Pupal polymorphism in the butterfly Danaus chrysippus (L.): environmental, seasonal and genetic influences

The pupae of the tropical butterfly Danaus chrysippus are either green or pink the switch being operated by a ‘greening’ hormone produced in the larval head. Both environmental and genetic cues are involved in controlling the endocrine mechanism. The environmental factors identified are of two distinct kinds: proximate factors influence pupal colour after the larva has selected its pupation site, whereas ultimate factors are effective at an earlier stage, either prompting choice of pupation site by the larva or priming pupation physiology in a particular direction. Genetic factors preadapt the larva to form a pupa which will be cryptic in the normal or average conditions, climatic or biogeographical, anticipated in its environment. The proximate factors demonstrated are background colour, darkness, light quality (wavelength) and humidity. There is some evidence that substrate texture may also be relevant. Ultimate factors are temperature, humidity and species of larval foodplant. Two closely linked gene loci which govern the phenotype of adult morphs and races either have a pleiotropic effect on pupa colour or are closely linked with other genes which do so. Moreover, the two loci interact epistatically with respect to their pupation effects. Factors producing predominantly green pupae are plant substrates, yellow background, darkness, yellow light, high humidity, high temperature, the b allele at the B locus when homozygous and, on non-plant substrates, the C allele at the C locus. High frequencies of pink pupae result on non-plant substrates, red backgrounds, in blue light, low humidity, low temperatures and in B- and cc genotypes. The C locus alleles, C and c, interact epistatically with the B alleles, their effect on choice of pupation site being determined by linkage phase. Of the two foodplants tested, Calotropis produced a high frequency of green pupae and Tylophora of pinks. The seasonal cycling of rainfall, temperature, availability or condition of foodplant, and gene frequencies are all correlated with oscillations in the frequencies of green and pink pupae. Though genotype influences pupa colour, all genotypes are capable of forming pupae of both colours. The variation can therefore be attributed to an environmental polyphenism superimposed upon a genetic polymorphism. The hormone producing green pupae emanates from the head during the prepupal period. Denied hormonal influence, the pupa is pink. Pupal colour is judged to be aposematic at close range and cryptic at distance.     

Colour morph related performance in the meadow grasshopper Chorthippus parallelus (Orthoptera,Acrididae)

Abstract 1. Polymorphism has been described for a number of herbivorous insects, but little is known about whether differences in body colour cause fitness differences. In Chorthippus parallelus, three main colour morphs occur, namely brown, green, and dorsally striped. 2. The present study examined colour morph abundances and morph‐related differences in body size, oviposition rate, and offspring numbers in females of C. parallelus collected in 15 montane grasslands. The study also examined the effect of plant species richness, composition, community productivity, and solar radiation on colour morph frequency and fitness. 3. The relative frequencies of the three colour morphs was 31.7% (brown), 33.1% (green), and 35.2% (dorsally striped), but the morphs were not evenly distributed across the 15 sites. 4. There was no effect of the habitat variables on the distribution of the green and the striped morph in the study sites, however 80% of the variation in the abundances of the brown morph was explained by plant species richness and composition. 5. Grasshopper size was equal among the morphs. Brown females laid significantly more egg pods than the green and dorsally striped morphs. There were no significant differences in offspring numbers among the colour morphs. 6. Body colour in C. parallelus seems to be a fitness‐relevant trait, raising the question of the evolutionary maintenance of polymorphism.     

Effect of light quality and larval diet on morph induction in the polymorphic caterpillar Nemoria arizonaria (Lepidoptera: Geometridae)

The emerald moth Nemoria arizonaria (Geometridae) is bivoltine, with distinct broods of caterpillars hatching in the spring and summer. Caterpillars of the spring brood develop into mimics of oak catkins, while caterpillars of the summer brood develop into mimics of oak twigs. Previous rearing experiments showed that all caterpillars reared on oak catkins developed into catkin morphs, while all caterpillars reared on oak leaves developed into twig morphs, regardless of temperature or photoperiod. However, those previous rearing experiments did not control the colour of light perceived by the caterpillars independently of their dict. Since wavelengths of light perceived by some species of polymorphic caterpillars can influence their colour, it is possible that morph induction in Nemoria arizonaria is due to the characteristics of light reflected from yellow catkins or green leaves, rather than larval diet itself. The experiments reported here independently varied larval diet and light characteristics to determine if light quality is involved in morph induction. Only larval diet influenced morph induction, since all caterpillars reared on catkins developed into the catkin morph, and all caterpillars reared on oak leaves developed into the twig morph, regardless of whether they perceived yellow light, green light, or were raised in the dark.     

