Ann. soc. entomol. Fr. (n.s.), 2006, 42 (1) : 99-105 ARTICLE

Morphological study of the Stridulatory Organ in two species of genus: Crematogaster scutellaris (Olivier 1792) and Crematogaster auberti (Emery 1869) (: Formicidae)

Eduardo Ruiz, M. Hoz Martínez, M. Dolores Martínez & J. María Hernández Dpto. de Zoología y Antropología Física, Facultad de Biología, Universidad Complutense de Madrid, 28040-Madrid, Spain

Abstract. The stridulatory organ of the Crematogaster scutellaris (Olivier 1792) workers is being described, comparing their pars stridens present in six nests of this species, with one nest of Crematogaster auberti Emery 1869 and with the bibliographical data regarding other neighbouring species at our disposal. Both species and some Crematogaster scutellaris nests have shown significant differences. We propose several hypotheses which could explain these differences. Résumé. Etude morphologique de l’organe stridulatoire chez deux espèces du genre Crematogaster : Crematogaster scutellaris (Olivier 1792) et Crematogaster auberti (Emery 1869) (Hymenoptera : Formicidae). L’organe de stridulation des ouvrières de Crematogaster scutellaris (Olivier 1792) est décrit. On compare les pars stridens issus de 6 nids de cette espèce avec ceux d’un nid de Crematogaster auberti Emery 1869 et avec les données de la littérature sur les espèces voisines disponibles. Les deux espèces et certains nids de Crematogaster scutellaris ont montré des différences significatives. Nous proposons plusieurs hypothèses pour expliquer ces différences. Keywords: Acoustic communication, stridulation, stridulatory organ, Crematogaster scutellaris, Crematogaster auberti, Iberian Peninsula.

nt social organization is based on very effective C. scutellaris (Olivier 1792), C. auberti Emery 1869, Asystems of communication (Grasso et al. 1998). C. laestrygon Emery 1869 and C. sordidula (Nylander Like most social , spend most of their time 1849). in the darkness of their nests, for this reason, vision Crematogaster scutellaris has a wide allocation in has a limited use in communication. Pheromones the Iberian Peninsula and a clearly Mediterranean and sounds are mainly used to exchange information distribution. It nests regularly in different trees (Soulié (Kirchner 1997). 1956); in the Iberian Peninsula it has preference for Acoustic communication plays a very important Quercus ilex and Quercus suber, although they can use role in several behaviours such as alarm, recruitment, other trees or big bushes (Casevitz- Weulersse 1972; mating ending and modulation of other signals Martínez et al. 1996). Crematogaster auberti generally (Hölldobler & Wilson 1990; Kirchner 1997). nests in the ground (Soulié 1956) and have nocturnal According to Markl (1973) acoustic communication habits (Bernard, 1968). can be accomplished by drumming or stridulating. The establishment of the colonies presents slight The most extended method in ants that nest in the differences between both species: in C. scutellaris the ground is stridulation, as drumming is for arboreal foundation is independent, whereas in C. auberti the ants. Crematogaster auberti Emery 1869, that inhabits regular method is fission (Soulié 1962a; Casevitz- in the ground and Crematogaster scutellaris (Olivier Weulersse 1991). 1792), that lives on trees, where chosen in order to Crematogaster scutellaris diet is mainly based on sweet establish differences between their stridulatory organs. substances and C. auberti is omnivorous. C. scutellaris Within the subfamily , the genus forms foraging columns while C. auberti randoms Crematogaster (Lund 1831) is distributed throughout alone or in small groups (Soulié 1962b). Temperature the world and includes 427 species (Bolton 1995); in may influence the collecting cycle; nevertheless other the Iberian Peninsula only four of them are present: climatic elements such as light intensity and relative humidity do not seem to be very important factors (Villagrán et al. 1992, 1998a, 1998b). E-mail: [email protected] Crematogaster is a unique genus due to the origin Accepté le 27 octobre 2005 of its track pheromones, which are not specific of the

