Thermoregulatory significance of wing melanization in Pieris butterflies (Lepidoptera: Pieridae): physics,posture, and pattern

Summary Pieris butterflies use a novel behavioral posture for thermoregulation called reflectance basking, in which the wings are used as solar reflectors to reflect radiation to the body. As a means of exploring the thermoregulatory significance of wing melanization patterns, I examine the relation of basking posture and wing color pattern to body temperature. A mathematical model of the reflectance process predicts certain combinations of dorsal wing melanization pattern and basking posture that maximize body temperature. Laboratory experiments and field observations show that this model correctly predicts qualitative differences in the relation of body temperature to basking posture based on differences in the extent of dorsal melanization on the wing margins, both between species and between sexes within species of Pieris. This is the first demonstration in insects that coloration of the entire wing surface can affect thermoregulation. Model and experimental results suggest that, in certain wing regions, increased melanization can reduce body temperature in Pieris; this effect of melanization is exactly the opposite of that found in other Pierid butterflies that use their wings as solar absorbers. These results are discussed in terms of the evolution of wing melanization pattern and thermoregulatory behavior in butterflies.     

EVOLUTION AND COADAPTATION OF THERMOREGULATORY BEHAVIOR AND WING PIGMENTATION PATTERN IN PIERID BUTTERFLIES

This paper addresses the question of how the relationship between morphological structure and functional performance differs in related groups of organisms. I describe the relationship between a suite of phenotypic characters (behavioral posture and the pattern of wing pigmentation) and one function of these characters (thermoregulatory performance) for two groups of butterflies in the family Pieridae, focusing on how behavior and wing pattern interact to affect specific aspects of thermoregulation. Using both natural and experimentally created variation in wing-melanization patterns, I develop and test a series of predictions about the relations among thermoregulatory posture, melanization pattern, body temperature, and flight activity. Results show that increased melanization in different wing regions has positive, negative, or neutral effects in increasing body temperature of Pieris butterflies. The angle of the wings used during basking alters the relative importance of different modes of heat transfer and thereby determines the contribution of different dorsal wing regions to thermoregulation. Experimentally increased dorsal melanization can either increase or decrease the onset of flight activity and can directly alter thermoregulatory posture. For Pieris, dorsal melanization affects basking and flight, while ventral melanization primarily affects overheating. These results are used to generate a functional map relating melanization pattern to thermoregulatory performance in Pieris. Reflectance-basking posture, white background color, and melanization pattern represent coadapted characters in Pieris that interact to determine thermoregulatory performance. The differences in thermoregulatory posture and background color between pierid butterflies in the subfamilies Pierinae and Coliadinae have led to a reorganization and partial reversal of the thermoregulatory effects of melanization pattern. I suggest that this change in the physical mechanism of thermoregulatory adaption in pierids has qualitatively altered the nature of selection on wing-melanization pattern.     

Thermal ecology of Pieris butterflies (Lepidoptera: Pieridae): a new mechanism of behavioral thermoregulation

Summary I document a new mechanism for behavioral thermoregulation, not previously described in animals, called reflectance basking. This behavior, described here for Pieris butterflies, involves the use of the wings as solar reflectors that reflect solar radiation onto the body to increase body temperature. Results show that Pieris require thoracic (body) temperature. between 29° and 40° C in order to take off and fly, and achieve these elevated temperatures by basking. Diurnal patterns of population flight activity are closely correlated with patterns of body temperature during basking. Behavioral studies indicate that 1) Pieris orient to solar radiation, 2) they use thermoregulatory postures consistent with reflectance basking, and 3) they do not use the basking postures found in other Pierid butterflies (i.e., the Coliadinae). There are consistent differences in wing angles used in reflectance basking between Pieris in different subgenera. Results are discussed with respect to thermoregulation and wing color in other Pierid butterflies, and suggest that a re-evaluation of the functional significance of melanization in Pieris is needed.     

