Thoughts on Aquatic Insects

Thoughts on Aquatic Insects G. Evelyn Hutchinson Aquatic adults seldom have terrestrialjuveniles. Respiratoryproblems may lead to the first one or two nymphal instars of tropical species being smaller than related temperate ones. Suctorial feeding on many aquatic insects such as the may- higher plants seems unknown. Cryptic coloration is common, aposematic, and epi- flies, the Hemiptera, and the aquatic rare. is but gamic Flightlessness common, potentially flying forms must be general- moth Acentropus, though later these de- ly available;this restricts tracheal gills to juveniles. 12 (Accepted for publication velop other respiratory mechanisms. January 1981) The unique pair of genera, Idiocoris and Paskia, the smallest of all known aquatic KINDS OF LIFE HISTORY ly rare, being exemplified only by certain Hemiptera, are apparently the only free- beetles of the families Hydraenidae and living adult apneustic insects; they are Among the insects of inland waters Dryopidae (Hinton 1955). Moreover, limited to the stony littoral of Lake Tan- there are certain peculiar situations though the aquatic bugs and beetles to- ganyika (Esaki and China 1927). which, though often obvious, at first gether include almost all the members of The large aquatic insects breathe ei- seem hard to explain. the adult aquatic and immature aquatic ther by tracheal gills, outgrowths of the We may group the animals under con- categories, the two orders differ in their body wall that are richly supplied with sideration according to whether they are relation to water: The endopterygote Co- trachea, or spiracles that may be fed terrestrial or aquatic in active immature leoptera have a pupa that is nearly al- from bubbles and replaced periodically and predominantly trophic stages or in ways formally terrestrial. It lies in a dry by visits to the surface or to photosyn- the mature and primarily reproductive cocoon in a cavity excavated above wa- thesizing plants. Such bubbles or air- climax of their lives. The following ma- ter-level in the bank of the water; the stores are held in various ways. Imma- trix shows the orders of magnitude of the larva lived in it, and all being well, the ture insects retain air-stores usually in four possible classes: adult may return to it. bunches of hair; in mature insects, the elytra of beetles or the hemel- Adult Adult equivalent ytra of bugs cover a large dorsal air- terrestrial aquatic RESPIRATION IN AQUATIC HABITATS store. There may also be very fine hairs air on the surfaces of various Immature carrying For a member of a terrestrial group parts of the body, which give the insect terrestrial n. 105 n. 102 entering freshwater, respiration proba- its "silvery-light" appearance, as Mary Immature bly poses a greater problem than does Ball (Ball 1846) called it 140 years ago. aquatic n. 104 n. 103 any other vital function. Small aquatic From the air-stores, air is usually passed insects (and other animals less than into the tracheal system much as in ter- about 1 mm in diameter) are apt to restrial insects. breathe by diffusion over the entire thin, Since the air in the air-store is in This reveals the fact that it is easier to be but largely unmodified, body wall. There contact with water, it can act as a immature in the water than to be mature. "phys- may be specialized hemolymph sinuses ical gill"-a universal, though rather The category of the aquatic mature stage below this wall. When the insect is a odd, appellation (Ege 1915, Popham that has grown up terrestrially is curious- relatively long and thin cylinder, one 1964, Thorpe 1950). As respiration takes may regard the whole body as a gill. place, oxygen is removed; the equivalent Hutchinson is Sterling Professor of Zoology (emeri- tus) and senior research biologist, Department of Many variations on this general theme carbon dioxide produced is much more Biology, Yale University, New Haven, CT 06511. are found in the Diptera, which common- soluble than the oxygen and quickly This paper is derived from a lecture given on 12 ly have blood gills or special diverticula into the so that the bubble September 1980 to Ruth Patrick's course on the passes water, biology of streams, School of Forestry and Environ- of the body wall containing sinuses. In constituting the air-store contracts. The mental Studies, Yale University. He is greatly in- many cases, however, these have now partial pressure of nitrogen in the bubble debted to her and to the school for giving him the been shown to be opportunity to present this material, which is based organs taking up chlo- therefore increases, so nitrogen diffuses on a far more extended treatment (with a much fuller ride ions rather than oxygen. In some out. Since oxygen is lost by respiration, set of references) to appear in the fourth volume, on chironomids, the hemolymph contains the partial pressure of that gas de- zoobenthos, of his Treatise on Limnology. ? 