It is evident from the presented facts, that notonectids can under certain conditions influence con siderably the structure of freshwater food webs.The fol lowing predaceous linkages are most important from an economic point of view: (1) backswimmers and mosqui toes, (2) backswimmers and fishes.
The notonectids are predators of mosquito and some other blood-sucking Diptera (e.g., Ceratopogonidae) larvae and pupae, which they attack as surface prey.The reports about that fact concern various species (especially of the genera Notonecta, Enithares, Martarega, Anisops, and Buenoa) and various regions of the World.Gittelman (1974a) characterized Martarega hondurensis and Buenoa antigone as predators of mosquito larvae in Costa Rica.Fox (1975) stated that Notonecta hoffmanni attacks surface prey of an average size of 2.2 (1st instar larvae) to 5.6 mm (adults).That behaviour corresponds with the sizes of various stages of mosquito larvae.In laboratory experiments the same author demonstrated the consump tion of 10-15 mosquito larvae per adult of Notonecta per day.Giller & McNeil (1981) showed that Old World spe cies, such as N. glauca, N. obliqua, and N. maculata, attack and feed on larvae of Aedes aegypti.Cockrell (1984b) noted that Culex larvae are prey of N. glauca.Murdoch et al. (1984) found that the North American spe cies N. hoffmanni and N. kirbyi prefer mosquito larvae as their natural diet.Giller (1986) examined the natural diet of N. glauca and N. viridis using the electrophoretic methods of gut contents analyses and showed, among oth ers, that the obligatory part of their food is composed by larvae of Anopheles sp., Aedes aegypti, Culicinae spp., and pupae of Ceratopogonidae.Aly & Mulla (1987) found that the prey of the North American species N. undulata are larvae of Culex quinquefasciatus.Neri-Barbosa et al. (1997) noted N. irrorata as an important predator of mosquito larvae.Wattal et al. (1996) described the predatory potential of Enithares indica against mosquito larvae.Other Oriental species of Eni thares and Anisops also attack mosquito larvae (M.P., unpubl.field observations of some species in Vietnam).These results and many other laboratory as well as field experiments and observations (e.g., Laird, 1947;Ellis & Borden, 1970;Hoy & Reed, 1970;Toth & Chew, 1972;Hazelrigg, 1974;Zalom, 1978;McDonald & Buchanan, 1981;Scott & Murdoch, 1983;Miura & Takahashi, 1987, 1988;Streams, 1992a) support the widespread opinion that mosquito density is reduced by notonectids and that the mosquito larvae are the preferable food for some spe cies of notonectids.The strategy of backswimmers' pre dation could, with some exaggeration, be declared as the "strategy of cleaning of the water surface".This behav ioral phenomenon means that notonectids may be used as control agents of mosquitoes on a worldwide basis.
Population density of mosquitoes, however, is reduced not only by direct predation but also by antipredatory behavior of mosquito females.Chesson (1984Chesson ( , 1989) ) and Blaustein et al. (1995) showed that reduction of mosquito density is influenced by negative selective oviposition at localities inhabited by backswimmers.
Notonectids are predators in the pond culture of various small fishes, as well as of the larvae of various fish spe cies living in still freshwater habitats.This fact was revealed by Torre Bueno (1905) and Clark (1928) in con nection with N. undulata.Berezina (1955Berezina ( , 1962) ) esti mated that great population of N. glauca in ponds with an area of about 0.01 ha can kill from 2,500-3,500 fish larvae per day.A laboratory experiment showed that one adult of N. glauca killed on average 2.6 fish larvae per day (Dahm, 1972).Gorai & Chandhuri (1962) recorded that Anisops bouvieri is a characteristic predator of fish larvae in India.A similar situation was found by Leong (1962) in the case of A. breddini, whose life history was studied in Singapore and Malaysia.
On the other hand, notonectids are the food for several predaceous or insectivorous fishes such as brown trout (Salmo trutta) (e.g., Macan, 1977;Sih, 1987), Lepomis cyanellus (Cook & Streams, 1984) or Gambusia affinis (Washino & Hokama, 1967;Farley & Younce, 1977;Miura et al., 1979Miura et al., , 1984)).Some studies indicate that notonectids in some areas prefer to settle in waters with a pH so low that fish cannot live there (Bendell, 1986;Bendell & McNicol, 1987).Bailey (1987) even described the formation of huge aggregations of Australian Anisops, with many hundreds of individuals (even of two different species), that are supposed to have an antipredaceous function.This behavioral strategy could be selected for only by long-lasting predatory pressure by fish.In spite of this fact the backswimmers are of only marginal impor tance as the food for fish.