Phytochemical correlates of herbivory in a community of native and naturalized cruciferae

Oviposition and larval feeding behaviors of the crucifer specialist Pieris napi macdunnoughii correlate with leaf glucosinolate profils of plant species in a natural community. Profiles are species-specific in this group of eight Cruciferae, but particular glucosinolates are shared by subsets of the community. Pieris accepts two lethal naturalized weeds whose glucosinolate profiles resemble that of an indigenous foodplant. The results suggest that specific glucosinolates constitute insect behavioral cues which are only loosely linked evolutionarily to foodplant suitability, and further suggest that allelochemically similar community associates influence the coevolution of individual plant species with insect herbivores.     

Nymphal colour/pattern polymorphism in the leafhoppers Eupteryx urticae (F.) and E. cyclops Matsumura (Hemiptera: Auchenorrhyncha): spatial and temporal variation in morph frequencies

Variation in colour/pattern morph frequencies in Eupteryx urticae and E. cyclops is described for various field populations. Eupteryx urticae populations in S Wales exhibit a steep morph-ratio cline, such that black morph frequencies are positively correlated with altitude. High melanic frequencies at high-altitude sites, and the absence of the two darker morphs in lowland populations, suggest a similar trend in E. cyclops , but the data are insufficient to confirm this statistically. No differences in morph frequencies were detected on different parts of the primary host plant or on alternative host species. Similarly, there were no consistent trends within or between the two annual generations of either species, although melanic morph frequencies in one E. urticae population were heterogeneous over 10 generations. It is suggested that the polymorphism in E. urticae is strongly influenced by climate selection, darker morphs being at an advantage in cooler environments where their coloration enhances absorption of solar radiation. The advantage gained through thermal melanism is probably balanced by visual selection against black morphs by entomophagous parasitoids.     

Biased predation could promote convergence yet maintain diversity within Müllerian mimicry rings of Oreina leaf beetles

Müllerian mimicry is a classic example of adaptation, yet Müller's original theory does not account for the diversity often observed in mimicry rings. Here, we aimed to assess how well classical Müllerian mimicry can account for the colour polymorphism found in chemically defended Oreina leaf beetles by using field data and laboratory assays of predator behaviour. We also evaluated the hypothesis that thermoregulation can explain diversity between Oreina mimicry rings. We found that frequencies of each colour morph were positively correlated among species, a critical prediction of Müllerian mimicry. Predators learned to associate colour with chemical defences. Learned avoidance of the green morph of one species protected green morphs of another species. Avoidance of blue morphs was completely generalized to green morphs, but surprisingly, avoidance of green morphs was less generalized to blue morphs. This asymmetrical generalization should favour green morphs: indeed, green morphs persist in blue communities, whereas blue morphs are entirely excluded from green communities. We did not find a correlation between elevation and coloration, rejecting thermoregulation as an explanation for diversity between mimicry rings. Biased predation could explain within‐community diversity in warning coloration, providing a solution to a long‐standing puzzle. We propose testable hypotheses for why asymmetric generalization occurs, and how predators maintain the predominance of blue morphs in a community, despite asymmetric generalization.     

The evolution of alternative morphs: density-dependent determination of larval colour dimorphism in a butterfly

Understanding the ultimate causes for the presence of polymorphisms within populations requires knowledge of how the expression of discrete morphs is regulated. In the present study, we explored the determination mechanism of a colour dimorphism in larvae of the butterfly Pararge xiphia (Satyrinae: Nymphalidae) with the ultimate aim of understanding its potential adaptive value. Last-instar larvae of P. xiphia develop into either a green or a brown morph, although all individuals are invariably green during the preceding three instars. A series of laboratory experiments reveal that morph development is strongly environmentally dependent and not the result of alternative alleles at one locus. Photoperiod, temperature, and in particular larval density, all influenced morph determination. The strong effect of a high larval density in inducing the brown morph parallels other known cases of density-dependent melanization in Lepidopteran larvae. Because melanization is often correlated with increased immune function, this type of determination mechanism is expected to be adaptive. However, the ecology and behaviour of P. xiphia larvae suggests that increased camouflage under high-density conditions may be an additional adaptive explanation. We conclude that the colour dimorphism of P. xiphia larvae is determined by a developmental threshold that is influenced both by heredity and by environmental conditions, and that selection for increased immune function and camouflage under high-density conditions may be responsible for maintaining the dimorphism.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 256–266.     

