Anisoptera: Gomphidae, Libellulidae)
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Odonatologica38(2): 167-172 June I, 2009 Overwintering dragonflies in an African savanna (Anisoptera: Gomphidae, Libellulidae) P. Van Huyssteen and M.J. Samways Department of Conservation Ecology and Entomology, Faculty of AgriSciences, University of Stellenbosch,Private Box X1, Matieland-7602,South Africa [email protected] Received January29, 2009 / Reviewed and Accepted February 16, 2009 Tobetter understand overwintering capability of dragonfliesin the African savan- observed individuals na, were placed into predetermined age categories at sites along the Mogalakwena river, Limpopo province, South Africa, during mid-winter. Age categories were determined by degree of wing wear each individual had sustained. The Biotic Index used to Dragonfly (DBI) was categorize spp. into rare, widespread generalists versus specialists. All the recorded rare, narrow-range spp. were common, microhabitats created the winter widespread generalists,occupying by dry season de- crease in water level and flow rate, and able to survive seasonal habitat changes. Sev- of the 1 en 8 spp. were libellulids,and gomphid. Their ability to thermoregulate by in addition to their habitat selecting appropriate perch sites, high tolerance, plays an role to important allowing them survive as adults throughout winter. It is confirmed that the libellulids observed here were highly habitat tolerant, common and wide- whose about least spread spp. success comes at partly from their ability to overwinter and be ready to take advantage of the first rains. INTRODUCTION is Thermoregulation an essential survival mechanism for many insects. They have to their regulate body temperatures between certain ranges, independently of the ambienttemperature, to sustain essential activities and reproductive fitness (CORBET, be 1999). Thermoregulation can achieved in two ways: ecothermically and endothermically, by either involving changes in metabolic heat production and/or heat exchange with the environment(COR BET, 1999). Thermoregulation allows insects to in of occur a variety environments, filling many microhabitats. However, a challenge for insects is to be able to survive the cold winter. Many insects do this by being in an immature state or going into diapause, while some 168 P. van Huyssteen & M.J. Samways thermoregulate as adults, using physical and/ or behavioural adaptations. Exam- include winter moths ples such as Eupsilia and Lithophane species which use an extensive ‘fur’ coat covering their thorax for heatretention(VERDU et ah, 2004). As dragonflies do not have thoracic ‘fur’, they use other mechanisms for increas- describedfor ing body temperature. Shivering or ‘vesper warming’ (a technique and their muscles activities which butterflies) perching to warm up flight are two dragonflies employ to increase body temperature before flight or before finding for cover the night (CLENCH, 1966; CORBET, 1999). Endothermic warming is assumed to be the most derived condition in dragonflies and is mostly among larger species (MARDEN et ah, 1996). Little information is available which on dragonfly species overwinter in the African where hot savanna, summers are generally and wet and winters dry and cold. We studied here the phenology of dragonflies in the savanna of SouthAfrica and ascertained their determinewhich ages to species were surviving through as adults and whether there also be eclosion to adulthood the win- may any during The and of the ter. age composition overwintering species were studied to create a platform from which furtherstudies can evolve. SITES AND METHODS AREA - STUDY A 3.54 km stretch of the Mogalakwena river in the northern Soutpansberg re- gion of the Limpopo province, was the studyarea (22°43’S, 28°46’E). The river flows through- of the the river decreases in and forms out most year. During winter, substantially size, many pools and slow flowing streams which connect the pools. - STUDY SITES Six study sites of 50 x 25 m were chosen for their accessibility, safety (from and most of crocodiles) importantly, occurrence dragonflies(Fig. 1). Samplingwas between 19 June and 7 July 2008. Within each study site, there wereboth largebodies of water and smaller pools, scat- tered with rocks and sand banks, in natural savanna. SAMPLING METHODS — Dragonfly species were observed and sampled between 11:00 h and 15:00 h lasted for 1.5 h each site. on windless, sunny days. Recording at sample Voucher specimens retained for Two observation methods used determine and were as age referencing. were to species of each individual. Individuals identified and close focus binoculars and age were aged using a guide (SAMWAYS, 2008). Photographs were also taken to confirm age of individuals. Individuals were viz. indicated 2: indi- aged according towing wear, 1; whole, perfect wingmargins young individuals, viduals with of 1 -5 mm wing wear were classified asmiddle aged (see Fig. 2), and 3; those with more than 5 mm of wing wear were classified asold (Fig. 3). Each species was also allocated a Dragonfly Biotic Index (DBI) value, which is