Morphometric analysis, mimicry, and color polymorphism infive species of Chauliognathus Hentz (Coleoptera, Cantharidae)

Vilmar Machado 1

Aldo M. de Araujo 2

Cieri S. Mosmann 1

ABSTRACT. This study presents data on morphometric variation [or length and widthor elytra in live sympatric species of the genus Chauliogna/hus I-lentz, 1830: C.j1avipes Fabricius, 1781, C. oc/omaclila/us Pic, 1915, C. expanslls Waterhouse, 1878,C. jallax Germar, 1824 and C. linea/lis Zwetsch & Machado, 2000. The meaning ofthis variation is discussed in the light o[ the theOly o[ mimicly and visual communi­cation between prey and predator. Females are larger than males in all species, exceptC. expanslis. The analysis of variance for length ofelytron as well as for width showedthat the differences between species are significant for males and females; significantinteraction (sex x species) was also found. The similarity in color pattern of thesespecies, as well as similarities in the morphometric analysis, suggests that they couldform a mimetic ring of the MUllerian type, which the authors suggest be called, a"yellow-black " complex.KEY WORDS. Coleoptera, Cantharidae, Chauliogl/o/hus, morphometlY, color po­lymorphism

The occurrence in a given place of different species of the same genus with asimilar color pattern suggests either plesiomorphism or convergence due to naturalselection, constituting then an example of mimicI)'. This could evolve as a responseto visually oriented predators or, on theoretical grounds, it could be a true convergence(ifthe prey is unpalatable-classic MUllerian mimicry) onhe evolution toward a patternof the protected species (Satesian mimicIY). On the other hand, morphologicalsimilarity by congeneric species is better explained by commom ancestrality. In thestate ofRio Grande do Sui (southemBrazil), several species ofChallliognalhlls I-lentz,1830 aggregate on their food sources and some of them show striking morphologicalsimilarity, including color pattern. For the species discussed herc, the backgrow1delytral coloration is yellow, with or without black spots, stripes, or even a black band.

Challiiognalhlls is distributed in the Americas and Australia, with more than250 species in the genus (DELKESKAMP 1939), but in spite of its wide geographicaldistribution studies have been made on the biology of only a few species as (e.g.McLAIN (1985, 1986) on assortative mating; HOWARD & SHIELDS (1990) onenzymatic variability and MACHADO & ARAUJO (1995, 1998, 1999) on colorpolymorphism and mating systems. .

1) Laborat6rio de Genetica, Universidade do Vale do Rio dos Sinos. Caixa Postal 275,93022-000 Sao Leopoldo, Rio Grande do Sui, Brasil. E-mail: machado@bios.unisinos.br

2) Departamento de Genetica, Universidade Federal do Rio Grande do SuI. Caixa Postal15053, 91501-970 Porto Alegre, Rio Grande do Sui, Brasil.E-mail: aldomel@portoweb.com.br

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712 Machado et al.

The present work is part of a long term study of color polymorphism inChauliognalhus species as well as their morphology, cytogenetics and molecularvariability. Some of the questions we are trying to answer are: I) What mechanismsare responsible for the maintenance of color polymorphism: differential survival ofmorphs and/or assortative mating? 2) Do the species involved belong to a mimeticcomplex, and if they do, are they MUllerian mimics? 3) Can these species bediscriminated on morphometric characteristics? In the present paper we try toanswer the third question and to comment on its possible role in the evolution of amimicry complex. This is accomplished by the study of about 300 individuals ofboth sexes belonging to five simpatric species in relation to their variability forelytral and pronotal measures.

