Some Insects and Their Adaptations to Erosional Habitats

Adaptations of aquatic insects to torrential or "rapid flow" habitats include the dorsoventral flattening of the body, which serves two purposes: it increases the organism's area of contact with the surface substratum, and it offers a mechanism by which animals can remain in the boundary layer when water velocity diminishes, thereby reducing drag under subsequent exposure to high velocities. However, this second idea may be an oversimplification. Indeed, some authors have suggested that the dorsoventrally flattened shape may actually generate lift in the insect. Examples of animals inhabiting stones in torrential habitats include a number of mayflies (Ephemeroptera) belonging to the families Heptageniidae (Fig. 1A) and Ephemerellidae; some Plecoptera, such as Perlidae (Fig. 1G); some Megaloptera (i.e., Corydalidae) (Fig. 1D); and caddisflies (Trichoptera), such as Leptoceridae (Ceraclea).

In addition to body shape, many mayflies and stoneflies have legs that project laterally from the body, thereby reducing drag and simultaneously increasing friction with the substrate. Most of these taxa are either scrapers or gatherers on surfaces of stones or predators on other aquatic insects. Undoubtedly, the diverse physical forces encountered in aquatic environments, especially streams, influence the array of morphologies found among aquatic insects.

In some caddisflies (e.g., Glossosomatidae), the shape of the case rather than the insect is modified. The larvae of Glossosomatidae in their tortoiselike cases are frequently seen grazing on the upper surfaces of stones in riffle areas. Another

FIGURE 1 Typical insects inhabiting lotic environments. (A) Ephemeroptera: Heptageniidae (Rhithrogena). (Photograph by H. V. Daly.) (B) Diptera: Simuliidae (Simulium), (C) Trichoptera: Limnephilidae (Dicosmoecus), (D) Megaloptera: Corydalidae (Corydalus), (E) Diptera: Tipulidae (Tipula), (F) Plecoptera: Pteronarcyidae (Pteronarcys), (G) Plecoptera: Perlidae, (H) Coleoptera: Psephenidae (Psephenus).

FIGURE 1 Typical insects inhabiting lotic environments. (A) Ephemeroptera: Heptageniidae (Rhithrogena). (Photograph by H. V. Daly.) (B) Diptera: Simuliidae (Simulium), (C) Trichoptera: Limnephilidae (Dicosmoecus), (D) Megaloptera: Corydalidae (Corydalus), (E) Diptera: Tipulidae (Tipula), (F) Plecoptera: Pteronarcyidae (Pteronarcys), (G) Plecoptera: Perlidae, (H) Coleoptera: Psephenidae (Psephenus).

curious caddisfly grazer on stone surfaces is Helicopsyche, whose larvae construct coiled cases of sand grains shaped like snail shells. Both glossosomatids and helicopsychids reach their greatest abundances in sunny cobble riffles, where they feed on attached periphyton or algae. Another lotic insect that relies on a rather streamlined case is the limnephilid caddisfly Dicosmoecus (Fig. 1C).

Larvae of the dipteran family Blephariceridae are unusual in that they possess hydraulic suckers. A V-shaped notch at the anterior edge of each of the six ventral suckers works as a valve out of which water is forced when the sucker is pressed to the substrate. The sucker operates as a piston with the aid of specialized muscles. In addition, a series of small hooks and glands that secrete a sticky substance aid sucker attachment as the larvae move in a zigzag fashion, releasing the anterior three suckers, lifting the front portion of the body to a new position, and then reattaching the anterior suckers before releasing and moving the posterior ones to a new position. These larvae are commonly found on smooth stones in very rapid velocities and are usually absent from stones covered with moss and from roughened stones that interfere with normal sucker function. Several other aquatic insects have structures that simulate the action of suckers. The enlarged gills of some mayflies (e.g., Epeorus sp. and Rhithrogena sp.: Fig. 1A) function as a friction pad, and Drunella doddsi has a specialized abdominal structure for the same purpose. Some chironomids have "pushing prolegs" represented by circlet of small spines that function as a false sucker when pressed to the substrate. Mountain midge larvae (Deuterophlebiidae) possibly use a similar mechanism to attach their suckerlike prolegs. Most of these animals are primarily grazers on thin films of epilithon (algae, associated fine organic matter, and microbes) found on the surface of stones.

Flowing water usually carries many organic (and inorganic) particles and a number of insects exploit these suspended particles. Filter-feeding collectors (Table IV) exploit the current for gathering food with minimal energy expenditure. For example, certain filtering collectors exploit locations where flows converge over and around substrates, thus allowing the animals to occupy sites of greater food delivery. Examples include caddisfly larvae belonging to the families Hydro-psychidae and Brachycentridae. Silk is used for attachment by a number of caddisflies (e.g., Hydropsychidae, Philopota-midae, and Psychomyiidae), which build fixed nets and retreats (Fig. 2A). Although the Philopotamidae are found in riffle habitats, their fine-meshed, tubelike nets are usually found in crevices or undersides of stones in low velocity microhabitats (Fig. 2C). The nets of the caddisfly, Neureclipsis, are limited to moderately slow (< 25 cm s-1) velocities and the large (up to 20 cm long), trumpet-shaped nets (Fig. 2D) are used to capture small animals drifting downstream. Neureclipsis larvae are often very abundant in some lake outflow streams where drifting zooplankton are abundant.