Backswimmers attack and feed on small juvenile cray fish (Dye & Jones, 1975;Hirvonen, 1992).Young cray fish are even killed by notonectids or resort to cheliped autotomy to escape predation, but this phenomenon causes a reduction of individual growth in these crusta ceans.
Notonectids can have a certain marginal economic importance even as predators or food for some protected, endangered, or vulnerable animals.There are no doubts about predation on amphibian tadpoles of Rana areolata (Cronin & Travis, 1986), Triturus spp.(Giller, 1986) and Bombina bombina (Dubský, 1991).The polarity of food relationships can change during the ontogeny of both par ticipants.Anuran adults feed on notonectid larvae (Morin et al., 1988).Notonectids are also a common part of the natural diet of some aquatic birds, for example, common heron (Ardea cinerea) (Exnerová & Boháč, 1991).

Corixidae
Distribution.Water boatmen are the nepomorphan group with the most species (35 genera, ca.550 species).Corixids are distributed worldwide, but only one genus, Sigara, is cosmopolitan, the other 34 genera inhabit smaller areas of distribution (for details see Mahner, 1993).
The number of generations per year varies according to species and climatic conditions.Species in tropical waters reproduce all the year round (e.g.Harrison, 1962;Peters & Spurgeon, 1971).Some species of the warmest regions of temperate areas produce three generations per year (e.g.Wróblewski, 1958).Most corixids of temperate areas are bivoltine, they produce one complete early spring generation and a partial or complete second late summer generation.A few species of temperate regions, for example Corixa spp., or species living in the northern most or southernmost areas or in high mountains, produce only one summer generation.Most corixids of temperate areas overwinter as adults.European Micronectinae over winter as larval stages 3 or 4. Some species can over winter in the egg stage -for example Corixa affinis in Europe (Poisson, 1935), and Trichocorixa verticalis inte riores in Saskatchewan (Canada) (Tones, 1975in Scud der, 1976).
Corixid sexes aggregate by pursuing acoustic signals.Both male and female stridulate, rubbing fields of pegs on the fore femora over flanges on the maxillary plate of the head (Jansson, 1972;Aiken, 1982Aiken, , 1985)).In some species spontaneous male stridulation concerns the aggregation of the sexes.The reproductive females swim towards the call of a male.Stridulation of corixids has specific audiospectrographic patterns.These signals play an important role in the ensuring of conspecific mating in sympatric species (see, e.g., Jansson, 1979;Aiken, 1982).The females of European species lay on the average some 5-10 eggs per day for a period of about one month (Jans son, 1986), but the total number of eggs laid by individual female varies from 10 to 1,000 (Young, 1965;Peters & Spurgeon, 1971).Eggs are attached and embedded in cement by a disc or pedicel on the substratum.The incu bation period of eggs varies according to temperature (e.g., Scudder, 1966;Martin, 1970).For instance, in sev eral European species of Corixa and Sigara the length of embryonic development recorded in laboratory is 16-25 days (M.P., unpubl.data).Postembryonic development depends both on the species and climatic conditions.Temperate Micronectinae, overwintering as larvae, have the longest postembryonic development -more than 7-8 months long.On the other hand, for example the larvae of the early spring generation of some temperate bivoltine species, have the shortest development -about 2-3 months long (cf., e.g., Savage, 1989).The maturation process is probably regulated by food, photoperiod, and temperature, although the exact mechanism remains unknown (cf., e.g., Pajunen, 1970;Jansson & Scudder, 1974).Thus even the adult longevity varies, depending on the species and generation.
Habitats and dispersal potential.Corixids have a high dispersal potential, which allows them to utilize various available habitats, both continental and insular, still and running, stable and temporary, fresh and saline (Jansson, 1986), including artificial waters (e.g., Williams, 1993) and very small puddles or lithotelms (cf., interesting studies, e.g., by Pajunen, 1979)*.Most corixids fly very well.Corixids have both flying and non flying morphs.Flightlessness may be the result either of the shortness of the wings or of a lack of flight muscles.It is most often combined with poor muscle development, which is linked with the development of sexual maturity and with the temperature at the time of attaining the adult stage (Scudder, 1975).The seasonal population dynamics of adult corixids in temperate areas conform to a general pattern in relatively stable communities.Numbers reach a maximum in early summer and in autumn (e.g., Crisp, 1962;Savage, 1979).Nevertheless, seasonal population dynamics is very varied, due mainly to both immigration and emigration in some habitats (e.g., Savage, 1981Savage, , 1989)).The irregular and seasonal migration of corixids has some common patterns, as in notonectids.