MATERIAL AND METHODS

Specimens were collected in January 1995 from an aggregate located at thecampus of the Universidade do Vale do Rio dos Sinos - UNISINOS (29°47'36"Sand 51 °09'03"W), Sao Leopoldo district, in the State of Rio Grande do Sui, Brazil.The insects were collected manually and conditioned separately in glass flasks andtaken to the laboratory where measurements were performed. Whenever possible,the same number of insects of each sex were collected and, for polymorphic speciesthe same number of insects for each morpho

The insects found in the study site belonged to five species: C. jlavipesFabricius, 1781, C. octomaculalus Pic, 1915, C. expansus Waterhouse, 1878, C.fallax Germar, 1824, C. lineatus Zwetsch & Machado, 2000 and C. tetrapllnctalusZwetsch & Machado, 2000. A sixth species, C. letrapunctatus Zwetsch & Machado,2000, was excluded from the present analysis due to the small number ofindividuals.Characterization as distinct species was based on examination of the male genitalia(ZWETSCH & MACHADO 2000) and color pattern of the elytra and, in females, bothcolor pattern of the elytra and of the abdomen. In species wi th color polymorphismfor the elytra (c. jlavipes, C. oclomaculatus and C. falIax), the individuals weredistributed according to a color pattern classification system (Fig. I).

Measurements of length and width of the elytron and of the pronotum wereperformed for each individual (Fig. 2), using a Mitutoyo, model CD-6 digitalprecision calliper. These structures were chosen because both exhibit colorationpolymorphism and also because their size and color are influential in matingpreferences (MCCAULEY & WADE 1978; McLA1N 1982). Statistical analysis wasperformed with NCSS software, version 6.0. Since all measures proved to becorrelated (Tab. I) multivariate analysis of variance or discriminant analysis wasexcluded. The original values were transformed into logarithms.

RESULTS

Table II shows the means and standard deviations (in the original units) forall measures in both sexes and in the different species. A bifactorial analysis ofvariance (species and sex) showed that, for all variables (except for length ofpronotum), significant interaction occurred between species and sex. Here only the

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Morphometric analysis, mimicry, and color polymorphism... 713

Soecies/Morphs LIGHT INTERMEDIATE DARK

C.jlavipes mm ffiffi(fJ W~(!)IC.jallax mm ffiffiffi ~C. octomaclIlatlls Glrnc. expansus ~~C. lineatus ffi

Fig. 1. Pattern of variation in elytral color for the five species of Chauliognathus dealt with inthe present paper. Chauliognathus expansus males atlef!, females at right.

LP

IIIIII

I Ir ----~-----------------.I II II II II II II I

~

WE

LE

Fig. 2. Schematic representation of the measures taken on the pronotum and elytron ofChauliognathus spp. (LE) Length of elytron, (WE) width of elytron, (LP) length of pronotum,(WP) width of pronotum.

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714 Machado et a/.

measures of elytra are discussed, since these structures correspond to most of thephenotype exhibited by the insects to possible predators and also because they showcolor polymorphism.

Analysis of variance for the length of elytron showed that the di fferences arehighly signiricant: males F= 143.00 (d.r. = 4, 135; P< 0.000 1); females F= 17.03 (d.f.= 4, 135; P < 0.00 I). Duncan's test separated the males into four groups (Fig. 3): CexpanslIs, WiUl the highest mean (11.02mm), followed by C linea/lis and the pair Cjlavipes/Cfallax and C oc/omacula/lIs, with the lowest mean (7.70 mm). For females,there are only three groups with C expanSl/S joining the pair C jlavipes/C fallax.

Concerning the variation in width of the elytron, the results of the ANOVAwas F= 306.60 (d.r. = 4, 125; P < 0.00 I) for males and F= 5.38 (d.r. = 4, 125; P <0.001) for females. In the males only three groups were formed and in this case Clinea/lis clustered with C. flavipes and C fallax; in the females only two groupswere formed, with C oc/omacllla/lIs being, as before, the smallest species.

