the larvae of many holometabolic insects. In the larvae of social wasps there is a change in the function of the glands from trophallaxis and digestion to cocoon ...
JOURNAL OF NATURAL HISTORY,
1990, 24, 1311- 1319
Chemical composition of the nest walls, and nesting behaviour, of Ropalidia (lcarielia) opifex van der Vecht, 1962 (Hymenoptera: Vespidae), a Southeast Asian social wasp with translucent nests. U. MASCHWITZ, W. H. 0. DO ROW and T. BOTZ
Zoologisches Institul der Johann Wo(fgang Goethe- Universitiit, Siesmayerstra}Je 70, D-6000 Frankfurt am Main I, FR Germany. (Accepted 12 February 1990) Ropalidia opifex nests between leaves of trees, and builds cellophane-like nest walls of pure secretion. Nest structure, nest building behaviour, and the properties and chemical composition of the secretion are reported. The secretion mainly consists of protein. No chitin was found . The selective advantages of the leaf region niche for nesting, and of the unusual type of nest, are discussed. Ropalidia ( Icarielia) opifex van der Vecht, 1962, Hymenoptera, Vespidae, nest, carton glue, labial gland, Southeast Asia, mimesis.
KEYWORDS:
Introduction Most social wasps build their nests with carton, which usually consists of fine woody material glued together with a secretion probably produced in the labial gland of the workers. The chemical composition of the secretion is difficult to investiga te, for it is mixed with other organic material in most nests. The use of pure secretion for nest building is a very rare phenomenon, only known from polistine species of the genera Pseudochartergus and Ropalidia (Jeanne, 1970). Schremmer et al. ( 1985) investigated the chemical composition of the pure secretion nest wall of the South America n species Pseudoclzartergus chartergoides (Gribodo, 189 1) and found that it consists mainly of chitin. The Southeast Asian wasp species R opalidia opffex also builds large nest walls solely produced with secretions. Vecht ( 1962) gave four photographs of the nest and a description, both of Pagden, Richards ( 1978) gave a key to the species of the subgenus Icarielia. We observed one of the highly cryptic colo nies of this species fo r several weeks and report here on the chemical composition of the secretion and on the nest building behaviour of R. opifex. Materials and methods The colony studied was found at Ulu Gombak (250 m NN) 30 km northeast of K uala Lumpur, West Malaysia. It was situated between leaves of a broad-leaved tree. The remains of two empt y secretion nests were also found in similar positions between leaves of ano ther broad-leaved tree, and o n a bamboo plant in the Gombak valley. Observations in the field were carried o ut during a 3-week period in 1988. Only pure nest wall material was used for chemical ana lysis. A preliminary chemical ana lysis o f the nest wall o f R. op({ex was carried out with Ponceau reagents. Quantitative determination of the to ta l protein co ntent was performed by employing the BCA-protein assay reagent (Pierce Chemical Company, R ockford, IL, 0022 · 2933/90 $5.()() ~ ? 1990 Taylor & Francis Ltd.
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USA) and alternatively by the Lowry method (Lowry et al., 1951) with bovine serum albumin as a standard. Hydrolysis of the samples was carried out with HCl (6 M) for 30 m in at I 00 ·c. The hydrolysate was further analysed for free amino aci ds by reversed-phase highperformance liquid chromatography (HPLC) using a Beckmann instrument (model 342) after pre-column derivatization with ophthaldialdehyde. The analytical procedure was carried out as described by Dohler ( 1985). Reducing sugars were analysed chromatographically on HPLC after derivatation with dansylhydrazine. Parameters for the hydrolysis of the sample material (ea. 200 !-lg) were adjusted carefully so as to give both complete hydrolysation and minimal monomer breakdown. T his was achieved by treating 200 !-lg portions of the sample material with H 2 So4 (6 M) for 4.5 h at 70 ·-c. Aliquots were titrated to pH 7 with BaOH , the solid was pelleted, and the supernatant was reduced under vacuum. Derivatisation procedures were adopted from Finden et al. ( 1985). HPLC conditions were essentially as described by Takeda et al. ( 1982). Instruments were from Biotronik , Maintal, FRG. The column was a Nucleosil 50- 7, 250 x 4 mm from Macherey-Nagel, Diiren, FRG . Possible decline of sugar monomers caused by the acid treatment was monitored wit h galactose as an internal standard . The detection limit was 20 pmol for glucose and 30 pmol for N-acetylglucosamine per injection, with a signal-to-noise ratio ofabout 10. Chitin was analysed as follows: to avoid breakdown of the acetyl residue the determination of N-acetyl glucosamine was performed after enzymatic hydrolysation of the sample matrix with protease (type Sigma PO 03849 in Tris (50 mM, pH 7·5). Subsequently chitinase (Sigma C-1650) was used to give free N-acetyl glucosamine. Monomers were analysed with the Elson-Morgan reagent according to the me thod of Reissig et al. ( 1955) modified as described by Botz et al. ( 1988) and with the HPLC method, described for reducing sugars. To check for the presence of fibre diffraction the standard crystallographic equipment and methods were used. Densely packed layers of the sample material were mounted in X-ray capillaries. Screen less precession photographs were taken at various orientations of the layers with a Huber 206 precision oscillation camera equipped with a colimator of0·5 mm diameter and 180 mm length. The so urce of the Cu Ka radiation was a Ringaku RU-200 rotation anode X-ray generator operated at 40kV and 160 mA. Double-coated high-sensitivity X-ray film CEA reflex 25 was employed, and exposure times reached 20 h. Results
