Megaloptera (Fishflies and Alderflies)
Larvae of most species occur only in relatively oxygen-
rich environments, because they usually obtain oxygen through their integument directly from the water in which they live. Their lateral filaments greatly increase surface area, allowing them to do this. Species inhabiting lentic habitats, which may experience low oxygen levels, have long caudal respiratory tubes to obtain oxygen from air at the surface. All larvae have spiracles and can live out of water in moist areas; larvae of some species survive in temporary streams.
Larvae can be readily collected with an aquatic net by using standard sampling procedures. Many stream species are often found underneath large rocks. Rearing of larvae is difficult because of long life cycles, cannibalism, and the need to supply prey species and a terrestrial environment for pupation. Rearing of oldest larvae, collected just before pupation, will alleviate some of these difficulties. Species of adults are well known, and as a result of recent studies larvae of most Nearctic species can also be identified. Larvae of Neotropical genera from Mexico
Megaloptera (Fishflies and Alderflies
This very small, holometabolous order (approximately 400 species worldwide) contains two aquatic families that once were included in the order Neuroptera. The aquatic larvae are distinctive, having seven or eight pairs of lateral filaments and large, conspicuous mandibles They are very distinctive and can be confused only with some aquatic Coleoptera larvae that have lateral filaments. The predaceous larvae often attain a very large size and may exert significant pressure on populations of macroinvertebrates living in the same habitat. Usually 10 or 11 larval instars are present, and life cycles range from one to five years, depending on the species, climate, and habitat. Larvae are aquatic; all other stages are terrestrial. Eggs are laid in masses on objects above the water, and pupation takes place in cells on land.
The terrestrial adults (Plate 16.16) are short-lived and generally secretive. However, because of their large size and often conspicuous mandibles, they attract attention when discovered. Adults are weak fliers, but are often attracted to lights. They fold their net-veined wings rooflike over their abdomen when at rest.
Aquatic Neuroptera (Spongillaflies)
Neuroptera is a fairly large terrestrial order, but larvae of only two families (Sisyridae and Nevrorthidae) are fully aquatic. Some species of the family Osmylidae have been reported in semiaquatic habitats along the margins of streams. Of the aquatic families, only Sisyridae, the spongillaflies, are found in North America. They live in and feed upon several genera of freshwater sponges (Spongillidae) that occur in permanent lotic or lentic habitats. Recent keys and some distribution records are available for larvae and adults[77] and also pupae[189] of North American spongillaflies. Information on the biology, habitat, and identification of larvae is presented next.
Aquatic L lepidoptera (Aquatic Caterpillars)
larvae that inhabit rocks in streams and also in some lentic case builders. Larvae of at least one species have hydrofuge hairs enabling them to maintain a plastron that is used as a physical gill. Pupation of aquatic species is in a cocoon, usually in the larval habitat. Emergence occurs after pupae swim or crawl to the surface of the water; the legs of species that swim have long swimming hairs. Adults are generally nondescript small moths that hold their wings rooflike over their body and remain in the vicinity of their larval habitat. Adults of stream species crawl under the water to oviposit on rocks, while those of most lentic species deposit rows of eggs just below the water surface on the preferred food plant. Most species are univoltine or bivoltine and have five larval instars.
Larvae inhabit a wide variety of permanent aquatic habitats. Some inhabit rocks in rapid water of larger streams where they feed on diatoms and other algae. Others inhabit aquatic macrophytes or duckweed in ponds or along the margins of streams and lakes. Here, they feed on the plants they inhabit and make cases from them. Respiration is cutaneous and is enhanced by numerous filamentous gills in
Aquatic L lepidoptera (Aquatic Caterpillars)
Larvae in several genera of this large, primarily terrestrial order are associated with aquatic habitats. Some feed on emergent parts of aquatic macrophytes and others mine stems of these plants. Still others feed on submerged parts of plants attached to other substrates or graze periphyton from around a taut layer of silk laid down atop hard substrates in the water, and only these will be considered here. All 17 aquatic genera in which aquatic larvae occur are in the family Pyralidae, and almost all are in the subfamily Nymphulinae.
Aquatic L lepidoptera (Aquatic Caterpillars)
Aquatic caterpillars have three pairs of thoracic legs and pairs of short prolegs ringed with hook-like crochets on abdominal segments 3–7 (Fig. 16.103). They are not likely to be confused with any other aquatic insect larvae, but often it is not possible to distinguish them from Lepidoptera that are inadvertently collected from emergent vegetation. Only those that have numerous filamentous gills covering their body and those that make portable cases of aquatic vegetation can be recognized with certainty as being aquatic. Larvae that live in cases or form in retreats on hard substrates are so well camouflaged that they often escape notice. Some species may become sufficiently abundant to have an impact on aquatic plant communities. While adults have been well studied and larvae of many of the species have been reared and described, reliable keys to species of larvae have not been constructed because larvae of many species remain unknown.