1981 American Institute of Biological Sciences. All rights hemoglobin. Comparable respiration increases, and oxygen diffuses into the reserved. volving the whole surface is common in bubble, at least from saturated water. July/August 1981 495 Ideally, the process can continue until The insects that have perfected this lova americana Desmoulins from the all the nitrogen has been lost; but in respiratory plastron live in habitats Lower Permian of Oklahoma. Because actual cases, as the bubble becomes where continualdisturbance, or the pho- the mandibleswere very large and bore small, the oxygen content becomes dan- tosynthetic activity of calcicole algae, teeth, Kukalova(1968) initiallyregarded gerously low, and the air-store must be keeps the water saturatedwith oxygen. the insect as a carnivore. Later, Hub- replaced at the surface, well before it Though in a perfectly developed case, bard and Kukalova-Peck (1979) com- disappears. The effectiveness of the such as that of Aphelocheirus, the plas- paredthe mouthparts with some modern process depends on the oxygen concen- tron obviates the necessity for surfacing, generalistfeeders. Though the evidence tration in the water, which is related it is evidently not a mechanismthat can is still inadequate,it seems that the may- inversely to the temperature.It also de- be effective in a wide variety of habitats. flies may exemplify a trend detectible in pends on the exposed surface of the A second specialization of the air- other freshwaterinvertebrates that have bubble, which, in a large animal, is store is found in the lesser backswim- evolved a kind of selective omnivory. smallerrelative to the mass of metaboliz- mers of the subfamilyAnisopinae in the For such animals,the haptobenthicalgae ing tissue than in a small one. Thus, the family Notonectidae:the generaAnisops and bacteria with the slime secreted by physicalgill works best for small animals living in the warmtemperate and tropical them, together with adherent detritus at low temperatures, but the effective- Old World and Buenoa in the New. and associated small animals, constitute ness varies greatly with the morphology These insects have an abdominalhemo- a universal pabulum covering all sur- and physiology of the organism under globinorgan, richly suppliedwith trache- faces, at least in the shallower parts of study. Under favorable circumstances, ae. The hemoglobinin the cytoplasm of temperate freshwaters (except perhaps the mechanism of the physical gill can the cells of the organ is charged with the most oligotrophic). A change from certainlyincrease severalfold the period oxygen at surfacingand slowly liberates carnivory to partial reliance on such a between surfacing,thereby reducingthe the gas as it is used in respiration.The source of food, together with any other risk of predation. insect thus can adjust its density, re- plant or animalmatter that may be avail- High temperaturesnot only decrease maining poised at the same depth for able, seems to characterizemany of the the physical gill's effectiveness, but also some minutes (Miller 1964). small Corixidae and some species of increase the metabolic demand for oxy- Most adultinsects living on the bottom Haliplidae. gen. Very little, unfortunately,is known or among weeds, if they use an air-store Development of a like kind of omniv- about respirationin tropical aquatic in- for respiration, will have to make fre- ory is known in the Lymnaeidae among sects, thoughMiller (1964)has clear evi- quent visits to the surface, except per- the watersnails. In all cases, some de- dence that the physical gill can work for haps when dormantin winter. Such ex- gree of selectivity is retained,though the Enithares sobria Stal, a backswimmer peditions are energetically wasteful and rangeover which it is exercised is greatly widespreadin CentralAfrica. In the Cor- limit the depths that can be colonized. extended. Some animals eat most algae ixidae, the largerCorixinae are predomi- Moreover, surfacing is bound to be a other than Cladophora, or most angio- nantly temperatewith a maximumnum- conspicuous act, increasing the dangers sperms other than Elodea. These two ber of species in the United States, of predation.In spite of these disadvan- plants are apparently largely inedible whereas the smaller Micronectinae are tages, no adult aquaticinsect respires by (and so most successful). The former is largely tropical with the richest local trachealgills, though such structuresare apparentlyprotected by lauric, myristic, fauna in Sri Lanka. In tropical uplands, obviously highly successful in the physi- and palmitoleic acids (Lalonde et al. such as the Nilgiri Hills in India and the ology of mayflyand stonefly nymphsand 1979, Larson 1981), which render the mountains of Ethiopia, Micronectinae, in more elaborateways in those of drag- plant poisonous, or at least inhibitoryin as in temperateregions, tend to be rarer onflies and damsel flies, in the larvae of variousways to mosquitolarvae, gastro- than Corixinae. These features of the alderfliesand caddisflies, and in those of pod mollusks, and crayfish.' Unfortu- distributionsof the Corixidae are, thus, some beetles and two-winged flies.

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