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Am.Midl. Nat. 127:348-354 Oviposition of the Dobsonfly(Corydalus cornutus, Megaloptera) on a Large River BRIAN P. MANGAN EcologyIII, Inc., R.R. 1, Berwick,Pennsylvania 18603 ABSTRACT.-Dobsonflyoviposition and theresults of twoannual surveys of oviposition sitesalong the Susquehanna River, near Berwick, Pennsylvania, in 1985-1986are described. Unlikeother megalopterans, none of the females prepared the substrate before egg deposition. Afterdeposition, females spread a clearfluid over the eggs,which dried to forma hard whitecoating. This coating appeared to protect the eggs from excessive heating and predation. Some4428 eggmasses were observed at 260 sitesin thetwo annual surveys. Trees were themost often used sites, but rocks and deadfallshad higheraverage numbers of egg masses on them.Aggregations of egg masseswere observed in surveyedareas and on individual sites(e.g., 339 on one tree),and Morisita'sDispersion Index showed that egg masses were contagiousin distribution. INTRODUCTION The dobsonflyCorydalus cornutus (L.) is a widely distributedaquatic insectand an im- portantpredator in many lotic ecosystems.Larvae (hellgrammites)are prized as bait for game fishes.Although the insect is well known among anglers,it has receivedrelatively littlescientific attention. Descriptions of the life history,and particularlyoviposition, are sparse and incomplete.Riley (1879) was firstto characterizeoviposition, noting egg masses on objectsoverhanging water. Parfin (1952) studiedpupation, adult emergenceand copu- lationof the insect,and Baker and Neunzig (1968) describedthe egg masses and first-instar larvae. Brown and Fitzpatrick(1978) reportedlife history and populationenergetics of this species. This study investigateddobsonfly oviposition on the Susquehanna River, near Berwick,Pennsylvania, in 1985-1986. My objectiveswere to describedobsonfly oviposition and surveyoviposition sites along a large river. STUDY AREA The studywas conductedalong a 6.4-kmsection of the Susquehanna River,approximately 11 km N of Berwick,Luzerne County,Pennsylvania. In the past,water quality and benthic macroinvertebrateswere depressedby coal mine drainage which enteredupriver from the studyarea (Deutsch, 1981). However, water quality has improvedsince 1976 (Soya et al., 1984) and benthicbiomass has increasedas much as fivefold(Zelenak et al., 1985). Mean riverwidth in this area was ca. 300 m and ranged from170-620 m. The river'sslope was about 0.3 m/km,and consequently,few natural rifflesoccurred in this area. Overhanging trees characterizedthe shoreline.Silver maple (Acer saccharinum)and riverbirch (Betula nigra) predominated;other species presentincluded sycamore(Platanus oxidentalis),red maple (Acer rubrum),weeping willow (Salix babylonica)and white oak (Quercus alba). Deadfalls and rocks (both on and offshore) were located intermittentlythrough the area. METHODS AND MATERIALS During eveningsof June, July and August 1985, between 1800-2400 h, searcheswere made froma motorboatalong the rivershoreline for ovipositing females. A high intensity light was used for illumination.When ovipositingfemales were found,information con- cerningsubstrate preparation, egg depositionand postovipositionactivity was recorded. 348 1992 MANGAN:DOBSONFLY OVIPISITION 349 In Augustand September1985 and Septemberthrough November 1986, six 1.6-kmareas of rivershoreline were surveyedfor oviposition sites. These areas were randomlyselected fromtwelve 1.6-kmareas available fromthis sectionof river.The surveyswere conducted duringdaylight hours and the same six areas were surveyedboth years. Sites were observed with fieldglasses froma motorboator fromshore. An ovipositionsite was definedas any objectwith an egg mass laid on it. Substratetype, numberof egg masses presentand theirplacement relative to the water were recorded.In 1986, heightsof individual egg masses above the water were measured using a 7.6-m surveyingrod (standardizedto the riverlevel measured on 30 September 1986). Also in 1986, an attemptwas made to quantify"potential" ovipositionsites in each surveyarea. These potentialsites were the numbersof trees,rocks and deadfallsin the surveyareas and areas withoutegg masses thatsatisfied the criterionof locationnear the water. The purpose of this was to investigatea possible relationshipbetween observednumbers of oviposition sites and relativesite availability. Egg mass dispersionpatterns were testedstatistically for departure from randomness by chi-square analysis,and were furtherevaluated using Morisita's Index of Dispersion: Id= n(2 x2- N)/N(N - 1) wheren is the numberof plots (actual and potentialoviposition sites), N is the totalnumber of individualscounted on all n plots,and sigma x2 is the square ofthe numberof individuals per plot summedover all plots (Id = 1.0 if dispersionis random,Id = 0 if perfectlyuniform, Id = n ifmaximally aggregated; Brower and Zar, 1984). The numbersof potential oviposition sites countedin 1986 were also used with 1985 surveydata so that the dispersionindex could be applied to thatyear. Change in potentialsites between these years was considered minimal. RESULTS AND DISCUSSION Ovipositionbehavior.