Proximate determination of male horn dimorphism in the beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

The existence of discrete phenotypic variation within one sex poses interesting questions regarding how such intrasexual polymorphisms are produced and modified during the course of evolution. Approaching these kinds of questions requires insights into the genetic architecture underlying a polymorphism and an understanding of the proximate mechanisms determining phenotype expression. Here we explore the genetic underpinnings and proximate factors influencing the expression of beetle horns – a dramatic sexually selected trait exhibiting intramale dimorphism in many species. Two relatively discrete male morphs are present in natural populations of the dung beetle Onthophagus taurus (Scarabaeidae, Onthophagini). Males exceeding a critical body size develop a pair of long, curved horns on their heads, while those smaller than this critical body size remain essentially hornless. We present results from laboratory breeding experiments designed to assess the relative importance of inherited and environmental factors as determinants of male morphology. Using father–son regressions, our findings demonstrate that horn length and body size of male progeny are not predicted from paternal morphology. Instead, natural variation in an environmental factor, the amount of food available to larvae, determined both the body sizes exhibited by males as adults and the presence or absence of horns. The nonlinear scaling relationship between the body size and horn length of males bred in the laboratory did not differ from the pattern of variation present in natural populations, suggesting that nutritional conditions account for variation in male morphology in natural populations as well. We discuss our results by extending ideas proposed to explain the evolution of conditional expression of alternative phenotypes in physically heterogeneous environments toward incorporating facultative expression of secondary sexual traits. We use this synthesis to begin characterizing the potential origin and subsequent evolution of facultative horn expression in onthophagine beetles.     

Hormonal control of male horn length dimorphism in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

Male dung beetles (Onthophagus taurus) facultatively produce a pair of horns that extend from the base of the head: males growing larger than a threshold body size develop long horns, whereas males that do not achieve this size grow only rudimentary horns or no horns at all. Here we characterize the postembryonic development of these beetles, and begin to explore the hormonal regulation of horn growth. Using radioimmune assays to compare the ecdysteroid titers of horned males, hornless males, and females, we identify a small pulse of ecdysteroid which is present in both hornless males and females, but not in horned males. In addition, we identify a brief period near the end of the final (third) larval instar when topical applications of the juvenile hormone analog methoprene can switch the morphology of developing males. Small, normally hornless, males receiving methoprene during this sensitive period were induced to produce horns in 80% of the cases. We summarize this information in two models for the hormonal control of male dimorphism in horn length.     

Dimorphism of both head and prothoracic horn morphologies in male Copris acutidens (Coleoptera: Scarabaeoidea)

Allometric scaling of male horn morphologies of Copris acutidens, including the head and prothoracic horns, was analyzed. The analyses of scaling relationships indicated a discontinuous increase in head horn length and prothoracic horn height but a linear increase in prothoracic horn length relative to body size. The different results for length and height of the prothoracic horn suggest that the height is functionally more important for ensuring mates in the nests. Furthermore, both the head and prothoracic horns were dimorphic, and this characteristic was not found in the scarabaeoid beetle Onthophagus. Similarity in the switch point values for the head and prothoracic horns suggests that the dimorphism of the two kinds of horns may result from the same developmental threshold mechanism.     

Diet alters male horn allometry in the beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae)

Darwin considered the horns of male beetles to be among the most striking examples of sexual selection. As with antlers in deer or elk, beetle horns scale positively with male body size, with the result that large males have disproportionately longer horns than small males. It is generally assumed that such scaling relationships (''static allometries'') are insensitive to short-term changes in the environment, and for this reason they are regularly used as diagnostic attributes of populations or species. Here I report breeding experiments on horned beetles that demonstrate that the scaling relationship between male horn length and body size changes when larval nutrition changes. Males reared on a low-quality diet had longer horn lengths at any given body size than sibling males reared on a high-quality diet. Such ''allometry plasticity'' may explain seasonal changes observed in this same scaling relationship in a natural population. These experiments demonstrate that scaling relationships of sexually selected traits can respond facultatively to variation in the environment, thereby revealing a new mechanism by which males regulate the production of exaggerated secondary sexual traits.     

