NRE 516 - Aquatic Entomology

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Doc Leonard's Lecture Notes on Hemiptera

The Hemiptera are a large and varied order of insects whose terrestrial representatives include some extremely serious pests such as the tarnished plant bug and the chinch bug, which are familiar plant pests , and a human pest important in a bygone era but now a practical rarity, the bedbug, which used to keep lumberjacks on the move. The order includes the Reduviidae, one of whose members, the infamous "kissing bug," transmits Chagas disease by injecting trypanosomes into the bloodstream of the unfortunate human victim.

Among other somewhat offbeat Hemiptera are the Polyctenidae, whose members are ectoparasites of bats in the Western United States, and the Cimicidae, a family which includes not only the vanishing bedbug, but various ectoparasites of both birds and mammals.

The number of aquatic families varies somewhat depending on which authority one consults. Pennak lists 15 families for North America. Usinger lists 16 families in his "Aquatic Insects of California." Polhemus in the latest work on aquatic Hemiptera lists 17 families. The truly aquatic families, that is the ones which live under water, are the Corixidae (water boatmen), Notonectidae (backswimmers), Nepidae (water scorpions), Belostomatidae (giant water bugs, toe biters or electric light bugs), and Naucoridae (creeping water bugs). Most authors now recognize a separate family, the Pleidae or pygmy back swimmers, distinguished from the Notonectidae by a 3-segmented beak, comparative size of the eyes and shape of the body. The remaining families are grouped as semi-aquatic and include the water striders in a broad sense (Gerridae or water striders, Veliidae or broad-shouldered water striders, and Mesoveliidae or water treaders) and the shore bugs in a broad sense (Hebridae or velvet water bugs, Gelastocoridae or toad bugs, Hydrometridae or marsh treaders, Saldidae or shore bugs, and 4 families without common names, Macroveliidae, Dipsocoridae, Schizopteridae, and Ochteridae).

In the Odonata, it was stated that there was a small group of workers throughout the period of modern biological study and perhaps because of this the order has maintained a fairly straightforward development of taxonomic problems. The number of workers concerned with aquatic Hemiptera, particularly in North America, was even smaller and more select. Starting with 1920, and for the next 40 years, H. B. Hungerford, of the University of Kansas and the University of Michigan Biological Station, dominated the field and most of our basic knowledge of aquatic and semi-aquatic Hemiptera comes either from his own synthesizing works or from the individual publications of a large group of graduate students who passed through his program. In England, T. T. Macan has published on the group, and it is of interest that G. Evelyn Hutchinson of Yale University, long known as a leader in advanced limnological thought in this country, got his start working on aquatic Hemiptera of South Africa, culminating in a definitive 300-page publication in 1929.

The aquatic and semi-aquatic Hemiptera occupy a wide variety of habitats from salt water pools to mountain lakes and from hot springs to large rivers. Usinger, who until his untimely death was Hungerford's successor as North America's authority, remarked that in general they fill the role of predators at intermediate stages in the food chains of their respective communities. Some, such as the water striders, appear to be complete masters of their environment, whereas others, such as the relatively defenseless Corixidae, are preyed upon by a wide range of organisms. The small corixids are partly responsible for the primary conversion of living plant material into animal food. Their mouth parts are, of course, greatly modified and bear little resemblance to the piercing beak characteristic of the order as a whole. However, the belief of earlier authorities that corixids subsisted on flocculent ooze alone, appears not to be entirely true. It is known that they take a certain amount of animal food, such as small dipterous larvae, rotifers, etc. as part of their diet.

The truly aquatic bugs appear in general to overwinter in the adult stage and to lay eggs in the spring which then hatch and develop through the nymphal instars (usually 5) during the summer. The eggs are laid in a wide variety of places, but so far as is known, none are deposited to float freely in the water. All are either glued to some object or inserted in plant tissue. Most of the aquatic Hemiptera fly quite readily (with the primarily exception of the marine Halobates) and upon occasion may be attracted in large numbers to lights. It is this phototropic habit which has led the large Lethocerus to receive the common name of "electric light bug."

Usinger points out that each of the truly aquatic families has a distinctive type of respiration. In general, water bugs depend on atmospheric air which they obtain either through breathing tubes (as in the case of water scorpions), breathing flaps (the belostomatids), directly through the tip of the abdomen (backswimmers and naucorids), or through the pronotum (water boatsmen). Several families mentioned maintain an air bubble or film on the underside of the body by means of hydrofuge hairs which allows them to carry an air supply plus take some oxygen directly from the water.

Aquatic families share two traits with their terrestrial relatives which serve to make them somewhat repugnant items of diet to fish and other predators. Most are capable of inflicting painful stings with their beaks and also possess what appears to be an offensive odor similar to the so-called stinkbugs (Pentatomidae). The corixids are fairly frequent items in fish stomachs and apparently are ingested deliberately and repeatedly. Backwimmers, which are capable of inflicting extremely painful bites on the hands of the unwary collector, probably can make things equally hot for the gullet and stomach of the unwary fish that devours them. Fragments of the large Lethocerus are rare in fish stomach, as are Belostoma. One finds Ranatra but never Nepa. Water striders do occur occasionally in fish stomachs; both naucorids and Plea (Pleidae) also show up although not very frequently.