Aquatic Coleoptera (Water Beetles)
Hydroscaphidae adults (Myxophaga) crawl to the surface to renew their air supply. Elmidae and Dryopidae adults (Polyphaga) usually live in oxygen-rich water and maintain a plastron of air that is carried on hydrofuge hairs. The plastron acts as a physical gill, with oxygen for respiration diffusing into it from the water, and carbon dioxide from respiration diffusing out into the water. Other Polyphaga adults (Helophoridae, Hydrochidae, Hydrophilidae, Hydraenidae, and Curculionidae) also maintain an air supply on hydrofuge hairs, but must renew the air by swimming (most Hydrophilidae) or crawling (Curculionidae, Hydraenidae, Helophoridae, Hydrochidae, and some Hydrophilidae) to the surface when the water contains too little oxygen for plastron respiration. Unlike Adephaga, adult polyphaga break the surface film with their antennae.
Adult water beetles range from 1–40 mm in length and vary greatly in structure. All are characterized by chewing or biting mouthparts and shell-like mesothoracic wings called elytra (Fig. 16.108). They have functional spiracles and rely on atmospheric oxygen for respiration. Adult Adephaga carry a bubble of air under their elytra, which is renewed at the surface by swimming or crawling (Amphizoidae) to the surface and breaking the surface film with the tip of the abdomen. This air supply must be frequently renewed, except when water temperatures are cold and metabolism is slowed, or when the water is well oxygenated.
Aquatic Coleoptera (Water Beetles)
Three suborders of Coleoptera have aquatic representatives. Adephaga contains five families in which all species are aquatic both as larvae and as adults. These families are often referred to as the Hydradephaga. The suborder Myxophaga is represented in North America by a single aquatic species, Hydroscapha natans, in which both larvae and adults are aquatic. Several families of Polyphaga have some species with one or more aquatic stages, but only in a few families are most species tied to the aquatic environment.
In late summer, large schools often appear in shallow bays of lakes and streams; and by herding them toward shore and sweeping through the school, large numbers often can be captured. In the north, where ponds and lakes freeze, lentic as well as lotic gyrinids congregate in streams in the fall to overwinter. Here, very large numbers of mixed species can be collected with a net from undercut banks where there is a current and the water is at least 0.5 m deep. Some overwintering lentic species of Dytiscidae, Haliplidae, and Hydrophilidae also may be found along stream banks in autumn and early spring. Large numbers of Hydrophilidae, Helophoridae, and Hydrochidae adults can often be collected by blacklight traps on warm nights in spring and summer.
Aquatic Diptera (Flies and Midges)
It is impossible to accurately estimate the number of species of aquatic Diptera, because in most families only terrestrial adults have been described taxonomically, and in many families it is not possible to know which species have aquatic larvae. It is also difficult to estimate the relatively large number of aquatic species that probably remain undiscovered and undescribed.
Diptera larvae lack segmented thoracic legs, and thus can be readily distinguished from larvae of other aquatic insects. In Nematocera larvae, a completely sclerotized, relatively round head capsule is present (except in most Tipulidae), and the mandibles, which usually have subapical teeth, move laterally. In Brachycera larvae the head is reduced to an internal skeleton (cyclorrhaphous Brachycera) or poorly formed and not rounded (orthorrhaphous Brachycera), and the mandibles (mouth hooks) move vertically and lack subapical teeth. Diptera larvae have many adaptations that allow life in a wide variety of environments. Those living in oxygen-rich environments usually rely on cutaneous respiration, which may be aided by gills that increase the surface area. In Chironomidae, larvae of some species have a hemoglobin-like pigment that aids in oxygen utilization under hypoxic conditions. Larvae in many families obtain oxygen at the water surface
Aquatic Diptera (Flies and Midges)
Diptera, or true flies, constitute a large, mostly terrestrial order, but larvae and pupae of numerous species are aquatic and it is the dominant order of insects in the aquatic environment. Diptera larvae inhabit all types of aquatic environments; and, because of the very short development periods in many species, they exploit ephemeral aquatic habitats as well as permanent ones.
The order is divided into two suborders, Nematocera and Brachycera[5], with Nematocera dominating the aquatic fauna. Formerly a third suborder, Cyclorrhapha, was recognized but it is now included in Brachycera as the infraorder Muscomorpha. In several families of Nematocera, all or most species have aquatic larvae and pupae. In Brachycera, most families that have species with aquatic larvae also have large numbers of terrestrial or riparian species. Larvae of the infraorder Muscomorpha are often referred to as cyclorrhaphous Brachycera, with the remaining Brachycera being called orthorrhaphous Brachycera.