Development of Copris tripartitus (Coleoptera: Scarabaeidae) in Two Different Rearing Media

ABSTRACT Like many other sub-social insects, Copris tripartitus Waterhouse requires parental care during the development period in the brood balls. Mass-rearing is achieved by using indoor rearing systems and adding more dung after taking away the brood balls containing eggs. Without parental care, larvae are often affected by desiccation, as well as by fungal or sciarid fly intrusion of the brood balls. This study developed a good substitute for brood ball rearing medium. A significantly greater number of progeny survived to adulthood from broods produced by beetles reared on vermiculite medium (90.69±10.98%) than on peat moss medium (44.82±13.92%). Results of the study suggest that to protect the brood balls from desiccation and to produce healthy adults, 15-20% moisture content vermiculite must be used as brood ball rearing medium for most species reared in the laboratory.     

Hormonal control of male horn length dimorphism in Onthophagus taurus (Coleoptera: Scarabaeidae): a second critical period of sensitivity to juvenile hormone

Male dung beetles (Onthophagus taurus) facultatively produce a pair of horns that extend from the base of the head: males larger than a threshold body size develop long horns, whereas males that do not achieve this size develop only rudimentary horns or no horns at all. Using topical applications of methoprene, we identified a sensitive period during the feeding stage of third (final) instar larvae when application of methoprene shifted the threshold body size for horn expression. Male larvae that received methoprene at this time delayed horn production until they attained a larger threshold body size than acetone-treated control larvae. This new sensitive period occurs earlier than a sensitive period previously reported for male horn regulation, and it coincides with a morph-specific pulse of ecdysteroid secretion described for this species. It appears that male horn expression is influenced by endocrine events at two different periods of larval development. We incorporate these results into an expanded model for the endocrine regulation of male horn expression.     

Adverse effects of ivermectin on the dung beetles, Caccobius jessoensis Harold, and rare species, Copris ochus Motschulsky and Copris acutidens Motschulsky (Coleoptera: Scarabaeidae), in Japan

Effects of the antiparasitic drug, ivermectin, on the dung beetles, Caccobius jessoensis Harold, 1867 and the rare species, Copris ochus Motschulsky, 1860 and Copris acutidens Motschulsky, 1860 were studied in laboratory and field experiments in Hokkaido, Japan. Ivermectin was detected in dung from 1 to 21 or 28 days following treatment, with a peak on the first day after treatment in two pour-on administrations (500 microg kg(-1)), although there were considerable differences between the two peaks. In C. jessoensis, brood balls constructed by the female were not reduced in the dung of treated cattle except for seven days after treatment in experiment 2. Also, there was no significant difference in the mean weight of brood balls between dung from treated and control cattle. However, the emergence rates were significantly reduced in dung 1-3 days after treatment. In the field study, brood balls constructed by C. jessoensis were more abundant in dung from treated cattle in experiment 1, but adult emergence was significantly reduced at one and seven days after treatments. Adult mortality of C. ochus Motschulsky at 90 days after the beginning of rearing was 11.1% in dung from control cattle with 22 brood balls constructed, whereas it was 84% in dung from treated cattle with no brood balls and/or ovipositioning. Also, in C. acutidens Motschulsky, adult mortality at 90 days after the beginning of rearing was 3.6% in dung from control cattle with 13 brood balls constructed, whereas it was 94.1% in dung from treated cattle with no brood balls or ovipositioning. The environmental risk in the use of ivermectin during breeding period of dung beetles in pasture is discussed.     

Male dimorphism in three sympatric species of Onthophagus (Coleoptera: Scarabaeidae)

Onthophagus trituber, O. taurinus, and O. proletarius, three sympatric dung beetle species native to southern Taiwan, exhibit normally distributed body size (pronotum width) but non-normally distributed horn length due to the presence of horned and hornless morphs. The scaling relationships between horn length and body size were established by using sigmoidal and segmented linear estimation, and the horned/hornless ratio in each of the species was estimated. The ratios estimated by sigmoidal curves showed that all three species were biased toward the hornless morph, whereas the ratios from segmented lines showed that only one species (O. taurinus) was biased toward the hornless morph. Nevertheless, the results of the two methods of estimation were concordant in ranking the horned morph proportions among the three species; O. proletarius had the largest proportion of horned morphs, whereas O. taurinus had the smallest, suggesting that the fewer horned morph in O. taurinus likely resulted from both intra- and inter-specific competition.     