The adaptive significance of alpine melanism in the butterfly Parnassius phoebus F. (Lepidoptera: Papilionidae)

Summary The adaptive significance of alpine melanism, the tendancy for insects to become darker with increased elevation and latitude, was investigated using the butterfly Parnassius phoebus. The effects on temperature dependent activity of five components of overall wing melanism, as well as size, were examined. The components of wing melanism examined were the transparency of the basal hindwing and distal fore-wing areas, the width of the black patch in the basal hind-wing area and the proportion of black to white scales in that area, and the proportion of the distal fore-wing covered by predominantly black scaling.The body temperature of dead specimens was correlated with air temperature, solar radiation, the width of the black patch at the base of the wings, and the proportion of black to white scales at the base of the wings. The minimum air temperatures and solar radiation levels required for initiation of flight did not vary with wing melanism of P. phoebus, in contrast to the results found for Colias butterflies by Roland (1982). However, under environmental conditions suitable for flight initiation, males with a higher proportion of black to white scales in the basal area of the hind-wing, and wider basal black patches, spent a greater proportion of time in flight at low air temperatures and low insolation. Increased basal wing melanism was also associated with increased movement of males within a population. In contrast, melanism in the distal area of the wings has no effect on activities which are dependant on body temperature. The amount of time spent feeding did not vary with differences in wing melanism. I suggest that in dorsal basking, slow-flying butterflies (Parnassius) basal wing color affects body temperature primarily during flight (rather than while basking), such that butterflies with darker wing bases cool down less rapidly because they absorb more solar radiation during flight.     

Why Small Is Beautiful: Wing Colour Is Free from Thermoregulatory Constraint in the Small Lycaenid Butterfly,Polyommatus icarus

We examined the roles of wing melanisation, weight, and basking posture in thermoregulation in Polyommatus Icarus, a phenotypically variable and protandrous member of the diverse Polyommatinae (Lycaenidae). Under controlled experimental conditions, approximating to marginal environmental conditions for activity in the field (= infrequent flight, long duration basking periods), warming rates are maximised with fully open wings and maximum body temperatures are dependent on weight. Variation in wing melanisation within and between sexes has no effect on warming rates; males and females which differ in melanisation had similar warming rates. Posture also affected cooling rates, consistent with cooling being dependent on convective heat loss. We hypothesise that for this small sized butterfly, melanisation has little or no effect on thermoregulation. This may be a factor contributing to the diversity of wing colours in the Polyommatinae. Because of the importance of size for thermoregulation in this small butterfly, requirements for attaining a suitable size to confer thermal stability in adults may also be a factor influencing larval feeding rates, development time and patterns of voltinism. Our findings indicate that commonly accepted views of the importance of melanisation, posture and size to thermoregulation, developed using medium and large sized butterflies, are not necessarily applicable to small sized butterflies.     

Thermal characterization of butterfly wings—1. Absorption in relation to different color, surface structure and basking type

1. 1. Spectral integral reflectance, transmittance and the resulting absorption of intact and descaled butterfly wings of the black-winged Pachliopta aristolochiae (Papilionidae), the white-winged Pieris brassicae (Pieridae), and the yellow-winged Gonepteryx rhamni (Pieridae) were determined between 350 and 800 nm.

2. 2. Whereas in the black forewing of the dorsal basking Pachliopta almost all incident light is absorbed nearly independent of the wavelength and thus converted into heat, the white forewing of the body basking Pieris absorbs less than 20% in the visible range of the spectrum.

3. 3. The yellow hindwing of the lateral basking Gonepteryx absorbs to a higher degree than the Pierid wing, but—due to the sparsely arranged scales—transmittance is clearly increased (40–50% between 525 and 800 nm).

4. 4. The varying thermal characteristics of the different wings with reference to the color and arrangement of the scales and the different basking strategies of the butterflies are discussed.