Phenotypic and genetic variation in emergence and development time of a trimorphic damselfly

Although colour polymorphisms in adult organisms of many taxa are often adaptive in the context of sexual selection or predation, genetic correlations between colour and other phenotypic traits expressed early in ontogeny could also play an important role in polymorphic systems. We studied phenotypic and genetic variation in development time among female colour morphs in the polymorphic damselfly Ischnura elegans in the field and by raising larvae in a common laboratory environment. In the field, the three different female morphs emerged at different times. Among laboratory-raised families, we found evidence of a significant correlation between maternal morph and larval development time in both sexes. This suggests that the phenotypic correlation between morph and emergence time in the field has a parallel in a genetic correlation between maternal colour and offspring development time. Maternal colour morph frequencies could thus potentially change as correlated responses to selection on larval emergence dates. The similar genetic correlation in male offspring suggests that sex-limitation in this system is incomplete, which may lead to an ontogenetic sexual conflict between selection for early male emergence (protandry) and emergence times associated with maternal morph.     

Trans-continental visible morph-frequency variation at homologous loci in two species of spider, Enoplognatha ovata s.s. & E. latimana

The spiders Enoplognatha ovata s.s. and E. latimana are sibling species which share a number of visible genetic polymorphisms. Data on colour and black-spotting morph frequencies in these species have been collected from 67 sites in western Europe. Sixty nine percent of the collections contained both species. In all adequately-sized samples, both species were polymorphic for colour and, in general, exhibited the same rank order of morphs lineata and redimita. (The top dominant morph, ovata, has not been found in E. latimana). Colour-morph frequencies are not correlated between species in sympatric populations from mainland Europe and from a previously studied area in Pembrokeshire, South Wales. Although associations with certain climatic variables are evident in E. ovata they are not consistent between transects, making their biological significance unclear. For black spotting, E. ovata s.s. is nearly fixed for spotting throughout mainland Europe but is highly variable in the Pembrokeshire populations. E. latimana is polymorphic in both areas. In Europe, spotting frequencies in E. latimana show significant associations with climatic factors but, again, their biological significance is not obvious. In E. ovata s.s. the variance in both colour and spotting frequencies among populations in Pembrokeshire is significantly greater than that in the whole of mainland Europe. The implications of these and previous results are considered in the context of the persistence of visible polymorphisms across species and the forces which determine morph frequencies in local populations.     

Mitochondrial genetic structure reflects the geographical variation of elytral polymorphism frequency in Cheilomenes sexmaculata (Coleoptera: Coccinellidae)

Many ladybird species are known to have an elytral colour polymorphism, which indicates geographical variation. The ladybird beetle Cheilomenes sexmaculata (Fabricius) exhibits elytral colour polymorphism and has expanded its distribution from 33°N to 36°N in Japan over 100 years since 1900. The mitochondrial COI gene haplotypes were integrated into two haplotype groups, with one group existing at higher frequencies in lower latitudes, the other group appearing at higher frequencies in higher latitudes. In addition, the dark morph types of this species increase with latitude, whereas the light types appear at higher relative frequencies in lower latitudes.In the present study, we first determined the morph types of individuals and examined the mitochondrial DNA COI gene. Second, we investigated the relationship between the genetic population structure based on the mitochondrial DNA COI gene and the morph types’ geographical variation. Results indicated that the mtCOI genetic structure was associated with the morph types by latitude; specifically, the haplotype group existing at higher frequencies in lower latitudes tended to be light morph types. In contrast, the haplotype dominant in higher latitudes more frequently exhibited dark morph types, indicating that dark morph types in the higher latitude genetic group may have led the distributional expansion toward higher latitudes since 1900 rather than the lower latitude light morph types.     

