Tracheal Gills of the Dobsonfly Larvae, or Hellgrammite Corydalus cornutus L. (Megaloptera: Corydalidae)

The dobsonfly, Corydalus cornutus (L.), is a medium to large insect found throughout most of North America east of the Continental Divide from Mexico to Canada (Brown and Fitzpatrick, 1978; Contreras-Ramos, 1998). The order Megaloptera is thought to be one of the oldest orders to exhibit complete metamorphosis (McCafferty, 1983). Adults are typically between 25 and 70 mm long (Lehmkuhl, 1979), but the species "... spends most of its life cycle as a predaceous lotic larva known as the hellgrammite" (Brown and Fitzpatrick, 1978:1091). Hellgrammites are found along the edges of lakes and ponds, under stones in streams (Pennak, 1953) and in riffles (Brown and Fitzpatrick, 1978; Epperson and Short, 1987). They are well known to fishermen and valued as live bait. They can be kept for days in damp moss (Pennak, 1953), perhaps because the larvae have a complete set of spiracles along the body (McCafferty, 1983) that are functional out of water. The name 'hellgrammite' is of uncertain origin and has been defined as simply as "Insect larva used for bait" (Eisiminger, 1981:147). Hellgrammites are noted for several characteristics besides their being used for fish bait. They have one of the highest rates of productivity known for a predatory aquatic insect, with an increase in biomass (up to 1000 times from the first to the final instar) that is more comparable to that of primary consumers (Epperson and Short, 1987). This might be related to their ability, rarely reported for insects, to compensate for temperature fluctuations to maintain relatively constant metabolic rates year round and thus allocate a high level of energy to growth (Brown and Fitzpatrick, 1978). The larval period is as long as 2-3 years (Pennak, 1953; Knight and Simmons, 1975) but likely one year for early-hatching individuals in Texas (Brown and Fitzpatrick, 1978; Epperson and Short, 1987). Lastly, hellgrammites have tufts of tracheal gills (Pennak. 1953) in addition to spiracles in pairs along the base of the first seven abdominal segments (Fitch, 1982; Kinnaman et al., 1984) that appear to be an important respiratory adaptation (Lee, 1929; Carey and Fisk, 1965; Fitch, 1982; Kinnaman et ai, 1984). Interest in the functional role of tracheal gills in respiration led to conclusions ranging from no known function to speculation that they were efficient respiratory structures (see review in McColl, 1943), or even used to trap food from the water stream that would later be collected by preening or combing the gills (Carey and Fisk, 1965). Hellgrammites introduced to hypoxic conditions protract their tracheal gills and begin ventilatory movements (Kinnamon et ai, 1984). A correlation between oxygen content of streams where aquatic larvae are found and gill surface area was found in mayflies and caseless caddisflies (Dodds and Hisaw, 1924). After experiments on mayfly tolerance to low oxygen content of the water, McColl (1943) stated that in time of emergency, tracheal gills might be the single most important factor to determine life or death. Lee (1929) stated that even though little was known about the respiratory physiology of the Megaloptera, "It is inferred that gas exchange takes place chiefly through these structures" (119). Kinnaman et al. (1984) stated that although "movement of tracheal gills facilitates gas exchange across gills and integument ... no data are available on the sites of gas exchange in hellgrammites, nor have effects of gill movements on the rate of oxygen flux into the animal been measured" (26). This study was conducted to provide some details of the morphology of tracheal gills in the hellgrammite to contribute to the discussion of respiration in these well-known, but less well-studied, animals.