Seasonal phenotypic plasticity of wing melanisation in the cabbage white butterfly, Pieris rapae L. (Lepidoptera: Pieridae)

Abstract.  1. Effective thermoregulation is crucial for the fitness of small flying insects. Phenotypic plasticity of the ventral hindwing of pierid butterflies is widely recognised as adaptive for effective thermoregulation. Butterflies eclosing in cooler environments have more heavily melanised wings that absorb solar radiation, thus allowing flight under these cool conditions.
2. Many pierids also exhibit phenotypic plasticity of dorsal forewing melanisation but in this case, cooler environments reduce melanisation. It has been hypothesised that this plasticity is also adaptive because it increases solar reflection from the wing surfaces onto the body in certain basking postures.
3. The degree of seasonal variation in ventral hindwing and dorsal forewing melanisation of wild-caught Pieris rapae was quantified to determine if it shows patterns of plasticity similar to that documented for other Pieris species.
4. Male wing melanisation on both wing surfaces shows the characteristic seasonal, adaptive plasticity. However, only some dorsal forewing pattern elements of females conformed to the predictions of the hypothesis of adaptive dorsal forewing melanisation. Sexual dimorphism of wing pattern plasticity may result from, and/or affect, sexual dimorphism of behaviour and physiology of these butterflies.     

Altitude and life‐history shape the evolution of Heliconius wings

Phenotypic divergence between closely related species has long interested biologists. Taxa that inhabit a range of environments and have diverse natural histories can help understand how selection drives phenotypic divergence. In butterflies, wing color patterns have been extensively studied but diversity in wing shape and size is less well understood. Here, we assess the relative importance of phylogenetic relatedness, natural history, and habitat on shaping wing morphology in a large dataset of over 3500 individuals, representing 13 Heliconius species from across the Neotropics. We find that both larval and adult behavioral ecology correlate with patterns of wing sexual dimorphism and adult size. Species with solitary larvae have larger adult males, in contrast to gregarious Heliconius species, and indeed most Lepidoptera, where females are larger. Species in the pupal‐mating clade are smaller than those in the adult‐mating clade. Interestingly, we find that high‐altitude species tend to have rounder wings and, in one of the two major Heliconius clades, are also bigger than their lowland relatives. Furthermore, within two widespread species, we find that high‐altitude populations also have rounder wings. Thus, we reveal novel adaptive wing morphological divergence among Heliconius species beyond that imposed by natural selection on aposematic wing coloration.     

Evidence for Partial Migration in the Southern Monarch Butterfly,Danaus erippus,in Bolivia and Argentina

Migration is a common life‐history strategy that includes traits such as directed flight, increased wing size, seasonal lipid deposition and reproductive arrest. The degree of investment in these traits ultimately determines the life‐history strategy of individuals. Partial migration is a common mixed life‐history strategy where species or populations consist of both migrant and resident individuals. While this phenomenon is widespread across taxa, the ecological factors that select for and maintain partial migration are poorly understood, especially among insects. Here, we investigate regional life‐history traits associated with migration in the southern monarch, Danaus erippus, and describe a mixed life‐history strategy in this butterfly. Individuals from the Bolivian lowlands were observed throughout the year exhibiting mate‐ and milkweed‐directed behaviors. These butterflies had smaller wings, lower wing loads and maintained constant lipid and egg loads across summer and autumn months. Danaus erippus in the highlands of the Bolivian Andes were observed only in the summer and autumn months, during which they also showed mate‐ and milkweed‐directed behaviors. These individuals possessed similar‐sized wings and maintained similar lipid and egg loads as the lowland butterflies. In contrast, individuals from northwest Argentina showed persistent, directed, southwesterly flight during the autumn (March–May), larger wing size, higher wing loads, and increased autumn lipid deposition along with decreased egg production. These data indicate that D. erippus utilizes a mixed life‐history strategy with a combination of residents and migrants in the Bolivian lowlands, elevational migrants in the Bolivian Andes, and latitudinal migrants in northwestern Argentina.     

Behavioural thermoregulation and the relative roles of convection and radiation in a basking butterfly

Poikilothermic animals are often reliant on behavioural thermoregulation to elevate core-body temperature above the temperature of their surroundings. Butterflies are able to do this by altering body posture and location while basking, however the specific mechanisms that achieve such regulation vary among species. The role of the wings has been particularly difficult to describe, with uncertainty surrounding whether they are positioned to reduce convective heat loss or to maximise heat gained through radiation. Characterisation of the extent to which these processes affect core-body temperature will provide insights into the way in which a species׳ thermal sensitivity and morphological traits have evolved. We conducted field and laboratory measurements to assess how basking posture affects the core-body temperature of an Australian butterfly, the common brown (Heteronympha merope). We show that, with wings held open, heat lost through convection is reduced while heat gained through radiation is simultaneously maximised. These responses have been incorporated into a biophysical model that accurately predicts the core-body temperature of basking specimens in the field, providing a powerful tool to explore how climate constrains the distribution and abundance of basking butterflies.     