Effect of light intensity on colour morph formation and performance of the grain aphid Sitobion avenae F. (Homoptera: Aphididae)

The grain aphid Sitobion avenae F., one of the major pest aphids of cereals in Central Europe, exhibits colour polymorphism, even within the same clones. Although there is evidence that green and brown morphs of S. avenae contain different carotenoids, the mechanisms determining the induction of colour morphs are unknown. The common understanding is that the formation of colour morphs is controlled by light and affected by genetic and environmental factors and by host plant species. So far, there is no unequivocal evidence that light intensity, photoperiod, or a mixture of several variables are involved in the induction of S. avenae colour formation, resulting in the induction of S. avenae colour formation and carotenoid synthesis.Here we determined the effect of light intensity on the colour formation and performance of ten clones of S. avenae with experiments that controlled for the effects of host plant and genetic factors. We found that some clones remained green under all test conditions. In other clones, colour morph formation was controlled by light. The synthesis of carotenoids correlated with changes in colour formation. Host plant did not affect colour formation in the ten clones studied. Although colour of the aphid clones did not affect their performance, high light intensity increased the fecundity and fresh weight of S. avenae clones, while low light intensity stimulated the production of alatae.     

Differential visual predation on morphs of Timema cristinae (Phasmatodeae:Timemidae) and its consequences for host range

Timema cristinae is a herbivorous insect that exhibits polymorphism for body coloration (green, red and grey morphs) and for pattern (striped, expressed only in the green morph, and unstriped). The striped green morph is associated with ceanothus ( Ceanothus spinosus ) and the unstriped green morph is associated with chamise ( Adenostoma fasciculatum ). This study examines the relative vulnerabilities to predation of the different pattern and colour morphs on their natural backgrounds. The vulnerabilities of the striped and unstriped morphs on their two food plants were tested using uncaged wild birds (Scrub Jays) and captive western fence lizards. Strong differential predation was observed suggesting that each morph is most cryptic on the food plant on which it is most common. Furthermore, in a mark-recapture experiment in a patch of ceanothus the unstriped and red morphs were recaptured in higher proportion than the other morphs. The vulnerabilities of the grey and green morphs on the ground and foliage were tested using lizards. The grey morph was more vulnerable on the plants than the green morph, but the inverse was observed on the ground (where they drop after a disturbance). This may be why the grey morph is not associated with specific food plants. The striped and colour polymorphisms in T. cristinae appear to be an evolutionary consequence of differential predation on different backgrounds. The implications of differential predation to food-plant utilization are discussed.     

Ontogeny of sexual dimorphism and phenotypic integration in heritable morphs

In this study we investigated the developmental basis of adult phenotypes in a non-model organism, a polymorphic damselfly (Ischnura elegans) with three female colour morphs. This polymorphic species presents an ideal opportunity to study intraspecific variation in growth trajectories, morphological variation in size and shape during the course of ontogeny, and to relate these juvenile differences to the phenotypic differences of the discrete adult phenotypes; the two sexes and the three female morphs. We raised larvae of different families in individual enclosures in the laboratory, and traced morphological changes during the course of ontogeny. We used principal components analysis to examine the effects of Sex, Maternal morph, and Own morph on body size and body shape. We also investigated the larval fitness consequences of variation in size and shape by relating these factors to emergence success. Females grew faster than males and were larger as adults, and there was sexual dimorphism in body shape in both larval and adult stages. There were also significant effects of both maternal morph and own morph on growth rate and body shape in the larval stage. There were significant differences in body shape, but not body size, between the adult female morphs, indicating phenotypic integration between colour, melanin patterning, and body shape. Individuals that emerged successfully grew faster and had different body shape in the larval stage, indicating internal (non-ecological) selection on larval morphology. Overall, morphological differences between individuals at the larval stage carried over to the adult stage. Thus, selection in the larval stage can potentially result in correlated responses in adult phenotypes and vice versa.     

Gene frequency changes in Cepaea snails on the Marlborough Downs over 25 years

In 1985 we resurveyed the sites on the Marlborough Downs in southern England at which Cain and Currey in 1960/61 sampled Cepaea snails and thence introduced the term 'area effects' to describe large areas of uniform morph frequency. Some sites no longer harboured Cepaea and at others the species composition had changed, with a general spread of Cepaea hortensis at the expense of Cepaea nemoralis. The majority, however, permitted comparison of morph frequencies between the two surveys. In C. nemoralis, we detected a significant overall decrease in the frequency of the brown morph and estimate selection as 5–9% per generation. There was no apparent change in frequencies of banded morphs. In C. hortensis we detected a significant overall increase in the frequency of unbanded shells (1–3% selection per generation) and an almost significant decrease in the frequency of fusions within the banded class. There was insufficient colour polymorphism in C. hortensis to allow analysis of colour morph frequencies. These changes—all in the direction of reduced absorption of solar energy—resemble others detected in both species at other localities in southern England. Possible explanations include large-scale climatic effects and changes in vegetation.     