Some case-making caddisflies (e.g., Brachycentrus sp.) also use silk for attaching their cases to the substrate in regions of moderately rapid flow. Many chironomid larvae construct fixed silken retreats for attachment or silken tubes that house the larvae, with a conical catchnet spun across the lumen of the tube. Periodically, the larva devours its catchnet with adhering debris that has been swept into the burrow by the water currents. Meanwhile, other chironomid larvae such as Rheotanytarsus spp. construct small silk cases that are attached to the stream substratum with extended hydralike arms. The arms project up in the current and are smeared with a silklike secretion to capture particles.

Larval blackflies (Simuliidae, Fig. 1B) use a combination of hooks and silk for attachment. The thoracic proleg resembles that of chironomids and deuterophlebiids, described earlier, and the last abdominal segment bears a circlet of hooks, which it uses to anchor itself to substrates. The larva moves forward, inchwormlike, spins silk over the substrate, and attaches the proleg and then the posterior circlet of hooks to the silken web. Most blackfly larvae possess well-developed cephalic fans, which are used to filter small particles from suspension. These attached larvae twist

FIGURE 2 Representative lotic insects in their environment: (A) Caddisfly larva (Macrostenum) in its retreat grazing on materials trapped on its capture net, (B) mayfly larva of Hexagenia (Ephemeridae) in its U-shaped burrow, (C) tubelike nets of philopotamid caddisfly larvae (Philotamidae) on the lower surface of a stone, (D) the caddisfly larva and cornucopia-shaped net of Neureclipsis (Polycentropodidae). [Habitat drawings modified and taken from Wallace, J. B., and Merritt, R. W. (1980). Filter-feeding ecology of aquatic insects. Annu. Rev. Entomol. 25, 103-132, (B); Merritt, R. W., and Wallace, J. B. (1981). Filter-feeding insects. Sa. Am. 244, 131-144 (A, C, D).]

FIGURE 2 Representative lotic insects in their environment: (A) Caddisfly larva (Macrostenum) in its retreat grazing on materials trapped on its capture net, (B) mayfly larva of Hexagenia (Ephemeridae) in its U-shaped burrow, (C) tubelike nets of philopotamid caddisfly larvae (Philotamidae) on the lower surface of a stone, (D) the caddisfly larva and cornucopia-shaped net of Neureclipsis (Polycentropodidae). [Habitat drawings modified and taken from Wallace, J. B., and Merritt, R. W. (1980). Filter-feeding ecology of aquatic insects. Annu. Rev. Entomol. 25, 103-132, (B); Merritt, R. W., and Wallace, J. B. (1981). Filter-feeding insects. Sa. Am. 244, 131-144 (A, C, D).]

their bodies longitudinally from 90° to 180° with the ventral surface of the head and fans facing into the current. The fusiform body shape of blackfly larvae reduces turbulence and drag around their bodies, which are often located in regions of relatively rapid flow. Blackfly pupae are housed in silken cases that are attached to the substrate.

Although unidirectional current is the basic feature of streams, most lotic insects have not adapted to strong currents, but instead have developed behavior patterns to avoid current. Very few lotic insects are strong swimmers, probably because of the energy expenditure required to swim against a current. Downstream transport or drift requires only a movement off the substrate to enter the current. Streamlined forms, such as the mayflies Baetis spp., Centroptilum, Isonychia spp., and Ameletus spp., are capable of short rapid bursts of swimming, but most lotic insects move by crawling or passive displacement. One characteristic of these latter mayflies is the possession of a fusiform, or streamlined, body shape: examples include several Ephemeroptera such as Baetis, Centroptilum, and Isonychia, as well as a number of beetle (Coleoptera) larvae. A fusiform body shape reduces resistance in fluids, and within the mayflies the shape is often associated with excellent swimming abilities.

The benthic fauna in streams often can be found in cracks and crevices, between or under rocks and gravel, within the boundary layer on surfaces, or in other slack-water regions. Another method of avoiding fast currents is living in debris accumulations consisting of leaf packs and small woody debris. This debris offers both a food resource and a refuge for insects and contains a diverse array of aquatic insects including stoneflies such as Peltoperlidae and Pteronarcyidae (Fig. 1F), caddisflies such as Lepidostomatidae and some Limnephilidae, as well as dipterans such as chironomids and tipulid crane flies (Fig. 1E).