Corixidae seem to be ecologically adaptable to a wide range of environmental conditions.But the results of Bosmans (1982), for example, from Belgium, and Savage (1994a) from the British Isles, document that individual species of Corixidae show distinct preferences for water habitats of a certain water quality.Savage (1994b) and Sladecek & Sladeckova (1994) summarize data docu menting the relationship of corixids to saprobity (organic pollution) of waters and show that, according to the occurrence of individual species (good indicators), and occasionally according to the proportions in the occur rence of individual species, it is possible to estimate the saprobic index.Presence or absence of some corixid spe cies can help in the general assessment of water-quality levels.
Corixids provide prey for many invertebrates, such as predaceous insects (e.g., Popham, 1941, 1943, Hutchin son, 1993;Stevens & Greven, 1999) and vertebrates in food webs.Predation by fish has been recorded several times (e.g., Frost & Macan, 1948;Fernando, 1956;Scudder, 1976).Some authors (e.g., Applegate & Kieckhefer, 1977) think that fish predation is not very significant, but Macan (1965) showed that predation could have a pro nounced effect on the regulation of both the abundance and distribution of Corixidae.The last mentioned author found that a part of the brown trout's food (Salmo truta) is formed by Callicorixa praeusta and Cymatia bonsdorffii, Hespercorixa castanea, H. sahlbergi, Sigara scottii, and S. distincta in small moorland ponds in England.Ahmed et al. (1970) found that insectivorous Gambusia affinis fed on various corixids.Oscarson (1987) stated that roach (Rutillus rutillus) fed on Glaenocorisa propinqua, Sigara distincta, and S. scotti.Rask (1983) noted that corixids are an obligatory part of the main food of Percafluviatilis living in small forest lakes.Henrikson & Oscarson (1978) believe that corixids are a suitable prey for fish and are not adapted to co existence with fish predators.Predation by fish can reduce the population of corixids (specifically G. propinqua).Various corixids have behavioral and color adapta tions that save them from predation by fishes.Some Corixidae, as well as some Notonectidae (cf.Bailey, 1987) , can form aggregations with many hundreds of individuals of various species.These aggregations have probably an antipredaceous function.Such aggregations, containing both adults and nymphs of different species, were found by field echo-sounding (Lyle & East, 1989;Kubečka et al., 2000).This fact proves the existence of a behavioral pattern of a stable adaptive character selected for over a long period and under the pressure of perma nent fish predation.
Salamanders (Griffith, 1945) and anurans (Morin et al., 1988) have also been noted as predators of corixids.Pre dation by water fowl and waders has been recorded, e.g., by Griffith (1945) and Munro (1945).Swimming corixids can be a prey for some water birds, such as goldeneye duckling (Bucephala clangula) (e.g., Eriksson, 1976), shoveller (Spatula clypeata) (Hutchinson, 1993) or common heron (Ardea cinerea) (Exnerová & Boháč, 1991).Flying corixids may fall prey to bats (Walton, 1943) or birds which catch flying insects (Fernando, 1959).Hungerford (1948) noted that the number of eggs of some corixids may be so great that they can be gathered and used as human food.Hutchinson (1993), referring to other authors, mentioned that in Mexico the eggs are obtained by allowing the enormous populations of Krizousacorixa and Corisella in Lake Texoco to oviposit on submerged plants, from which the eggs can be scraped; this has been a traditional source of human food known as "ahuautle" or water grain, they are favored on some vigils (e.g., Christmas Eve).The adults and eggs were also eaten by the Aztecs and by humans living in the Nile basin in Egypt.The publications about the corixid eggs as human food were summarized also by Devey (1957).The adults and eggs of some corixids have been dried and used as food for aquarium fish, caged birds, and other birds bred by humans.