Altered genotypic and phenotypic frequencies of aphid populations under enriched CO2 and O3 atmospheres

Environmental change is anticipated to negatively affect both plant and animal populations. As abiotic factors rapidly change habitat suitability, projections range from altered genetic diversity to wide-spread species loss. Here, we assess the degree to which changes in atmospheric composition associated with environmental change will influence not only the abundance, but also the genotypic/phenotypic diversity, of herbivore populations. Using free-air CO₂ and O₃ enrichment (FACE) technology, we assess numerical responses of pea aphids (Acyrthosiphon pisum) exhibiting a pink–green genetic polymorphism and an environmentally determined wing polyphenism on broad bean plants (Vicia faba) under enriched CO₂ and/or O₃ atmospheres, over multiple generations. We show that these two greenhouse gases alter not only aphid population sizes, but also genotypic and phenotypic frequencies. As the green genotype was positively influenced by elevated CO₂ levels, but the pink genotype was not, genotypic frequencies (pink morph : green morph) ranged from 1 : 1 to 9 : 1. These two genotypes also displayed marked differences in phenotypic frequencies. The pink genotype exhibited higher levels of wing induction under all atmospheric treatments, however, this polyphenism was negatively influenced by elevated O₃ levels. Resultantly, frequencies of winged phenotypes (pink morph : green morph) varied from 10 : 1 to 332 : 1. Thus, atmospheric conditions associated with environmental change may alter not just overall population sizes, but also genotypic and phenotypic frequencies of herbivore populations, thereby influencing community and ecosystem functioning.     

Range limits,large‐scale biogeographic variation,and localized evolutionary dynamics in a polymorphic damselfly

Studies of heritable colour polymorphisms allow investigators to track the genetic dynamics of natural populations. By comparing polymorphic populations over large geographic areas and across generations, issues about both morph stability and evolutionary dynamics can be addressed, increasing our understanding of the potential mechanisms maintaining genetic polymorphisms. In the present study, we investigated population morph frequencies in a sex‐limited heritable colour polymorphic damselfly (Ischnura elegans, Vander Linden), with three discrete female morphs. We compared the frequencies of these three female morphs in 120 different populations from ten European countries at differing latitudes and longitudes. There were pronounced differences in morph frequencies both across the entire European biogeographic range, as well as at a smaller scale within regions. We also found considerable between‐population variation at the local scale within regions, particularly at the edges of the range of this species. We discuss these findings in the context of recent models of adaptive population divergence along the range of a species. This polymorphism is thus highly dynamic, with stable morph frequencies at the core of the species range but fluctuating morph dynamics at the range limits. We finish with a discussion of how local interactions and climatic factors can be expected to have a strong influence on the biogeographic patterns in this species and other sexually selected polymorphisms. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 775–785.     

Spatial analyses of two color polymorphisms in an alpine grasshopper reveal a role of small‐scale heterogeneity

Discrete color polymorphisms represent a fascinating aspect of intraspecific diversity. Color morph ratios often vary clinally, but in some cases, there are no marked clines and mixes of different morphs occur at appreciable frequencies in most populations. This poses the questions of how polymorphisms are maintained. We here study the spatial and temporal distribution of a very conspicuous color polymorphism in the club‐legged grasshopper Gomphocerus sibiricus. The species occurs in a green and a nongreen (predominately brown) morph, a green–brown polymorphism that is common among Orthopteran insects. We sampled color morph ratios at 42 sites across the alpine range of the species and related color morph ratios to local habitat parameters and climatic conditions. Green morphs occurred in both sexes, and their morph ratios were highly correlated among sites, suggesting shared control of the polymorphism in females and males. We found that in at least 40 of 42 sites green and brown morphs co‐occurred with proportions of green ranging from 0% to 70% with significant spatial heterogeneity. The proportion of green individuals tended to increase with decreasing summer and winter precipitations. Nongreen individuals can be further distinguished into brown and pied individuals, and again, this polymorphism is shared with other grasshopper species. We found pied individuals at all sites with proportions ranging from 3% to 75%, with slight, but significant variation between years. Pied morphs show a clinal increase in frequency from east to west and decreased with altitude and lower temperatures and were more common on grazed sites. The results suggest that both small‐scale and large‐scale spatial heterogeneity affects color morph ratios. The almost universal co‐occurrence of all three color morphs argues against strong effects of genetic drift. Instead, the data suggest that small‐scale migration–selection balance and/or local balancing selection maintain populations polymorphic.     

