Anisoptera: Gomphidae)
Total Page：16
File Type：pdf, Size：1020Kb
Odonalologica II (I): 1-14 March I. 1982 Factors affecting microdistributionof two species of burrowingdragonfly larvae, with notes on their biology (Anisoptera: Gomphidae) D.G. Huggins andM.D. DuBois State Biological Survey of Kansas, 2045 Avenue A, Campus West, Lawrence, Kansas 66044, United States Received May 21, 1981 / Accepted July 16, 1981 and Progomphus obscurus Gomphus externus larvae were studied in both laboratory and field during July, 1976, regarding their burrowing habits and their burrow beneath association with substrates. They were found to completely bottom substrates by using their front and middle legs, never deeper than 2 cm; small larvae burrowed less deeply. — Field studies revealed that both spp, and either occurred in the stream in relatively high numbers were distributed in a both and contagious or random manner. Populationstructure of spp. was similar consisted mainly of late instars. Statistical analysis of physical factors measured (substrate particle size, digestedorganics and detritus) and the occurrenceof each and each sp. demonstrated a correlation between particle size occurrenceof sp. P. obscurus lends to inhabit microhabitats in stream where sand substrate (1-0.625 mm) predominates. Microdistribution of G. externusis most strongly correlated with the amount of silt/clay substrate present (less than or equal to 0.0625 mm), of also environmental but percentage organics was significant. Although other factors may have had an effect on the distribution of these 2 spp., the primary factor was substrate particle size. INTRODUCTION This investigation attempts to determine some physical factors affecting microdistribution of Progomphus obscurus (Rambur) and Gomphus behavior of externus (Hagen). Burrowing and population structure these species are also examined. Most factors affecting distributionof freshwater aquatic organisms which burrow in fine substrates are poorly known (LYMAN, 1943, 1956; ERIKSEN. 1964; GALE, 1971, 1973; MARZOLF, 1965). LYMAN (1943) 2 D.G. Huggins & M B. DuBois showed that substrate composition was of great importance in the distribution of of burrowing mayfly nymphs the genus Hexagenia. Laboratory substrate preference experiments by GALE (1971) showed that the fingernail clam, Musculium transversum, selected mud substrates most frequently and sand least frequently. Adults of another species of fingernail clam (,Sphaerium striatinum) showed no clear preference for mud, sand, or sandy mud, although small clams exhibited preference for mud over sand or sandy mud (GALE, 1973). Little is known about the burrowing behaviorand factors affecting habitat selection by burrowing larval Odonata. Information about habitat selection by gomphid larvae is mostly restricted to observations which lack quantifiable data (RICH, 1917; HARRISON, 1951; WRIGHT, 1943; indicated MILLER, 1964). CORBET (1963) odonate larvae may select the of sediment ”to which type they are adapted” using substrate particle size as a but little documentation basis, provides to support many observations. Only KEETCH & MORAN’s (1966) work with the South African sand- burrowing dragonfly, Paragomphus cognatus (Rambur), provided quantita- tive data on the relationship between substrate particle size and habitat selection. of They showed that larvae P. cognatus were able to distinguish substrates of differing particle size. In laboratory experiments they demonstrated that P. cognatus generally burrowed in coarse sands (approximately 0.50-0.21 mm) and avoided fine sands (< 0.15 mm), but their findings were unsubstantiated by field data, which showed no significant difference between particle size profiles from stream areas in which larvae in which were present and those they were absent. They suggest that initial choice of habitat is governed by particle size but other factors might leadto a patchy distribution within this habitat. It has long been suggested that Progomphus larvae prefer sandy bottom streams (BYERS, 1930), sandy beds, or sandy reaches within various lotic or lentic habitats (BYERS, 1939; NEEDHAM, 1941; NEEDHAM & WESTFALL, 1955). Needham & Westfall briefly discussed morphological adaptations of P. obscurus larvae which allow- them to live in shifting sand habitats and account for their great burrowing abilities. No quantitative American of studies are available on of the North species Gomphus but any , casual field observations suggest that most species prefer habitats where fine sediments prevail. CORBET (1963) suggested that burrowing dragonfly larvae with dorso-ventrally flattened abdomens typically inhabit finer sediments since the shape of the abdomen helps prevent passive sinking. G. externus possess this morphological adaptation and others common to Corbet’s "shallow borrowers”. Corbet further characterized gomphids, in is normally living coarse sediments, as having an elongate abdomen (which concave ventrally and strongly convex dorsally) and short, robust legs. These Microdistribution of burrowing gomphid larvae 3 larvae, which live in sand, are usually smoothand pale in color. P. obscurus