Pleidae and Helotrephidae
Pleidae and Helotrephidae are closely related families from a morphological, phylogenetic, and biological point of view.They are minute predaceous insects which swim in an inverted position as do notonectids, but they can swim with the dorsum up like corixids.The knowledge of the biology of these bugs is fragmentary, but many aspects of their biology are interesting and unique among all water insects.
Pygmy backswimmers of temperate regions are usually univoltine, although some may also be partly bivoltine (cf.Wefelscheid, 1912;Bare, 1926;McPherson, 1986).They overwinter in the adult stage, in an inactive dormant state.Diapause during overwintering is caused by low temperatures (Gittelman, 1975;Kovac, 1982).Overwin tering adults can survive even in a habitat completely frozen to the bottom (Hilsenhoff, 1984).In spring they start to become active again.Females, which are activated by rising temperature during overwintering, are not able to reproduce, as their maturation cannot be completed (Kovac, 1982).They may live two years (Papacek, 1993).Mating and oviposition periods are relatively long, for example from 2 to 2.5 months in Plea minutissima (see Papacek, 1985).In Central Europe P. minutissima usually starts to breed in May and June; in western Europe P. minutissima, and in eastern North America N. striola, mate in June and July, later than the sympatric species of other genera of water bugs (Wefelscheid, 1912;Gittelman, 1974b;Papacek, 1985).Eggs are laid in slits in the tissues of submerged aquatic plants.Embryonic development lasts about 3 weeks, and postembryonic development from 40 days to 2 months depending on conditions in the habitat (cf.Papacek, 1985;McPherson, 1986).P. minutissima can survive even in a habitat dried up for a short time, finding shelter among the roots of the plants in the bottom mud (facultative aestivation) (Papacek, 1993).It is known that pleids survive very well under other stresses, for instance low temperatures during overwintering or low oxygen concentration.Their stress tolerance and success is supported by their small size, a relatively ample air bubble, a physical gill mechanism, and partial replenishing of the air bubble with oxygen produced by the peroxydic metathoracic scent gland (cf.Gittelman, 1975;Kovac, 1982;Kovac & Maschwitz, 1989).
Habitats and dispersal potential.Pygmy backswim mers live for the most part in both stable and temporary still fresh waters with rich submerged vegetation; their occurrence in running waters is very rare.Nothing is known about the migration of pleids.Most known species are pteridomorphic, with a macropterous morph (long hindwings and claval suture of forewings present), and a micropterous morph (strongly reduced hindwings, claval suture of forewings absent).Macropterous pleids fly rela tively well, and can colonize temporary or newly formed water bodies.
Food relationships with possible economic impor tance.Pleidae are cannibalistic predators which attack and feed on small water animals, such as ostracods, cladocerans, and copepods; small larvae of water insects, including own larvae; annelids like Tubifex or Stylaria; and animals that fall onto the water surface, such as Collembola and small Diptera.Below the water, pleids hunt larger prey than themselves, for example, tubificids and chironomid or mosquito larvae.They find their prey by visual, vibratory and probably also chemical signals.Takahashi et al. (1979) stated that Neoplea striola feeds on mosquito larvae and does not attack other mosquito predators, and thus suppresses mosquito populations.Attack on mosquito larvae by pleids was also observed in the field in southeast Asia (M.P., unpubl.field observa tion).Some laboratory experiments show that in central Europe Plea minutissima prefers zooplankton as food, but attacks mosquito larvae as well (Šafář, 1991).

Helotrephidae
Distribution.Most helotrephids (they have no ver nacular name) (19 genera, more than 120 species) are dis tributed, except for two endemic Palearctic species, in the Oriental, Afrotropical, and Madagascan regions.Only a few species of these bugs (Neotrephinae) are known from the Neotropical region, especially from Middle and South America.
Life history.Helotrephidae are a relatively poorly known family, but there is a growing scientific interest in them.Literature dealing with the Helotrephidae considers mostly their taxonomy, morphology, and distribution.Except for Usinger's (1937) note, the only more or less monothematic study is that by Miyamoto (1952), devoted to the biology of Helotrephes formosanus.However, some fragmentary data on the biology occur in some taxonomic papers (e.g., Esaki & China, 1928;Lundblad, 1933;China, 1935China, , 1940;;Papácek et al., 1988Papácek et al., , 1989)).