In some streams with unstable sandy or silt substrates, woody debris can represent a "hot spot" of invertebrate activity. Wood debris provides a significant portion of the stable habitat for insects in streams when the power of the flowing water is insufficient to transport the wood out of the channel. In addition to the insect component using wood primarily as a substrate, there is often a characteristic xylophilous fauna associated with particular stages of wood degradation. These include chironomid midges and scraping mayflies (Cinygma spp. and Ironodes spp.) as early colonizers, and larvae and adults of elmid beetles. In western North America, an elmid (Lara avara) and a caddisfly (Heteroplectron) are gougers of firm waterlogged wood, chironomids are tunnelers, and the tipulids, Lipsothrix spp., are found in wood in the latest stages of decomposition. Woody debris is most abundant in small, forested watersheds, but it is also an important habitat in larger streams with unstable beds. In the southeastern coastal plain of the United States and in low gradient mid- and southwestern streams and rivers with unstable bottom substrate, woody debris or "snags" often represent the major habitat for aquatic insect abundance and biomass. High populations and biomass of filter-feeding animals such as net-spinning caddisflies (Hydropsyche spp., Cheumatopsyche spp., and Macrostenum) (Fig. 2A), and blackflies occur in these streams and rivers. In addition to filter feeders, other groups such as odonates, mayflies, stoneflies, elmid beetles, nonfiltering caddisflies, and dipteran larvae can be locally abundant on large pieces of woody debris. Invertebrate shredders and scrapers promote decomposition of outer wood surfaces by scraping, gouging, and tunneling through wood. In fact, wood gouging habits of some net-spinning caddisflies have been blamed for the failure of submerged timber pilings that had been supporting a bridge!

Sand and silt substrates of rivers and streams are generally considered to be poor habitats because the shifting streambed affords unsuitable attachment sites and poor food conditions. An extreme example of this instability is the Amazon River, where strong currents move the bedload downstream, resulting in dunes of coarse sand up to 8 m in height and 180 m in length, thus largely preventing the establishment of a riverbed fauna. However, sandy substrates do not always result in poor habitat for all aquatic insects: some sandy streams are quite productive. Blackwater (i.e., high tannic acid concentrations from leaf decomposition) streams of the southeastern United States have extensive areas of sand, with some of insects, such as small Chironomidae (< 3 mm in length), exceeding 18,000/m-2 in abundance. Their food is derived from fine organic matter, microbes, and algae trapped in the sandy substrate. Numerically, the inhabitants of sandy or silty areas are mostly sprawlers or burrowers, with morphological adaptations to maintain position and to keep respiratory surfaces in contact with oxygenated water. At least one insect, the mayfly Ametropus, is adapted for filter feeding in sand and silt substrates of large rivers. Ametropus uses the head, mouthparts, and forelegs to create a shallow pit in the substrate, which initiates a unique vortex (flow field in which fluid particles move in concentric paths) in front of the head and results in resuspension of fine organic matter as well as occasional sand grains. Some of these resuspended fine particles are then trapped by fine setae on the mouthparts and forelegs. Many predaceous gomphid (Odonata) larvae actually burrow into the sediments by using the flattened, wedge-shaped head and fossorial (adapted for digging) legs. The predaceous mayflies Pseudiron spp. and Analetris spp. have long, posterior-projecting legs and claws that aid in anchoring the larvae as they face upstream. Some mayflies (e.g., Caenidae and Baetiscidae) have various structures for covering and protecting gills, and others (e.g., Ephemeridae, Behningiidae) have legs and mouthparts adapted for digging. The predaceous mayfly Dolania spp. burrow rapidly in sandy substrates and have dense setae located at the anterior—lateral corners of the body as well as several other locations. The larva uses its hairy body and legs to form a cavity underneath the body where the ventral abdominal gills are in contact with oxygenated water.

Dense setae also are found in burrowing mayflies belonging to the family Ephemeridae that are common inhabitants of sand and silt substrates. They construct shallow U-shaped burrows and use their dorsal gills to generate water currents through the burrow (Fig. 2B), while using their hairy mouthparts and legs to filter particles from the moving water. Hairy bodies seem to be a characteristic of many animals dwelling on silt substrates, which include other collector mayflies such as Caenis, Anepeorus, and some Ephemerellidae.

Many dragonflies (e.g., Cordulegaster spp., Hagenius spp., and Macromiidae) have flattened bodies and long legs for sprawling on sandy and silty substrates. Some caddisflies, such as Molanna, have elongate slender bodies but have adapted to sand and silt substrates by constructing a flanged, flat case. They are camouflaged by dull color patterns and hairy integuments that accumulate a coating of silt. The eyes, which cap the anterolateral corners of the head, are elevated over the surrounding debris. The genus Aphylla (Gomphidae) is somewhat unusual in that the last abdominal segment is upturned and elongate, allowing the larvae to respire through rectal gills while buried fairly deep in mucky substrate. Some insects burrow within the upper few centimeters of the substratum in depositional areas of streams. This practice is found among some dragonflies and a number of caddisflies, including Molanna, and various genera of the families Sericostomatidae and Odontoceridae.

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