Table I. Correlation matrix for the variables length of eiytron, width of elytron, length ofpronotum, and width of pronotum in C. f1avipes (other species showed the same results). Allcorrelations are significant at oc =0.01

Width of elytron Length of pronotum Width of pronotum

Males Length of elytron 0.70 0.76 0.62Width of elytron 0.73 0.62Length of pronotum 062

Males Length of elytron 0.83 0.81 081

Width of elytron 0.86 0.83Length of pronotum 0.88

Table II. Mean and standard deviation (sd, mm) for elytron and pronotum in the five speciesof Chauliognathus. (LE) Length of elytron, (WE) width of elytron, (LP) length of pronotum, (WP)width of pronotum.

Species LE Sd LE sd LE sd LE sd

MalesChauliognalhus lavipes 8.84 0.36 2.77 017 2.46 0.13 3.61 0.26 50Chauliognalhus oclomaculalus 7.70 0.45 2.38 0.10 223 0.13 3.20 0.20 14

Chauliognalhus expansus 1102 037 4.46 015 2.71 0.10 427 017 20Chaul,ognalhus sp. 1 979 0.50 2.84 0.19 2.85 0.17 385 026 20Chauliognalhus fallax 8.81 0.44 2.74 015 245 0.12 3.61 021 36FemalesChauliognalhus lavipes 9.42 0.49 3.12 0.21 2.62 016 882 025 44

Chauliognalhus oclomaculalus 8.92 0.47 2.91 0.14 2.56 014 372 0.27 14

Chauliognalhus expansus 969 0.40 3.16 0.18 2.58 0.12 3.62 0.20 20Chauliognalhus sp. 1 10.20 0.58 3.09 0.18 2.95 0.14 399 022 20Chauliognalhus fallax 9.52 0.40 3.12 017 2.61 0.15 3.79 0.22 36

DISCUSSION

The differences found among the species are signi licant but do not allow thefull separation of species based on the measurements performed, for example, Cjlavipes and Cfiilla.y were not separated in either analysis. This similarity could beimportant, were they mimetic species; moreover, these species present a marked

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Morphometric analysis, mimicry, and color polymorphism ... 715

parallel polymorphism for elytra color pattern. In C flavipes nine morphs are foundand in CfaLIax six (Fig. I). The morphs found in these two species are very similarindeed to each other, and even similar to other morphs found in the other threespecies. Two species are monomorphic, C. expansus and C lineatus although easilydistinguished (Fig. 1).

The differences in size between males and females is a common feature ininsects. The larger size of C expanslls males is due to the presence of lateralexpansion in their elytra, which are transparent and do not change the male colorpattern. Females, on the contrary, are indistinguishable morphometrically from C.fhll'ipes and C faLIax (Fig. 3 and Tab. II).

Males

LE

WE

Females

LE

WE

_o_ct----JI < If_a_1__fla 1< I sp 1 I<~

__o_ct-----J, < l_fa_1__f_la s....:....p---,1I< I exp

~_o_ct-----JI < If_a_1__f_la__ex_p_~1< I sp 1

__o_c_t_I < l,-fa_I fla__e--,xp_s--'P--J1 IFig. 3. Clustering of species according to the significant differences between the means of thevariables; Duncans' test with a = 0,01 (see figure 2). Species: (fla) C. f1avipes (exp) C.expansus, (oct) C. octomaculatus, (sp) Chauliognathus sp. 1, (fal) C. (aI/ax.

The similarity in color patterns in all these species, as well as that obtained frommorphometric analysis, suggests that they could form a MUllerian-type mimicry ring,which could be called a "yellow-black" complex. Although palatability tests withthese beetles are not reported in the literature, an unpublished investigation by 1.YASCO CELLOS-NETO (personal communication) suggests that they are distasteful tosome insectivorous birds. We also performed some palatability experiments withdomestic chickens. The percentage ofavoidance towards C flavipes, C. fallax, and C.oclon/aeulalus was strikingly high, being greater than 87 % (FERNANDES el af.unpublished data). The presence ofdefensive secretions and theirchcmical nature werestudied by MEtNWALD et af. (1968) for C. leeonlei Champ, 1914; MOORE & BROWN(1978) for C. pulchellus Pic, 1930; EISNER et af. (1981) for C penmylvanicl/s De Geer,1774 and by BROWN et af. (1988) for C. lugubris Pic, 1930.