Our idea of the life cycles of helotrephids is largely speculative, and is based especially on phenologic data.Miyamoto (1952) found larvae of Helotrephes formo sanus in Taiwan from January to June.According to his data, we can not determine precisely if the species is univoltine with a long oviposition period and postembry onic development, or if it is partly bivoltine.Papacek (1993) provided some data on the ontogeny of Idiotrephes asiaticus from laboratory cultures.*Mating and ovi position periods of this species were observed throughout December and January.Females laid eggs on the surface of water plant leaves in clusters containing 2-4 eggs.The shortest time of embryonic and postembryonic develop ment was 14 and 70 days, respectively.The last hatched larvae finished their development in the middle ofMay.
Habitats and dispersal potential.Helotrephids have been found in large and small, stable and temporary, quiet and running waters, in rock pools, water falls, seeping rocks, hot springs, in water tanks, and outdoor ceramic bowls.Various species of helotrephids live in unstable habitats.They can probably cope with stress conditions, an opinion supported by the discovery of Mixotrephes hoberlandti in South Iran, so far the only helotrephid found in an arid zone.Circumstances of the find suggest that the species lives there in ephemeral waters, and that it can aestivate as an adult, probably for a considerable part of the year in a nonaquatic environment (Papacek et al., 1989).
Food niche.Myiamoto (1952) fed Helotrephes formo sanus during laboratory rearing with the same small ani mals he found where he collected the helotrephids: larvae of Chironomidae, Tipulidae, Dryopidae, Ephemeridae, Perlidae, and Gerridae.Reared helotrephids attacked all the prey as well as their own larvae.Papacek (1993) found that Idiotrephes asiaticus from Vietnam, reared in the Czech Republic, reproduced successfully when fed on Central European species of cladocerans and copepods, and their own larvae.They attacked and killed redundant prey without eating it.No prey preference was discovered.These bugs, about 1.3-1.4mm long, even attacked moving prey far bigger than themselves.Based on the observations of this rearing, and the field observa tions of feeding strategy of Helotrephes shepardi in Vietnam (M.P., unpubl.data), the conclusion can be made that helotrephids feed also on mosquito larvae, and can help in mosquito population control together with Corixidae, Notonectidae, and Pleidae.

Aphelocheiridae
Distribution.All species (1 genus, 60 species) are dis tributed in the Old World, mostly in the Paleotropic area.
Life history.The biology of the Palearctic species Aphelocheirus aestivalis is the best known, but published data are often controversial (cf.Krajewski, 1966;Kanyukova, 1974;Messner et al., 1980;Saettem, 1986;Papacek, 1989).The species is probably semivoltine or univoltine with "cohort splitting populations" and a twoto three-year long postembryonic period.The life cycles of individual populations, living in various regions of their distribution, are modified or even differentiated.Females lay eggs from spring to autumn on the surface of various objects on the bottom, on the cuticle of crusta ceans, and shells of Bivalvia.Late summer and autumnal eggs overwinter.Larvae of instars 1-3 appear from spring to autumn, larvae of instars 4 and 5, and adults all the year long (cf.Krajewski, 1966).Adults live at least two years, some females have two ovipositional periods, and males have continual multiwaved spermatogenesis (Papacek & Soldan, 1996).
Habitats and dispersal potential.Aphelocheirids occur especially in lakes, streams, and rivers.They have a relatively wide ecological valence, but these bugs, having plastron respiration, prefer aerated water.They live on a gravel-sand bottom, under the stones, in the river rapids and calms, on lake bottoms, among roots of plants, or buried in the bottom.Most specimens or populations of some species (e.g., A. aestivalis) are micropterous and flightless.Some species in the tropics and subtropics are macropterous (see Polhemus & Polhemus, 1988) and fly very well (M.P. field observation).Individual populations in rivers and streams migrate seasonally (Messner et al., 1983).
Food niche.Aphelocheirids feed on various benthic insect larvae including larvae of simuliids, and small crustaceans and bivalves.
Life history.The major works on the biology of gelastocorids are by Hungerford (1922) and Todd (1955).The life cycle of the Old World species Gelastocoris oculatus was studied by Hungerford (1922).This species is univoltine, with overwintering adults.Females lay up to 200 eggs (1-14 per day) on the surface of various objects.Postembryonic development lasts 60-100 days.
Habitats.Toad bugs are saltatory riparian or amphibian predacious insects.They live in various habitats such as under water, under stones, on muddy or sandy banks, or rock faces while some species are found in soil remote from water, particularly in rain forests.