Parental behaviour in Copris lunaris (Coleoptera, Scarabaeidae): care and defence of brood balls and nest

Abstract. 1. Nesting female beetles righted brood balls (so as to replace the egg or larva in the uppermost position) and repaired damaged balls. This behaviour required the presence of an egg or larva in the ball, or of a short-lasting material found just after oviposition. The shape of the ball was also a righting stimulus since artificial ellipsoids were stood on end.
2. Balls containing dichloromethane extracts of C.lunaris brood were righted and repaired. Eggs and larvae of several other Scarabaeidae did not release these responses but were destroyed.
3. Righting behaviour was released when brood was absent from the top of the ball. The beetle then crawled vertically downwards and, if it encountered the displaced apex, a novel rolling action followed which automatically turned the ball towards the correct position.
4. An opening made in the nest was repaired with soil excavated from the chamber floor. Clunaris adults and Aphodius fossor larvae were attacked if they were encountered in the nest.     

The mesostigmatic mites associated with Copris hispanus (L.) (Coleoptera, Scarabaeidae) in Israel

Works dealing with the association of searabaeid beetles and mesostigmatic mites are shortly reviewed. In the present paper the mite fauna of 106 specimens of Copris hispanus (Scarabaeidae) is investigated. A total of 3732 mites was collected, belonging to 17 species (10 genera, 6 families). The following species are described (or redescribed) and figured: Parasitus coleoptratorum (L.) Berlese, 1906; Parasitus copridis n. sp.; Parasitus hyper-setosus n. sp,; Pachylaelaps hispani Berlese, 1908; Proctolaelaps parascolyti n. sp.; Alliphis halleri (G. & R. Can., 1881); Alliphis gurei n. sp. The degree of specifity of the mite-beetle association is discussed. It is suggested that there is some relation between the social organization of the beetles and the degree of mite infestation, comparable to the mite-hymenoptera association.     

Sexual dimorphism in Diabrotica speciosa and Diabrotica viridula (Coleoptera: Chrysomelidae)

Diabrotica speciosa (Germar) and Diabrotica viridula F. (Coleoptera: Chrysomelidae) are the two most abundant species of the genus in South America, and belong to the fucata and virgifera groups, respectively. Here, we characterize the dimorphism of the setae present on the basitarsi of males and females of these species. Dimorphism was confirmed in both species, and it was related to the presence of adhesive setae exclusively in males, which possess these structures on the basal tarsomeres of the pro- and mesothoracic legs.     

Sexual dimorphism and allometry in two seed beetles (Coleoptera: Bruchidae)

Male Callosobruchus chinensis (Coleoptera: Bruchidae) have elaborated, pectinate antennae, which are absent from conspecific females and both sexes of a congener, Callosobruchus maculatus. To begin to unravel the mechanisms producing this striking dimorphism, we examined which morphological traits best explain body size variation in bruchid beetles and quantified sexual dimorphism of antenna size through allometric analyses. Using principal component analyses, we found that elytron length and pronotum width were significantly correlated with the first principal component, which was interpreted as explaining variation in body size. Regressions of log‐transformed body size measures on log‐transformed antenna length revealed that males of both species had longer antennae than conspecific females for any given body size, although most of this effect was attributable to higher intercepts, rather than increased allometry, in males. Comparisons among heterospecific males revealed that C. maculatus males have noticeably longer antennae than C. chinensis males at large body sizes. Callosobruchus chinensis males, thus, appear to have increased the receptive area of their antennae by adding to the width of, rather than further elongating, their antennae. Finally, we found evidence for positive allometry between log‐transformed antenna length and log‐transformed antenna width in C. chinensis males. We discuss our results in the context of evidence supporting the presence of an additional, and potentially unique, sex pheromone in C. chinensis females.     