By and large, the aquatic Hemiptera are not particularly important from the standpoint of fisheries management. Most are predatory in habitat and presumably may be somewhat competitive with fish. However, there is no indication that they are ever numerous enough to make their presence particularly felt in terms of fish population density.

In Mexico, corixids are a common item in commerce. Tons of Corixidae, together with some specimens of Notonecta, are collected, dried, and shipped out to serve as bird food or fish food. The eggs of some of the corixids are gathered for human food in Lake Texcoco. Both bugs and eggs are offered for sale over the counter throughout Mexico and the adults are packaged in cellophane envelopes and sold as food for birds and pet turtles here in the United States. The eggs of certain Corixidae in Mexico are laid in enormous numbers on submerged objects, each egg attached by a short petiole. Commercialization is effected by placing reeds in the water and returning at a later date to harvest.

The backswimmers, or Notonectidae, are common inhabitants of lakes in this part of the world. The genera are Notonecta and Buenoa. Quite a lot of Hungerford's earlier work on aquatic Hemiptera was done at Douglas Lake where he was a long time member of the summer faculty. He was the first to report, back in 1922, that Buenoa is one of the few insects (along with certain chironomid fly larvae) which possess hemoglobin, apparently as a respiratory pigment. The eggs of Notonecta usually are laid in plant tissue but sometimes are glued to plant or rock surfaces under water. Notonectids are voracious feeders and consumer chironomid larvae, microcrustacea and even fish fry.

The Naucoridae, or creeping water bugs, are represented in this part of the world by a single genus, Pelocoris. It clambers among submerged vegetation and very infrequently shows up in fish stomachs.

The giant water bugs or electric light bugs, Belostomatidae, attract a great deal of attention because of their large size and because of their unquestioned ability to prey on small fish. They are fitted for life in open water with their well-developed swimming legs and reduced siphon. Three genera are common in the Eastern United States. Benacus and Lethocerus are about the same size and are separable chiefly on the basis that Lethocerus has a front femora with a groove in which the tibiae can fold, while Benacus lacks this groove. A third genus, Belostoma, is somewhat smaller and is characterized by the interesting fact that the female glues her eggs to the backs of the males which then carry them in this position until they hatch. It has been noted that the female may add to the batch of eggs on successive days so that a single male may carry more than 100. This, of course, puts effective stoppage to flight during the egg rearing period, and it may be assumed that the trait evolved for egg protection. If severely provoked, the male may shed the entire egg mass and fly. The two larger genera can become pests around fish hatcheries. Not only can they tackle small fish, but are also known to feed on tad-poles and soft-shelled crayfish. If disturbed and unable to flee, they may feign death. A large species of Lethocerus is boiled and eaten in China and Southeastern Asia.

The Nepidae includes two genera in this part of the country, Ranatra and Nepa. Although fitting into the same family, they are very different in appearance. Ranatra is long and slender with a greatly elongated breathing tube at the end of its abdomen. It lives among weeds in deeper water. Nepa, on the other hand, is much more robust and bears a superficial resemblance to Belostoma where it lives in shallow water weeds near shore.

Gerrids, or water striders, are familiar and scarcely need mention. One may note that Halobates, one of the few strictly oceanic or marine insects, is a member of this family as is Trichocorixa which lives in salt marshes and brackish water. An interesting behavioral trait of water striders in general is that of gregariousness. The Veliidae are sometimes known as the small water striders or riffle (ripple) bugs. They are small, 1-1/2 to 5mm long, black or brown in color, sometimes with silvery markings, which usually occur in or near riffles of small streams. Rhagovelia is perhaps commonest in this portion of the country, although one may have to look hard to find them.

The marsh treaders, or Hydrometridae, are rather weird looking insects whose economic role is not understood. One author has commented that they are seldom found in large bodies of water and may perhaps be excluded by their vulnerability to fish. They certainly are not capable of rapid evasive action, and apparently they lack the scent glands which are generally characteristic of Hemiptera.

To summarize, one may say the aquatic and semi-aquatic Hemiptera are interesting insects whose role in the scheme of things is essentially that of predator. In general, they are predators at intermediate stages in the food chains of their respective communities. They probably are not numerous enough to make themselves particularly felt so far as fish populations are concerned. The fisheries biologist is more likely to notice them simply as active members of various aquatic situations and also for the occasional belligerence of some. As indicated earlier, notonectids will sting the hand of the collector if given a chance, as will the large belostomatids. Furthermore, the toad bugs (Gelastocoridae) are rather aggressive types which will bite the bare toes of bathers if given opportunity. For those who wish to go further into this subject, it is suggested that one read the aquatic Hemiptera chapter of Usinger and some of the items which may be found in the references cited below.

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