To provide protection, the similarity between species should be such as toremind the predator of an unpleasant experience with similar prey, since selectionfor mimicry does not necessarily affect all aspects ofthe morphology ofan organism,but rather only those involved in signaling properties unpleasant to predators. Inthis context the differences in species size cannot be perceived by predators or does

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716 Machado et al.

not interfere in the signaling of their unpalatability, and in this case selection formimicry would be more marked on the color patterns. In C pennsylvanicus, a nearticspecies, the frequency of color patterns is not different between sexes, with femaleslarger than males (MASON 1976). According to him this simi larity is the result ofselection for MUllerian mimicry between males and females.

Experimental studies have demonstrated that even superficial similaritiesconfer some advantage to the mimics (PILECKI & O'DONALD 1971; EDMUNDS1974). Furthermore, predators apparently generalize a color pattern, recognizing anorganism by its general appearance (IKTN & TURNER 1972).

The small differences observed in width and position of the spots possiblydoes not interfere in signaling unpleasant properties in these species. The messagesent to the predators could be associated with the contrast between yellow and black.The color pattern of an organism could also be involved in intra-speci fic commu­nication (TURNER 1977; ENDLER 1980, 1988), which could influence the differencesin shape and position of the black spots on the elytra of these species.

Our data shows that three, or at most four, groups can be morphometricalyrecognized (Fig. 3) and, interestingly, morph variability can be grouped into threecategories as far as coloration is concerned: light, intermediate and dark, though theseare independent of species (Fig. I). Light morphs are those without spots or bandsoccurring in Cjl.avipes and Cfal/ax; intermediate morphs occur in all species exceptC expansus which has only dark morphs (50% or more of elytral area dark) similarto those in Cjl.avipes and Cfal/ax.lt should be pointed out that part of this similarityamong species could be due to a common ancestor, that is to say, it could correspondto a plesiomorphism. We are not at present able to distinguish between this po sibilityand the possibility of mimicry, although we believe that the available data point tomimicry. The simple fact that they share common ancestry do not invalidate thepossibility of evolution of mimicly in these species (a well known example is givenby the pair of butterfly species, Heliconius erato and H. melpomene).

The stabilizing selection exerted by predators should theoretically preventcolor polymorphism in MUllerian rings, but even so there are several examples ofthe occurrence ofpolymorphisms in these rings (e.gOwE 1970; BROWN & BENSO1974; PAPAGEORGIS 1975). We do not currently have information which allows theidentification of the set of factors enabling color polymorphism in the three speciesofChauliognathus analyzed in this study. An important possible factor could be thedensity of aggregates formed by these species. According to BROWN & BENSON(1974) very common unpalatable species release themselves from the normalizingselection for pattern uniformity exerted by predators. This release is a result of thefact that the number of insects consumed during the learning period of the predatorswould be small compared with the total number of insects in the aggregate. A morecomplex situation is that modeled by ENDLER (1988) in which a polymorphism canevolve if more than one species of predator is involved.Finally, the polymorphismcould be maintained if some sort of assortative mating is occurring. Despite thisprocess being stated in a previous paper on C.ralla.;r (DIEHL-FLEIG & ARAUJO 1991)it was not supported in more detailed analysis (MACHADO & ARAUJO 1999).

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Morphometric analysis, mimicry, and color polymorphism...

ACKNOWLEDGMENTS. Thanks are due to Couselho Nacional de Desenvolvimento Cienti­

fico e Tecnologico (C Pq) and to Financiadora de Estudos e Projetos (FINEP) for the financial

support.

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717

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Recebldo em 29.VI.2000: aceilo em 16.v11.2001.

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