Nest structure We found the studied colony at a height of 2 m on a broad-leaved tree, nesting at the tip of a branch between three living leaves of approximately 12 cm x 5 cm x 4 cm in size (Figs I and 2): it contained about 100 workers. Because of its translucent colourless nest wall it was barely visible during daytime . At night, however, the cellophane-like nest wall glistened in the light of a torch and so was found by chance. The nest chamber measured 10·5cm x 6·2cm x 1·7cm. All gaps between the leaves- up to 1·5cm widewere closed by 5- 10 J.lm thick nest walls of pure secretion, only one semicircular entrance was left open (Figs I and 2). The leaves themselves were not lined with the material. The onl y oval comb (4· 5 cm x I· 5 cm x 0·5 cm), which consisted of a grey carton material, was fixed with a few petioles on the lower side of the nest's top leaf (Fig. 2). The petioles also consisted of carton, but were covered and glued to the leaf
Nest walls and nesting behavio ur of R opalidia op!f'ex
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Nest o f Ropalidia op((ex a t the tip of a bra nch between three living leaves (length of worker approximately 7 mm).
surface with pure secretion. As shown by scanning electron microscopy the carton did not consist of plant fibres but of parenchymatic plant tiss ues glued together by the secretion. The comb contained 180 cells; of these, 30 at the periphery were built of carton, wh ile the bottoms of most of the remaining cells consisted of translucent secretion. Such tra nsparent cell bottoms are known from several Old World polistine genera. Adult wasps chew openings in the bottoms of the brood cells, remove the meconium-conta ining peritrophic sac fro m guts of the post-feeding larvae and then close the hole in the cell bottom with a translucent oral secreti o n (Kojima and Jeanne, 1986). In the empty nes t found on another broad-leaved tree three combs were present on different leaves. but the architect ure of the nest could no lo nger be recognized. Nest building helzaviour For evaluating the nest building behaviour, pieces of the nest wall near the entrance were removed. At this disturbance m any wasps left the entrance and assembled at the nest surface (Fig. I). Some of them took flight and a ttacked and stung the observer. When the colony was calm again, some workers began to repair the nest wa ll. They started at the leaf surface where the broken-off part of the nest wall had been fixed before. Then they moved forward, parallel to the border of the remainin g nest wall (Fig. 3). The edge of this bo rder was tak en between the mandibles and sm all amounts of a fluid appea red between the moving mouth parts. By up-and-down movements of
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Ftc;. 2. Nest of Ropalidia opilex ··- schematic drawing of the same nest as in Fig. 1. D ouble arrow indicates nest en trance. D otted area: sal ivury wall. Inset shows transverse section th rough the nest a long the marked line. Numbers 1- 3 refer to the leaves involved in nest const ruction. Drawing by Margot Kre uder.
the head, while the wasp slowly went forward, a narrow strip of fres h material was attached to the o ld nest wa ll with the mandibles. Marking with colo ured dots showed that the wall was enlarged 0·5 mm per day. After20h the new material had not yet fully hardened. When the wasps had finished repairing the nest, we again broke off a piece of the wall. Within half a day we observed four building wo rkers, which we removed from the nest. After this treatment nest building stopped totally for 3 d ays, then a few workers again started to repair the nest wall.
Properties and chemical composition (){the secretion The thin. smooth, colourless and translucent nest wall was similar in appearance to a cellophane sheet. Its surface appeared smooth in scanning electron microscopy at 2000-fold magnificatio n (Fig. 4). In the humid air of the rainforest it was stable and clastic, bu t when dried it became brittle. The nest wa ll material did not dissolve in water, ethanol, diet hylether, or xylol wi thi n 5 days at room temperature. Qualitative protein analysis with Ponceau reagents was s trongly positi ve for t he R. op({ex ma terial as well as for fresh carton material from the European Polistes dominulus (Christ, 179 1), wh ich was assayed as a con trol. Quantitative protein analysis of the secretion with two independent assays revealed 88.6% wfw (SEM (standard error of the mean )