Is the colour dimorphism inDactylorhiza sambucina maintained by differential seed viability instead of frequency-dependent selection?

The European rewardless, bee-pollinated orchidDactylorhiza sambucina commonly produces yellow-flowered and purple-flowered individuals in frequencies that range from balanced (per population) to very unbalanced, with parts of the species’ range entirely monochromatic. We studied male and female reproductive success of the two morphs in 22 populations in the Czech Republic, relating it to morph frequency, population size and density, and presence and abundance of yellow and purple co-flowering nectar-providing species visited by the same bee species. Cumulative abundances of yellow nectar-producing co-flowering species (of which, on average,Primula veris made up 56%) had a negative effect on male reproductive success of the yellow morph, and spectral analyses showed that to bumblebees the colours ofP. veris and yellowD. sambucina are different, permitting ready visual discrimination. The cumulative abundance of purple co-flowering species had no significant effect on morph reproductive success. Morph frequencies were unrelated to reproductive success and population size, and there was no evidence of frequency-dependent selection except in one highly unbalanced population. Density of flowering conspecifics was negatively correlated with male reproductive success of the purple morph. Seed mass, viability, and germination success depended on whether seeds resulted from outcrossed or selfed matings and on morph colour. Selfed seeds and seeds produced by the yellow morph from yellow × yellow and yellow × purple crosses had zero germination (after three months), providing the first hint that differential vegetative fitness, rather than differential reproductive fitness via pollinator selection, may explain morph frequencies inD. sambucina.     

Opening a can of worms: unprecedented sympatric cryptic diversity within British lumbricid earthworms

Earthworms play a major role in many aspects of soil fertility, food web ecology and ecosystem functioning, and hence are frequently the subjects of, for example, ecological and toxicological research. Our aim was to examine the genetic structure of common earthworm species, to identify cryptic lineages or species that may be distinct ecotypes or biotypes (and hence confound current research based upon morphotypes) and to try to explain the massive cryptic diversity that eventually emerged. We demonstrated that species such as Allolobophora chlorotica, Aporrectodea longa, Aporrectodea rosea and Lumbricus rubellus all comprise highly divergent lineages with species-level divergence at the mitochondrial cytochrome oxidase I (COI) gene. In Allo. chlorotica alone, we found 55 haplotypes for COI, with 35 of these being found in pink and 20 in green morph worms. There were no cases of the two colour morphs sharing COI haplotypes. Phylogenetic analyses of mitochondrial COI and 16S genes showed the presence of five highly divergent lineages, suggesting the presence of multiple cryptic species within Allo. chlorotica. There was no clear geographical pattern to lineage distribution and many populations were polymorphic for both mitochondrial DNA lineage and colour morph. Amplified fragment length polymorphism results, based on two primer combinations, were broadly congruent with mitochondrial DNA results with one significant exception. Despite showing over 14% divergence at COI, amplified fragment length polymorphism markers showed that the two green morph lineages may be interbreeding and therefore represent a single taxon. The cryptic diversity revealed by these results has profound consequences for all areas of earthworm research.     

Shell colour variation in Bullia digitalis, a sand-dwelling, intertidal whelk (Gastropoda: Prosobranchia)

Bullia digitalis is an intertidal whelk that lives on sandy beaches in South Africa. It is highly variable in shell colour, with individuals varying from white to dark brown. This paper describes shell colour variation of B. digitalis at seven sites, along a 230 km coastline east of the Cape Peninsula. Seven colour forms were found: striped, violet, banded violet, banded brown, orange, pale yellow and white. These forms are probably genetically determined morphs. The striped form is the most common at all sites, constituting 53–62% of each sample. The violet is the second most common morph. Its frequencies are remarkably stable at 15–17%. The striped form blends well into the sandy environment and may therefore be of considerable cryptic value in concealing B. digitalis from predators. The violet form is highly conspicuous. Its stable frequency throughout the study area may represent a genetic balance that is not relevant to any visual advantages of the violet colour.