99 E. Ruiz, M.H. Martínez, M.D. Martínez & J. M. Hernández

Crematogaster ants are aggressive. C. scutellaris is the most hostile of the species studied in this paper (Soulié 1960). Crematogaster ants are unique among Myrmicinae in the defence of their nests because they use the secretion of Dufour gland as a defensive contact secretion (Daloze et al. 1987; Pasteels et al. 1989). Stridulation sounds have been detected in four subfamilies of ants: Ponerinae, Pseudomyrmecinae, Mymicinae and Nothomyrmecinae. Two structures are involved in the stridulatory organ: the plectrum or scraper, located in the back edge of the third abdominal tergite and the pars stridens or file, placed in the fourth abdominal segment (Grasso et al. 1998; Hernandez et al. 2002). Sound occurs when gaster moves up and down; causing rubbing of the scraper with the file (Spangler 1967). Some descriptive studies of the stridulatory organ for species have been completed: Grasso et al. (1998) for Messor minor (André,1883) and Messor wasmanni Krausse,1910; Schilliger & Baroni Urbani (1985) for Messor capitatus (Latreille 1798) and Messor structor (Latreille 1798); Hernández et al. (2002) for Messor barbarus (L. 1767); Le Roux (1976) for Myrmica laevinodis Nylander 1846; Hickling & Brown (2000) for Solenopsis richteri Forel 1909; Kermarrec et al. (1976) for Acromyrmex octospinosus (Reich 1793); and Álvarez et al. (in press) for Aphaenogaster senilis Mayr 1853. However, no detailed studies have been undertaken with regards to the Crematogaster genus, with the exception of Raignier in 1933. All the reasons mentioned above have impulsed us to study the Crematogaster scutellaris and the Crematogaster auberti stridulatory organ. Material and Methods Worker ants were sampled during the months of September and October 2001. Crematogaster scutellaris specimens were collected at “Casa de campo” in Madrid, UTM: 30TVK 364 749, altitude: 645 m, nests were located in the trunks of oak trees. Six different nests were sampled, collecting 50 specimens in each. 50 workers of Crematogaster auberti were collected from a single nest, located in the ground under stone, at Majadahonda (Madrid), UTM: Figure 1 30TVK 242 744, altitude: 712 m. Location of stridulatory organ. For the study of the stridulatory organ, the gaster of each specimen was detached and fixed on the same cardboard to which the ant had been mounted previously. A mould of the first gaster tergite was obtained by means of covering it with species included in this paper. Espadaler & Martí transparent nail lacquer; it was dried for 24 hours before (1994) explain that the reason for this is that the C. proceeding to separate the obtained layer. This was mounted on microscope slides, fixing them with a cover slide by means auberti and the C. scutellaris inhabit different ecological of two strips of sticky paper. niches. Measurements of the stridulatory organ were obtained from the

100 Stridulatory Organ in Crematogaster digitalized images of preparations. We used a Presentco CCD organ for Formicidae, conformed by a scraper or 3630V camera with microscope adapter and connected to a plectrum in the third abdominal tergite and a file or digitized card 3400 ASUS AGP TNT for PC. pars stridens placed on the medial dorsal region of The studied specimens and preparations obtained are deposited in the collection of the “Department of Zoology” at fourth abdominal pretergite (Fig. 1). “Complutense University of Madrid” (UCME Collection). General pattern is similar in all the observed Three variables were measured from each stridulatory organ: specimens. Plectrum is shaped by a thickening of the Maximum width, maximum length, and distance between back edge on the third abdominal tergite. It has not ridges. This last variable was taken considering it as the sum any other modified structure (Fig. 1). of the width of a ridge and its corresponding inter-ridge. From Pars stridens is an elliptical area that fills the medial each specimen twenty measurements were performed and the mean value was computed. The relation between Maximum region of the fourth abdominal pretergite. It reaches, length/Maximum width was also calculated for each specimen. almost, the anterior or posterior edges of the former. In In order to compare the size of the stridulatory organ with this area, tegumentary sculpture adopts the structure the size of the ant, the width of the head was also considered. of fine parallel ribs, showing a uniform striated aspect According to Elmes (1976) it represents a good indicator of (Fig. 1). the full-size of the specimen. The measurements were made The stridulatory organ is similar to the one present following Dubois’ criterion (1998). A binocular microscope with a micrometric eyeglass was used to measure the width of in other Myrmicinae species, both in its shape and in its the head, right behind the eyes. structure and position, although remarkable differences Inter and intra-specific relationships between the nests were may appear concerning the number of ridges of pars calculated using a one way ANOVA test, and a Tamhane T2 stridens, (Le Roux 1976; Schilliger & Baroni Urbani test of multiple comparisons. 1985; Hernández et al. 2002). The shape and general structure is similar for all Results and Discussion the nests under study and differences have only been The two studied species present a typical stridulatory found in the full size of pars stridens and in the distance

Table 1. One-way ANOVA results for pars stridens and head variables.