The cost of melanization: butterfly wing coloration under environmental stress

Evolutionary studies typically focus on adaptations to particular environmental conditions, thereby often ignoring the role of possible constraints. Here we focus on the case of variation in dorsal wing melanization in a satyrine butterfly Pararge aegeria. Because melanin is a complex polymer, its synthesis may be constrained if ambient conditions limit the resource budget. This hypothesis was tested by comparing melanization among butterflies that fed as larvae on host grasses experiencing different drought-stress treatments. Treatment differences were validated both at the level of the host plant (nitrogen, carbonate, and water content) and of the butterfly (life-history traits: survival, development time, and size at maturity). Melanization rate was measured as average gray value of the basal dorsal wing area. This area, close to the thorax, is known to be functionally significant for basking in order to thermoregulate. Individuals reared on drought-stressed host plants developed paler wings, and development of darker individuals was slower and less stable as estimated by their level of fluctuating asymmetry. These results provide evidence that melanin is indeed costly to synthesize, and that differences in environmental quality can induce phenotypic variation in wing melanization. Therefore, studies dealing with spatial and/or temporal patterns of variation in wing melanization should not focus on adaptive explanations alone, but rather on a cost-benefit balance under particular sets of environmental conditions.     

The rôle of butterfly wings in regulation of body temperature

Free and unnarcotized butterflies in a vertical basking position were exposed to radiation from a halogen lamp. Warming rate and equilibrium excess temperatures were recorded by means of microthermistors on the cuticle. Living, dead, and dried specimens were irradiated partly and totally. If the wings are shaded, the excess body temperature is reduced by about 30 per cent. The major portion of the heat transferred from the wing to the body originates from 15 per cent of the wing surface nearest to the body. There is no significant difference in excess thoracic temperatures of living and freshly dead specimens. After drying, the body temperature level rises about 1·4 to 2·2°C, remaining almost constant between 15°C (not radiated) and 37°C (radiated). The influence of air convection was tested with dried specimens under varying spatial orientation, keeping incident radiation constant. In an approximately horizontal position the heat arising from the wing increases to about 40 per cent by accumulation of warm air under the wing base. The ecological implications of heat supply by the wings and adaptive significance of wing pattern are discussed with respect to different modes of heat transport     

Thermal physiological ecology of Colias butterflies in flight

Summary As a comparison to the many studies of larger flying insects, we carried out an initial study of heat balance and thermal dependence of flight of a small butterfly (Colias) in a wind tunnel and in the wild.Unlike many larger, or facultatively endothermic insects, Colias do not regulate heat loss by altering hemolymph circulation between thorax and abdomen as a function of body temperature. During flight, thermal excess of the abdomen above ambient temperature is weakly but consistently coupled to that of the thorax. Total heat loss is best expressed as the sum of heat loss from the head and thorex combined plus heat loss from the abdomen because the whole body is not isothermal. Convective cooling is a simple linear function of the square root of air speed from 0.2 to 2.0 m/s in the wind tunnel. Solar heat flux is the main source of heat gain in flight, just as it is the exclusive source for warmup at rest. The balance of heat gain from sunlight versus heat loss from convection and radiation does not appear to change by more than a few percent between the wings-closed basking posture and the variable opening of wings in flight, although several aspects require further study. Heat generation by action of the flight muscles is small (on the order of 100 m W/g tissue) compared to values reported for other strongly flying insects. Colias appears to have only very limited capacity to modulate flight performance. Wing beat frequency varies from 12–19 Hz depending on body mass, air speed, and thoracic temperature. At suboptimal flight temperatures, wing beat frequency increases significantly with thoracic temperature and body mass but is independent of air speed. Within the reported thermal optimum of 35–39°C, wing beat frequency is negatively dependent on air speed at values above 1.5 m/s, but independent of mass and body temperature. Flight preference of butterflies in the wind tunnel is for air speeds of 0.5–1.5 m/s, and no flight occurs at or above 2.5 m/s. Voluntary flight initiation in the wild occurs only at air speeds 1.4 m/s.In the field, Colias fly just above the vegetation at body temperatures of 1–2°C greater than when basking at the top of the vegetation. These measurements are consistent with our findings on low heat gain from muscular activity during flight. Basking temperatures of butterflies sheltered from the wind within the vegetation were 1–2°C greater than flight temperatures at vegetation height.     