Normal and atypical nesting behaviour of Copris lunaris (L.): comparison with related species (Coleoptera, Scarabaeidae)

Abstract. 1. The lifecycle, mating and larval behaviour of Clunaris are described. Adults appeared in the autumn and nested in the following spring. The female beetle remained in the nest with the brood and could nest again the following year.
2. Nesting was initiated when virgin females were mated in the spring. Brood balls were formed by techniques sirnilar to those used by Scarabaeini. The female beetle left the nest soon after the first imagos broke out of the brood balls.
3. Nesting behaviour was readily modified by external conditions. Many parts of the sequence could be repeated or omitted. The female beetle left the nest if the brood was removed, but she remained for longer than usual if younger brood was substituted near the end of the normal nesting period.
4. Certain experimental conditions released behaviour patterns typical of other species. These were formation of superficial nests or of two-chambered nests, oviposition before completing the brood ball, and coating of the brood balls with soil (all found in other Coprina), as well as ball rolling and ball burial (found in Scarabaeini). The results are discussed in relation to the evolution of Copris nesting behaviour.     

Population genetic characterization of the endangered dung beetle Copris tripartitus (Coleoptera: Scarabaeidae) using novel microsatellite markers

The dung beetle Copris tripartitus (Coleoptera: Scarabaeidae) has long been considered an endangered insect in South Korea; the detection of recent population increases leaves its endangered status uncertain. Population genetic analysis subsequent to development of molecular markers is essential for establishing proper conservation strategies. In this study, we developed ten microsatellite markers specifically for C. tripartitus. Sixty-eight individuals of C. tripartitus collected from six South Korean localities were genotyped to validate these markers and preliminarily assess population genetic characteristics. Per-locus observed number of alleles, observed heterozygosity (H_O), and expected heterozygosity (H_E) ranged from 5–12, 0.499–0.958, and 0.54–0.743, respectively. All populations showed higher H_O than H_E, negative values of inbreeding coefficient, and, overall, no sign of recent population bottlenecks (excluding one population, Seosan). This suggests that C. tripartitus did not suffer from genetic drift and inbreeding, which are typically severe in small, isolated populations. Nevertheless, detection of only one of the two gene pools in some populations and resultant genetic subdivision into two population groups may suggest that the population size is not enough to cover both gene pools. Thus, a more extended period of protection may be required to ensure higher genetic diversity of widespread populations and achieve the long-term conservation goal.     

Genetic diversity of the dung beetle,Copris tripartitus (Coleoptera: Scarabaeidae), that is endangered in Korea

The dung beetle, Copris tripartitus (Coleoptera: Scarabaeidae), is an endangered insect in Korea. In order to establish a conservation strategy, a preliminary investigation on the genetic diversity of Korean populations was performed using mitochondrial COI (658 bp), CytB (433 bp), and nuclear ITS2 (411 nucleotide positions). Sequencing of 69 individuals collected from five localities showed substantially higher variability (5.02% for COI, 4.62% for CytB, and 8.03% for ITS2). The resulting networks for mitochondrial DNA (mtDNA) haplotypes exhibited two star‐like phylogenies, which might indicate that Korean populations have recently expended from two small populations. The ITS2 network, which was presented in the form of a star‐like phylogeny, confirmed that a recent population expansion occurred. Considering the high genetic diversity and gene flow in C. tripartitus populations, one issue regarding conservation seems to be the recovery of previous habitats.     

Distribution and fine structure of antennal olfactory sensilla in Japanese dung beetles,Geotrupes auratus Mtos. (Coleoptera : Geotrupidae) and Copris pecuarius Lew. (Coleoptera : Scarabaeidae)