Food niche.Toad bugs very actively attack their prey, usually small insects living in the same habitats.

Ochteridae
Distribution.Velvety shore bugs (3 genera, ca.50 spe cies) are distributed world-wide in the tropics and the subtropics and in the warm parts of temperate regions.
Life history.The major data on the biology of velvety shore bugs were published by Takahashi (1923), Jaczewski (1934), Schell (1943), Bobb (1951), andPoisson (1957).The biology of Ochterus marginatus is probably the best known.This species is univoltine, and adults and instars 4 or 5 overwinter.Only adults compose the early spring population.Females have a relatively long oviposi tional period; they lay 2-3 eggs per day in groups of 25-30 eggs on the surface of sandy grains, plants, or roots ofplants.Larvae develop for 2 months.
Habitats.Ochterids are terricolous or littoral bugs which live along the shores of ponds or streams, they occur especially in sandy stretches near the water, but they also run on aquatic plants and hunt their prey even under water.
Food niche.Poisson (1957) states that these predators hunt small larvae of Diptera and small crustaceans, such as Gammarus spp.

DISCUSSION AND CONCLUSIONS
Although Notonectidae and Corixidae, also Pleidae and Helotrephidae, differ from each other in their life strate gies, they have a similar spatial distribution in individual habitats.All have a somewhat similar function in food webs.Their behaviour in nature and the consequences arising for their economic importance for humans are summed up.Notonectidae and Corixidae are surely eco nomically the most important of these families.All notonectids are definitely aggressive, size-limited preda tors, which kill small animals beyond their food needs.
The feeding strategy and niches of corixids are more complicated.Although food relationships of economic importance have been documented only in some dozen corixid species, it is probable that more species of the family have similar function in food webs.
The economic importance of Pleidae and Helotrephidae is not mentioned in the literature.Yet it is probable that under certain conditions pleids and helotrephids can be useful in controlling mosquito larvae.
The economic importance of these "pelagic families" of water bugs can be characterized from these points of view: (a) Positive economic importance.(1) Notonectids, corixids, pleids, and helotrephids function as biological mosquito control agents.(2) Corixidae especially are food for insectivorous fish, amphibians, birds, and bats.(3) Humans use the eggs, nymphs, and adults of corixids as food for poultry, dried feed for aquarium fish; and rarely the eggs are used as a food for humans themselves.(4) It seems that corixids can be good indicators of water qual ity.
(b) Damage.( 5) Both Notonectidae and some Corixidae attack fish eggs and fry, and under certain conditions the enormous overpopulation (due to migration or successful reproduction) can cause serious damage to fish culture.( 6) Notonectids attack and feed on small larvae of amphibian species, some of which are listed as endan gered or protected.
The economic importance of small benthic or ripicolous bugs varies.Benthic Aphelocheiridae, on the one hand, could suppress populations of some blood-sucking dipterans (larvae of Simuliidae); on the other hand, they also suppress populations of predators (larvae of Trichoptera), which also feed on simuliids.Ripicolous Gelastocoridae and Ochteridae also participate in suppressing popula tions of these blood-sucking dipterans with (a) swimming larvae breathing air on the water surface, (b) larvae living in small water bodies in holes in the sand or in rock pools, and (c) saprophagous larvae living on the banks.
The economic effect of these heteropterans (especially of notonectids and corixids) for human activity and health has yet not been calculated, either as a worldwide average, or under extreme conditions.They can be very damaging in ponds where fish fry are bred.Several papers follow and estimate the damage to fish culture (for example -the number of killed fish larvae/per bug/per day or per square area-unit) (e.g., Berezina, 1955Berezina, , 1962;;Dahm, 1972;Sokolovskaya & Zhitneva, 1973).The financial effects are not presented either.The studies fail to deal with the energy demands of the bugs' production (with the exception of some papers -e.g., Toth &Chew, 1972 andWaitzbauer, 1976), or the energy needs of food relationships that do have an economic effect.We also do not know the percentage of the biomass of mosquito larvae that water bugs can destroy under various condi tions.Missing, therefore, are reliable quantitative data for the financial expression of the damage or for the eco nomic benefit.Getting better knowledge of the biology of water bugs, and acquiring the quantitative characteristics of some food relations, are interesting and important tasks for future research.