-Oviposition began in June and lasted throughSeptember in 1985 and 1986. The egg masses were composedof threelayers of eggs similarto the description given by Baker and Neunzig (1968). Average numberof ova per mass was 920 (range = 561-1491; SD = 268; n = 12), comparedto reportsof 3000 per mass in Missouri (Riley, 1879) and averages of 1080 in North Carolina (Baker and Neunzig, 1968) and 1309 in Texas (Brown and Fitzpatrick,1978). The average dimensionsof 68 egg masses laid on horizontaland diagonal surfacesof a boat dock in the riverwas 18.4 mm long and 15 mm wide, very similar to those reportedfrom Texas (Brown and Fitzpatrick,1978). The predominantshape was oval and all masses were white. Ovipositionbehavior of eightfemales was observedin the field.All ovipositionsoccurred aftersundown. None of the femalesprepared the substratebefore egg deposition.As in- dividual eggs exited the abdomen,they were coated with a clear fluidwhich had adhesive properties.This adhesion assistedegg attachmentto the substrateand to othereggs in the mass. In contrastto Corydalus,another megalopteran, an alderfly,Sialis cornuta,was reported by Pritchardand Leischner (1973) to secretea stickysubstance from its abdomen onto the substratebefore egg deposition. The threelayers of eggs were depositedconcurrently; the bottomlayer containedthe most eggs, and the second and thirdlayers subsequentlyfewer. Eggs in the bottomlayer were laid parallel to the substrate,while those in the upper layers were angled away fromthe substrate(some almost perpendicular).Females stopped oftenduring depositionto probe the incompletemasses with the tips of theirabdomens, suggesting that a tactilestimulation 350 THE AMERICAN MIDLAND NATURALIST 127(2) is involvedin egg placement.The head and thorax remained motionlessas the abdomen was moved laterally to deposit eggs in rows. Egg depositionrequired about an hour to complete. When egg depositionwas completed,a clear fluidwas secretedfrom the abdomen and spreadover the eggs. The femalesbrushed the fluidover the eggswith the tip ofthe abdomen in a lateral motion,often extending the fluidbeyond the eggs and onto the substrate.This fluiddried to forma hard whitecoating on the egg mass. This is probablythe same material that coated individualeggs duringdeposition. I moved two femalesfrom their egg masses beforeapplication of the egg coating. Seeminglydisoriented, they moved around on the substratefor ca. 10 min,then applied the egg coatingdirectly to the substrate.Subsequently, the eggs of both uncoveredegg masses turnedbrown when exposed to directsunlight the followingday and the eggs did not hatch. Other egg masses laid on this same substrateand near these masses hatched successfully.This suggestedthe abilityof the white coatingto protectthe eggs fromexcessive heating, as proposed by Brown and Fitzpatrick(1978). Other observationssuggested that the coatingprotected the eggs frompredators as well. I foundonly fouregg masses that appeared damaged by predators,as indicatedby a hole in the protectivecoat. Eggs in the area of the hole were partiallyor wholly removed.The remainingundamaged eggs of these masses hatchedsuccessfully. The hard surfaceformed by this coatingis probablyimpenetrable to most potentialinsect predators. A Californian dobsonfly,Neohermes aridus (Maddux), lays eggs withoutprotective coats (Maddux, 1952), as do Sialis alderflies,and both were reportedto have large percentagesof theiregg masses parasitized(Azam and Anderson,1969; Pritchardand Leischner,1973). Brown and Fitz- patrick (1978) reportedthat only three Corydalusegg masses were attacked,of several thousandobserved in theirstudy. The females I observeddeposited only one egg mass beforemoving off the substrate. Brown and Fitzpatrick(1978) reportedthat Corydalus in Texas usually laid two egg masses each and sometimesa thirdsmaller one, and calculated an average of 2.3 egg masses per femalefrom numbers of ova per femaleprior to ovipositing.I collected10 femalesby light trap and counteda mean of 1293 ova, with a range of 633 to 1635 ova per female (SD = 298). I do notknow whether these females oviposited before collection. However, considering the range of ova per egg mass noted earlier,these femalescould lay one large egg mass or multiplesmaller ones. Ovipositionsites.-River surveysrevealed 3204 egg masses laid on 149 sites in 1985 and 1224 masses on 111 sites in 1986 (Table 1).

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