The distribution and fine structure of olfactory sensilla on the antennal flagella of the Japanese dung beetles, Geotrupes auratus (Coleoptera : Geotrupidae) and Copris pecuarius (Coleoptera : Scarabaeidae) were studied by scanning and transmission electron microscopy. In both types of dung beetles, sensilla on the antennal segments were most abundant on the apical 3 lamellar segments, where sensilla basiconica were particularly concentrated. The sensilla were solely concentrated in the central part of the distal side of both the 9th and 10th segments in G. auratus and the 7th and 8th segments in C. pecuarius, respectively. The numbers of sensilla in these areas (channels) of the 9th and 10th segments in G. auratus were approximately 4,500 and 3,000, respectively. In C. pecuarius, there were about 1,900 sensilla on the 7th segment and l,700 sensilla on the 8th. The cuticles of the sensilla in both species were perforated by numerous elongate pores, with a pore density of about 45/ μm² in G. auratus and 30/ μm² in C. pecuarius. Each sensillum basiconicum in C. pecuarius was generally associated with 2 sensory neurons. The sensory cilia passed into the sensillum lumen and branched profusely, occupying most of its space. On the basis of the fine structure and dense location, these sensilla are considered to have an olfactory function for food detection.     

Sexual dimorphism of guava weevil Conotrachelus psidii Marshall (Coleoptera: Curculionidae)

We investigate structural and behavioral differences between male and female adults of guava weevil, Conotrachelus psidii Marshall, 1922 to help in sex determination. Weevils collected from a commercial guava orchard were individually caged and maintained under laboratory conditions. Every day ten individuals were grouped and their behavior was observed. During mating the males and females were identified. Fifty weevils of each sex were killed and they were observed with stereoscopic microscope. The antenna insertion on the rostrum and hair on the thorax and abdomen surfaces were examined. Length and width of body, pronotum, last abdominal sternite and rostrum were also taken. Behaviorally, the male weevils produce audible sound by stridulation of abdomen while females do not. In the females, the body and pronotum width and body a rostrum length were higher than in the males, but the last abdominal sternite was smaller. Three other structural differences were visible with the naked eye: 1) the anterior region of pronotum have scarce hair or have not in the females and hair is dense in the males; 2) the last tergite of females is hidden by the penultimate tergite whereas in the males is visible; 3) the first abdominal sternite of females is more prominent, very convex and without or with little bristle. In the males it is slightly concave and with abundant hair. The male-female differences found in this work allow, without hurt or change insect behavior, an efficient sex determination of guava weevil.     

Discrimination of alternative male phenotypes in Scapanes australis (Boisduval) (Coleoptera: Scarabaeidae: Dynastinae)

Abstract  The palm pest Scapanes australis has been the object of considerable recent research to develop control measures that target male reproductive behaviour. This study investigated horn size scaling relationships and frequency distributions in two subspecies of S. australis for evidence of alternative male reproductive strategies. Indeed, a sigmoidal horn size allometry in both populations was detected as a significant vertical discontinuity by a piece-wise regression model, which indicates that distinct minor and major male phenotypes are expressed in both taxa. A non-linear regression algorithm showed that these subspecies differ significantly in two modelled parameters of horn allometry. A likelihood model was used for probabilistic discrimination of the alternative male phenotypes by estimating their ratios across the bimodal frequency distribution. The latter is intended to aid field investigation of the reproductive biology of S. australis by statistically discriminating the alternative male phenotypes on the basis of easily measured parameters in live samples.     

Rearing Costelytra zealandica (Coleoptera: Scarabaeidae)

Interaction between larvae can be a cause of mortality when scarabaeid larvae are concentrated in a confined volume of soil. Larvae of Costelytra zealandica (White), the New Zealand grass grub, were held for 30 days at population densities between 1 and 50 larvae per 200 ml of medium, and the effect of crowding on their survival and weight change was assessed. Larval density had no effect on survival in soil plus chopped sheep dung (3 :1), with up to 10 2nd‐instar larvae or 5 3rd‐instar larvae per 200 ml of medium. Larval survival and weight gain were increased by adding germinating ryegrass seed (Grasslands Ruanui) to the medium base, but the effect of density did not diminish. Weight gain of 3rd‐instar larvae decreased as their density increased. When the amount of germinating ryegrass seed in the medium was varied, there was no significant difference between the survival of larvae held in treatments with 5, 25, and 50 g of seed per litre of medium. Survival was significantly lower with 0 and 100 g of seed per litre of medium. It is concluded that, in rearing conditions, the final density of fully grown 3rd‐instar larvae should be a maximum of about 20 per litre of medium to prevent mortality caused by larval interaction—provided that adequate food is available.