Variations in the arrangement of sensory bristles along butterfly wing margins

The surfaces of insect wings exhibit numerous sensilla, which have been suggested to have a behavioral function. Some evidence suggests that the sensory bristles along the wing margin of lepidopteran insects (butterflies and moths) are involved in the regulation of wing movement. We investigated the arrangement of sensory bristles along the wing margins of 62 species of papilionoid butterflies, using light-microscopic examination of mounts of whole wings after removing the scales surrounding the bristles. In the majority of the wings examined, bristles were located on the ventral wing surfaces and were continuously distributed along the wing margins, except in the vicinity of the wing bases. In some wings, bristles were also located on the dorsal wing surfaces, and were continuously or discontinuously distributed along the wing margins of different species. In a minority of the species studied, we observed bristle distribution in the vicinity of the wing base, discontinuous bristle distribution on both the dorsal and ventral wing surfaces, or an absence of bristles along the wing margins. This variation in the arrangement of bristles along the wing margins is discussed in relation to the reception and transmission of sensory information in the wings.     

Avoiding Male Harassment: Wing‐Closing Reactions to Flying Individuals by Female Small Copper Butterflies

Males of many butterfly species persistently court and attempt to mate with females even if the females reject courtship. This male harassment almost certainly has negative effects on female fitness. Therefore, females have likely evolved strategies to avoid such encounters. To investigate the harassment avoidance strategy of females of the small copper butterfly, Lycaena phlaeas daimio, I observed the reactions of females to other individuals flying nearby in the field. In response to the conspecific butterflies, females closed their wings if they had previously been open and did not exhibit any action if the wings had been closed. Females that closed their open wings in response to a conspecific received fewer mating attempts than did females that held their wings open. These results indicate that the wing‐closing behaviour of L. phlaeas females functions to deter male mating attempts. The wing‐closing reaction occurred primarily in mated females. Because females of L. phlaeas copulate only once during their lives, this behaviour is not considered an indirect mate choice but rather an attempt to avoid persistent mating attempts (i.e. sexual harassment) by males.     

Colour pattern homology and evolution in Vanessa butterflies (Nymphalidae: Nymphalini): eyespot characters

Ocelli are serially repeated colour patterns on the wings of many butterflies. Eyespots are elaborate ocelli that function in predator avoidance and deterrence as well as in mate choice. A phylogenetic approach was used to study ocelli and eyespot evolution in Vanessa butterflies, a genus exhibiting diverse phenotypes among these serial homologs. Forty‐four morphological characters based on eyespot number, arrangement, shape and the number of elements in each eyespot were defined and scored. Ocelli from eight wing cells on the dorsal and ventral surfaces of the forewing and hindwing were evaluated. The evolution of these characters was traced over a phylogeny of Vanessa based on 7750 DNA base pairs from 10 genes. Our reconstruction predicts that the ancestral Vanessa had 5 serially arranged ocelli on all four wing surfaces. The ancestral state on the dorsal forewing and ventral hindwing was ocelli arranged in two heterogeneous groups. On the dorsal hindwing, the ancestral state was either homogenous or ocelli arranged in two heterogeneous groups. On the ventral forewing, we determined that the ancestral state was organized into three heterogeneous groups. In Vanessa, almost all ocelli are individuated and capable of independent evolution relative to other colour patterns except for the ocelli in cells ?1 and 0 on the dorsal and ventral forewings, which appear to be constrained to evolve in parallel. The genus Vanessa is a good model system for the study of serial homology and the interaction of selective forces with developmental architecture to produce diversity in butterfly colour patterns.     