1-way ANOVA (BETWEEN SPECIES) 1-way ANOVA (BETWEEN NETS) SPECIES 1 Sum of Mean Sum of Mean df F Sig. df F Sig. squares square squares square Inter-groups 79.78 1 79.78 0.53 0.47 Inter-groups 5976.24 5 1195.25 10.84 0 MAXIMUM MAXIMUM Intra-groups 25134.69 167 150.51 Intra-groups 12566.71 114 110.23 WIDTH WIDTH Total 25214.47 168 Total 18542.95 119 Inter-groups 53.00 1 53.00 0.49 0.48 Inter-groups 4330.98 5 866.20 11.23 0 LENGTH Intra-groups 17968.33 167 107.59 LENGTH Intra-groups 8792.46 114 77.13 Total 18021.34 168 Total 13123.44 119 Inter-groups 1.43E-03 1 1.43E-03 0.08 0.77 Inter-groups 0.16 5 3.14E-02 1.70 0.14 SHAPE Intra-groups 2.82 165 1.71E-02 SHAPE Intra-groups 2.07 112 1.84E-02 Total 2.82 166 Total 2.22 117 Inter-groups 9.33E-02 1 9.33E-02 13.05 0 Inter-groups 8.11E-02 5 1.62E-02 2.54 0.03 RIDGES Intra-groups 1.19 167 7.15E-03 RIDGES Intra-groups 0.73 114 6.38E-03 Total 1.29 168 Total 0.81 119 Inter-groups 0 1 0 0.01 0.91 Inter-groups 1.048 5 0.21 16.66 0 HEAD HEAD Intra-groups 3.69 167 0.02 Intra-groups 1.43 114 0.01 WIDTH WIDTH Total 3.69 168 Total 2.48 119 Inter-groups 0.04 1 0.04 2.05 0.15 Inter-groups 0.47 5 0.09 6.98 0 HEAD HEAD Intra-groups 3.12 167 0.02 Intra-groups 1.52 114 0.01 LENGTH LENGTH Total 3.16 168 Total 1.99 119 Inter-groups 56.28 1 56.28 2.39 0.12 Inter-groups 533.97 5 106.79 8.01 0 WIDTH/ WIDTH/ Intra-groups 3926.07 167 23.51 Intra-groups 1519.32 114 13.33 LENGTH LENGTH Total 3982.35 168 Total 2053.28 119 Significant difference between means at level 0.05

101 E. Ruiz, M.H. Martínez, M.D. Martínez & J. M. Hernández between ridges, although the two species have different length and width, whilst the index relating both variables nesting behaviours (edaphic and arboreal),their does not show significant differences (Table 2). This stridulatory structures are similar. implies that, while the size seems to be significantly The first ANOVA results (Table 1) show significant variable between nests, the general proportions of the differences only in one variable among the two species stridulatory organ remain the same. under study: the distance between ridges (p<0.05), but The Tamhane’s multiple comparisons test places the significant variations in the size of pars stridens were length and width differences in nests 4 and 6. Table 3 not detected. shows a gradation in shape and length in workers from Cephalic width does not confirm significant inter- different nests, being those from nest 4 the smallest species differences. This indicates a similar size on both and those from nest 6 the largest. This explains the groups of workers. reason why these nests show Tamhane test significant As differences in the size of pars stridens are not differences when compared to other nests. Several present, the variation in the distance between ridges hypotheses that could be verified later, may show that may determine a different number of ridges, which probably the smaller size of the stridulatory organ of in turn could mean certain specificity in the sound nest 4 workers, is due to the fact that the individuals produced. collected conformed the first generation of the colony, A second ANOVA for the six C. scutellaris nests which is usually smaller in size (Gray 1971; Passera shows significant differences (p<0.01) in the maximum 1984).

Table 2. Tamhane T2 test results for pars stridens variables.