possess all characteristics of coarse sediment dwellers. STUDY AREA AND METHODS STUDY AREA Mud second-order km north in Creek is a stream originating 22.5 of Lawrence, Kansas Jefferson County. Itflows southward to the Kansas River flood plain where it turns and flows in a southeasterly direction until it enters the Kansas River in northeastern Douglas County, Kansas. Before its lower reaches were channelized, Mud Creek was 32.8 km long with a It differential elevation of 121.6 m. is normally a slow-moving, warm water stream. The amount the of flow varies seasonally with water becoming very turbid during high water stages. HUGGINS & MOSS (1974) described this stream and its basin along with ananalysis of the fish population structure and a listing of the more common aquatic macroinvertebrates. The present study was conducted in a 100 m section of M ud Creek adjacentto the Kaw Valley Fish Farm 1 km north of Lawrence Municipal Airport. This section of Mud Creek is characterized small riffle and small number ofshort, Over 80 of by a single a deeppools. percent reaches this section consists of long of normally shallow stream runs (characterized by a usually These and unbroken flow pattern). runs include areas oferosion depositionwhich provideareas ofvarying velocity and substrate composition. Substrates in stream runs are various mixtures of small gravel, sands and silt/clay with quiescent areas containing various amounts of detritus. BIOLOGICAL METHODS Field and laboratory observations were made on burrowing activities and patterns of P. obscurus and G. externus prior to initiation of quantitative study of their microdistribution. Larvae were collected from one area ofthe stream and released for observation in an undisturbed the measured area of the stream visually similar to collection site. Burrowing depth was in laboratory and field. Laboratory measurements were made by placing various size groups in 20.5 cm x 8.25 cm specimen dishes (finger bowls) filled with 4 cm of substrate and 7 cm of stream Size consisted of sizes: water. groups were based on total length and the following G. externus: medium, 16-18 mm; large. 25-28 mm; and P. obscurus: medium, 16-20 mm; large 23-26 mm. Two of five of each introduced into each dish and groups, consisting specimens species were wooden dowel measurements were made after an hour by probing with a calibrated 2 mm in larva detectable diameter. Striking a buried generally resulted in a movement which was checked the by excavating insect. Field measurements were made with a similar wooden dowel in several shallow sandy reaches of the stream which allowed for observation of trails left in the smooth sand the P. surface by burrowing activity of obscurus. No G. externus trails were visible. Gut of 17 medium- and larvae each contents large-sized of species were examined by removing crop, proventriculus. and ventriculus of preserved larvae and identifyingtheir contents using dissecting and compound microscopes. Quantitative field samples were taken during the first two weeks of July. 1976. Climatic conditions characterized during the study period were stable; by sunny skies, warm days (mean A air temperature 27°C) and noprecipitation. stovepipe sampler similar to the collecting cylinder substrate described by CUMMINS (1961), which retained both materials and organisms, was used to obtain all samples. The sampler was constructed from galvanized stovepipe 55 cm long and 22.2 in diameter with handles attached the of the his cm two near top pipe. T sampler enclosed 2 The down 0.039 m . sampler was brought rapidly at arm’s lengthin front of the collector and 4 D.G. Huggins & M.B. DuBois 10 the bottom substrates forced approximately to 20 cm into at randomly selected points. A of the 2 of subsample top 2.5 cm substrate was removed from the sampler, placed in a small, white checked for then returned and plastic tray, organisms, to the laboratory in a labeled plastic bag dried in an oven at 30°C for 24 hr. Substrate samples were then sealed and stored for later The the with analysis. rest of stovepipe sample was sieved a strainer (approx. I mm mesh). All odonates in were collected and placed vials containing 80 percent ethanol. The larvae were returned to the laboratory where they were enumerated and identified to Table I species. The two most abundant Substrate particle size terminology and categories species, P. obscurus and G. for finer particle sizes externus, were classified by instar width using head measurements WENTWORTH (1922) since this is of certain one body This paper CUMMINS Name Panicle size measurements which serve as (1962) range (mm) reliable indicators of age (or In the instar) in many species. 32-64 maximum head present study Pebble Pebble Pebble 16-32 width was plotted against fre- 8-16 of both quency occurrence for 4-8 species. Exuviae collections were made in order to identify the final Gravel Gravel Granule 2-4 instar stage. Instars were design- ated in Coarse sand numerically a manner Very coarse Very coarse similar to LUTZ (1968): final sand sand 1-2 = (F=0), penultimate (F-I l), an- Coarse sand Coarse sand 0.5-1 tepenultimate (F-2=2), etc.