The responses of wild jacamars (Galbula ruficauda,Galbulidae) to aposematic,aposematic and cryptic,and cryptic butterflies in central Brazil

1. This article reports the responses of wild, adult jacamars to butterflies with distinct coloration types in central Brazil. Fully aposematic species, i.e. those exhibiting bright and/or contrasting colours on both wing surfaces (= A/A), were predominantly sight‐rejected by birds and, with one exception, the few butterflies attacked and captured were taste‐rejected afterwards. 2. Aposematic and cryptic butterflies, i.e. those exhibiting bright and/or contrasting colours on the upper and cryptic colours on the underwings (= A/C) were sight‐rejected while flying, when they show their conspicuous colours to predators. This suggests that birds associate butterfly colours with their difficulty of capture, as in the case of Morpho and several Coliadinae species. These butterflies, however, were heavily attacked at rest, when they are cryptic. 3, Fully cryptic butterflies, i.e. those exhibiting cryptic colours on both wing surfaces (= C/C) did not elicit sight rejections by birds. Comparisons involving the number of attacks and the capture success of flying and resting individuals showed no significant differences in species more frequently observed like some cracker butterflies (Hamadryas feronia and H. februa) and Taygetis laches. Compared with the A/C Coliadinae, these butterflies showed a lesser, although not significantly different, ability to escape while flying, but a greater and significantly different ability to escape while at rest. 4, A hunting tactic of jacamars, which consists of following flying A/C and C/C butterflies on sight, and waiting until they perch to locate and attack them, is described for the first time.     

Rearing the pale grass blue Zizeeria maha (Lepidoptera,Lycaenidae): Toward the establishment of a lycaenid model system for butterfly physiology and genetics

Although some nymphalid butterflies have been intensively used to study mechanisms of the colour pattern formation on butterfly wings, lycaenid butterflies are equally attractive, having easily identifiable distinct spot patterns and highly diverse colour patterns among species. To establish a lycaenid model system for physiological and genetic experiments, we here describe a series of methods for rearing the Japanese pale grass blue Zizeeria maha (Kollar) (Lepidoptera, Lycaenidae) in a small laboratory space with an artificial diet for generations. Adult individuals readily mated and oviposited in a small cage with sufficient light, flowers, and host plants. Eggs were harvested in the cage, and larvae were successfully reared to normal adults with an artificial diet made from fresh leaves (AD‐F), although they were smaller than those reared with a natural diet. Feeding an artificial diet made from dried leaves (AD‐D) frequently produced adult individuals with aberrant wing colour patterns. Using our rearing methods, it is now possible to rear this species in a laboratory and to establish specific strains for physiological and genetic experiments on the wing colour pattern development, diversity, and evolution.     

EXPERIMENTAL ANALYSES OF WING SIZE,FLIGHT, AND SURVIVAL IN THE WESTERN WHITE BUTTERFLY

Butterflies have distinctively large wings relative to body size, but the functional and fitness consequences of wing size for butterflies are largely unknown. I use natural and experimentally generated variation in wing surface area to examine how decreased wing size affects flight and survival in a population of the western white butterfly, Pontia occidentalis. In the laboratory, experimental reductions in wing area (reduced-wings manipulation) significantly increased wingbeat frequencies of hovering butterflies, whereas a control manipulation had no detectable effects. In contrast, behavioral observations and mark-release-recapture (MRR) studies in the field detected no significant differences in flight activity, initial dispersal rates, or recapture probabilities among treatment groups. Estimated selection coefficients indicated that natural variation in wing size, body mass, and wing loading in the population were not significantly correlated with survival in the two MRR studies. In two mark-recapture studies with manipulated butterflies, survival probabilities were not significantly different for reduced-wings individuals compared with control or unmanipulated individuals. In summary, experimental reductions in wing area significantly altered aspects of flight in the laboratory, but did not detectably alter flight or survival in the field for this population. The large wing size typical of butterflies may reduce the functional and survival consequences of wing size variation within populations.