DISTANCE BETWEEN RIDGES PARS STRIDENS MAXIMUN WIDTH (I) (J) Difference Standard (I) (J) Difference Standard Significa. Significa. Nest Nest Mean (I–J) error Nest Nest Mean (I–J) error 1 2 2.53E-02 2.52E-02 0.99 1 2 -0.69 3.32 1 3 5.39E-02 2.52E-02 0.50 3 -9.32 3.32 0.19 4 3.73E-02 2.52E-02 0.64 4 10.59 3.32 0.05 5 -9.90E-03 2.52E-02 1 5 -0.51 3.32 1 6 -1.80E-02 2.52E-02 1 6 -10.88 3.32 0.06 2 1 -2.53E-02 2.52E-02 0.99 2 1 0.69 3.32 1 3 2.86E-02 2.52E-02 1 3 -8.63 3.32 0.29 4 1.20E-02 2.52E-02 1 4 11.28 3.32 0.03 5 -3.52E-02 2.52E-02 0.96 6 0.18 3.32 1 6 -4.34E-02 2.52E-02 0.82 7 -10.19 3.32 0.09 3 1 -5.39E-02 2.52E-02 0.50 3 1 9.32 3.32 0.19 2 -2.86E-02 2.52E-02 0.99 2 8.63 3.32 0.29 4 -1.66E-02 2.52E-02 1 4 19.91 3.32 0 5 -6.38E-02 2.52E-02 0.40 6 8.81 3.32 0.01 6 -7.19E-02 2.52E-02 0.22 7 -1.56 3.32 1 4 1 -3.73E-02 2.52E-02 0.64 4 1 -10.59 3.32 0.05 2 -1.20E-02 2.52E-02 1 2 -11.28 3.32 0.03 3 1.66E-02 2.52E-02 1 3 -19.91 3.32 0 5 -4.72E-02 2.52E-02 0.55 6 -11.10 3.32 0.004 6 -5.54E-02 2.52E-02 0.28 7 -21.47 3.32 0 5 1 9.90E-03 2.52E-02 1 5 1 0.51 3.32 1 2 3.52E-02 2.52E-02 0.96 2 -0.18 3.32 1 3 6.38E-02 2.52E-02 0.40 3 -8.81 3.32 0.10 4 4.72E-02 2.52E-02 0.55 4 11.10 3.32 0.004 6 -8.13E-03 2.52E-02 1 7 -10.37 3.32 0.02 6 1 1.80E-02 2.52E-02 1 6 1 10.88 3.32 0.06 2 4.33E-02 2.52E-02 0.82 2 10.19 3.32 0.09 3 7.19E-02 2.52E-02 0.22 3 1.56 3.32 1 4 5.54E-02 2.52E-02 0.29 4 21.47 3.32 0 5 8.13E-03 2.52E-02 1 6 10.37 3.32 0.02

102 Stridulatory Organ in Crematogaster

The results of the cephalic size analysis are similar be a sign that sound communication in these species to the ones obtained for the measurements of the pars exists, even at short range stridens. Significant differences have been observed in Finally, the stridulatory organ structure is similar nests 4 and 6. When comparing them with the rest to the Messor species ants (Hernández et al. 2002) and of the nests, workers of nest 4 are smaller in size and other groups of insects with functional stridulation, like workers of nest 6 are bigger. some Coleoptera families (Jing-Qiu 1991; Hernández For that reason, the differences in size of the pars et al. 1997). In beetles, structures like the ones stridens in the Crematogaster scutellaris could be described above, appear in other corporal segments. associated with the general size of ant. This could be This convergence with other species that also have a explained by several variables, like the worker’s age for well known sound communication is another sign of each studied nest or the food availability. the stridulatory organ functionality in Crematogaster. The ANOVA forC. scutellaris nests also shows a More studies in the analysis of emitted acoustic signals difference in distance between ridges (p>0.01). This may confirm all these aspects. difference is not explained by the Tamhane’s multiple comparisons test, as the two by two associations do not show a significant difference. Although the presence of a developed stridulatory organ does not constitute evidence in itself, it could

PARS STRIDENS LENGTH PARS STRIDENS - SHAPE (Length/Width) (I) (J) Difference Standard (I) (J) Difference Standard Significa. Significa. Nest Nest Mean (I–J) error Nest Nest Mean (I–J) error 1 2 2.60 2.78 0.10 1 2 3.38E-02 4.30E-02 1 3 -2.25 2.78 1 3 9.60E-02 4.35E-02 0.26 4 9.13 2.78 0.06 4 -2.2197E-02 4.30E-02 1 5 2.60 2.78 1 5 3.84E-02 4.35E-02 1 6 -10.72 2.78 0.01 6 2.34E-02 4.30E-02 1 2 1 -2.60 2.78 1 2 1 -3.38E-02 4.30E-02 1 3 -4.85 2.78 0.59 3 6.23E-02 4.35E-02 0.86 4 6.52 2.78 0.30 4 -5.60E-02 4.30E-02 0.97 5 7.5E–04 2.78 1 5 4.62E-03 4.35E-02 1 6 -13.32 2.78 0 6 -1.04E-02 4.30E-02 1 3 1 2.25 2.78 1 3 1 -9.60E-02 4.35E-02 0.26 2 4.85 2.78 0.60 2 -6.23E-02 4.35E-02 0.86 4 11.38 2.78 0.005 4 -0.12 4.35E-02 0.02 5 4.85 2.78 0.63 5 -5.76E-02 4.40E-02 0.95 6 -8.47 2.78 0.08 6 -7.26E-02 4.35E-02 0.39 4 1 -9.13 2.78 0.06 4 1 2.22E-02 4.30E-02 1 2 -6.52 2.78 0.292 2 5.60E-02 4.30E-02 0.97 3 -11.38 2.78 0.005 3 0.12 4.35E-02 0.02 5 -6.52 2.78 0.32 5 6.06E-02 4.35E-02 0.97 6 -19.85 2.78 0 6 4.56E-02 4.30E-02 0.98 5 1 -2.60 2.78 1 5 1 -3.84E-02 4.35E-02 1 2 -7.5E–04 2.78 1 2 -4.62E-03 4.35E-02 1 3 -4.85 2.78 0.63 3 5.76E-02 4.40E-02 0.95 4 6.52 2.78 0.32 4 -6.06E-02 4.35E-02 0.97 6 -13.32 2.78 0 6 -1.50E-02 4.35E-02 1 6 1 10.72 2.78 0.01 6 1 -2.34E-02 4.30E-02 1 2 13.32 2.78 0 2 1.04E-02 4.30E-02 1 3 8.47 2.78 0.08 3 7.26E-02 4.35E-02 0.39 4 19.85 2.78 0 4 -4.56E-02 4.30E-02 0.98 5 13.32 2.78 0 5 1.50E-02 4.35E-02 1