a composite measure of geographical distribution,Red List status and sensitivity to changing environmental conditions (SAMWAYS, 2008; SIMAIKA & SAMWAYS, DBI 2008). The ranges from 0 to 9, with a low value (0-3) indicating a geographically-widespread, non-threatened, habitat-tolerant species. RESULTS In total, 142 individuals in eight species were sampled (Tab. I). All were ani- sopterans, and only one species was not in the Libellulidae.All had a low Drag- Overwintering dragonfliesin African savanna 169 onfly Biotic Index (DB1), rang- ing between 0 and 3, indicating widespread generalists. The most abundantspecies was Orthetrum chrysostigma, making up 28% of all individualsobserved. O. chrys- has of 2 ostigma a DBI (Tab. I). of observed indi- Average age viduals of this species, based on wing damage, was category 2. Ic- tinogomphus ferox and Trithemis annulata were the least abundant species and only two individuals of each species were observed. each of these For an average of of 1.5 de- age category was termined. I. ferox has a DBI of 2 and T. annulata 1. The species with the highest DBI was Croco- themissanguinolenta, with a DBI of and of 3 average age category 1.5 (Tab. I). This species was not Fig. 1. The study area showing the Mogalakwena river at all sites. T. arteriosa and C. and the matrix of dry savanna. erythraea had the oldest average ancf age, 2.3 and 2 respectively. Both species have a DBI of 0 (Tab. I). T.furva T. both kirbyi have a DBI of 0 and average age category of 1.6 and 1.8 respective- ly. individuals (I.Overall, average age of ranged from 1.5 to 2.3. Four species fer- O. C. and ox, chrysostigma, sanguinolenta T. annulata) had an average age of 1.5. These species also had DBI values above 0 (2, 2, 3, and 1 respectively) (Tab. I). Virtually all individuals were males, but both femaleand male individuals of O. chrysostigma, C. erythraea and T. kirbyi were at certain sites, and engaged in courtship. T. kirbyi, T. furva, O. chrysostigma and C. erythraea males were highly territorialamong themselves and witheach other. DISCUSSION For anadult dragonfly to maintain vital activities, it must maintainbody temper- ature withinaspecific range, independent of ambientair temperature(CORBET, 1999; MAY, 1978). MAY (1976) found that the minimum temperaturea dragonfly must maintainto sustain flight in a laboratory experiment was 21.3 ± 1.6°C. In order achieve this of to temperature, independent the ambient temperature, drag- P. & M.J. Sam 170 van Huyssteen ways 2-3. Figs Examplesof dragonfly individuals of different age: (2)a middle-agedOrthetrum chrysostigma male (agecategory 2), with some wearin the anal area ofthe hindwing; — (3)an old Trithemis arteriosa male (age category 3), with extensive wing wear along the margins of both the fore- and hindwings. Overwintering dragonfliesin African savanna 171 onflies mustthermoregulate Table I Abundance, and DBI of observed (MAY, 1976). Previous stud- average age species ies have shown that adult No. dragonflies can be divided SpeciesSpecies individuals DBI AverageAverage age into two general groups, observed perchers (which regulate their body ectothermically) IctinogomphusIctinogomphusferox 2 1.5 2 Orthelrum Orthetrum chrysosligmachrysostigma 35 1.5 2 and fliers (which regulate en- Crocothemis erythraea 2222 2.0 0 dothermically) (CORBET, Crocothemis sanguinolenta 20 1.5 3 1999). TrithemisTrilhemis annulataamulata 2 1.5 1 Out of the eight species Trithemis arteriosa 1717 2.3 0 Trithemis furva 2424 1.6 0 here, only one, the gomphid Trithemis kirbyi 2020 1.8 0 I.ferox, was not in the Libel- lulidae. In general, individualsin the Libellulidae do not endothermically gener- heat ate by shivering. In addition, they cannot increase the rate of heat loss by blood circulation the because of the increasing through abdomen, high energy is small-bodied cost which incurred by dragonflies when they attempt to regulate their body temperature endothermically (HEINRICH & CASEY, 1978; McGE- OCH & SAMWAYS, 1991). Individualshere dam mostly basked on the ground, rocks, walls, or otherperch- and made es, short trips between perching sites, and longer, territorialand foraging raise lower flights, returning to perch to ectothermically or body temperatures. able to utilise the the it By being sunny winter days (although nights are cold), is possible for certain Libellulidae to occur in this savanna all species region year round. Winter in these is characterized savanna areas by dry atmospheric con- ditions, very little rain, dropping water levels, and change in rapid-flowing river conditions into and in a slow stream, pools places even a dry river bed, with a concurrent decrease in insect abundance and species diversity (SUH & SAM- WAYS, 2000). able However, by overwintering as adults, species are to complete at least two in This allows them have univoltine life generations one year. to or multivoltine cycles (CORBET, 1999). As the savanna here is a sub-tropical, regulated ecosys- the have tem, dragonflies are likely to an A.2.1.2. or A.2.2 life cycle (CORBET, where the winter months 1999), species spend dry as pre-reproductive adults or as eggs.