103 E. Ruiz, M.H. Martínez, M.D. Martínez & J. M. Hernández Significa. error Standard 5.605.53 1.316.50 1.48 0.004 1.49 0.01 0.002 -5.60 1.31 0.004 -5.53 1.48-6.50 0.01 1.49 0.002

HEAD WIDTH / LENGTH Difference Mean (I-J) 34 -0.065 0.90 -1.57 0.88 0.91 0.75 1 1 0.50 34 1.495 2.466 -0.02 -4.04 1.092 1.114 0.99 -1.495 1.46 0.95 0.97 -1.51 0.40 1 1.092 0.13 1.143 1.02 -2.465 0.95 -0.97 -2.47 1 0.90 1.112 1.143 1.04 0.024 0.40 1.506 1 2.47 0.29 -4.03 0.99 1.02 1.04 1.40 1 0.91 0.30 0.11 4 6 6 6 23 4.045 1.46 4.03 0.13 1.40 0.11 (J) Nest 1 2 -1.55 0.85 0.71 2 1 1.563 0.85 1 0.07 0.71 4 0.88 1 -0.90 1 5 0.91 1 1.57 1 61 0.75 0.50 (I) Nest Significa. error Standard HEAD LENGTH 0.190.12 0.03 0.030.16 0 0.005 0.03 0.001 0.13 0.04 0.02 -0.19-0.16 0.03 0.03-0.13-0.12 0 0.001 0.04 0.03 0.02 0.005

Difference Mean (I-J) 3 0.09 0.04 0.32 2 4 5 6 0.0534 0.065 0.046 0.09 0.04 0.022 0.90 4 -0.06 0.035 0.89 0.04 0.106 0.03 0.04 -0.03 0.10 1 0.043 0.045 0.89 0.04 -0.106 -0.07 0.25 1 2 1 0.043 0.03 -0.094 -0.036 0.25 0.07 0.49 0.03 -0.062 0.043 -0.02 0.034 0.10 0.04 0.035 1 0.04 0.49 0.06 0.72 0.04 1 0.04 1 0.72 (J) Nest 1 2 0.03 0.03 0.99 41 51 2 1 -0.033 0.03 1 -0.09 0.99 0.04 0.32 6 1 -0.05 0.04 0.90 (I) Nest . Significa. error Standard HEAD WIDTH 0.21 0.040.22 0 0.0400.12 0 0.04 0.05 0.13 0.040.170.29 0.02 0.16 0.04 0.04 0.03 0 0 0 -0.17-0.21-0.22 0.04-0.12 0.04-0.13 0.04-0.29 0.04 0 0.04 0 0.04 0 0.05 0.02 -0.16 0 0.03 0

Difference Mean (I–J) 3 0.09 0.03 0.11 2 3 5 6 6 4 5 4 56 0.08 -0.0834 0.03 0.105 0.036 0.09 -0.07 0.03 0.17 2 0.27 4 -0.10 0.035 0.04 0.09 6 -0.01 0.03 0.14 0.55 0.03 0.09 1 4 3 23 -0.09 0.01 0.032 0.03 0.07 0.14 1 0.04 0.55 Tamhane T2 test results for head variables (J) Nest 1 2 -0.01 0.03 1 2 1 0.013 0.03 1 -0.09 1 0.03 0.12 5 1 -0.086 0.03 1 0.08 0.17 0.03 0.27 41 (I) Nest Table 3. Table

104 Stridulatory Organ in Crematogaster

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