Tom D. Schultz

Lack of innate preference for morph and species identity in mate-searching Enallagma damselflies

Insect mate recognition is often viewed as stereotypic, innate, and species-specific. However, male damselflies can learn to identify female-specific color morphs as potential mates. A suite of male mimicry hypotheses assume that heteromorphic females, which differ from males in color pattern, are more easily recognized as “female” and thus lack the inherent, anti-harassment advantage that the more male-like signal provides for andromorphs. Using two measures of male preference, we investigated whether naïve males have a preexisting sensory bias for a given morph color in Enallagma civile, a species that appeared to exhibit extreme plasticity in morph expression across generations within a breeding season. E. civile males raised in the absence of females exhibited no preference for either morph, whereas males raised with one female type exhibited a learned sensory bias for that morph. Male Enallagma also lacked a bias toward conspecific females over a congeneric sister species. In a naturally naïve population of Enallagma ebrium, males reacted sexually to both morphs of Enallagma hageni as often as they did to conspecific females, whose thoracic spectra were nearly identical with those of E. hageni. Moreover, despite the similar thoracic spectra of males and andromorphs, both of which reflected UV, males rarely reacted sexually to other males. Our results falsified implicit assumptions of male mimicry hypotheses, supported learned mate recognition, and suggested a scenario for speciation via sexual conflict.

The evolution and frequency of female color morphs in Holarctic Odonata: why are male-like females typically the minority?

Abstract We compiled data on the occurrence and frequency of distinct female variants among Holarctic Odonata and interpreted the data in light of harassment-based hypotheses. The major source of male confusion for male mimicry hypotheses is predicted to be signal similarity between andromorphs and male distractors; for the learned mate recognition hypothesis (LMR), it is predicted to be variation in female signals. Mapping morphism state onto molecular phylogenies of Ischnura and Enallagma failed to resolve the general ancestral female condition. However, it appeared that the andromorphic state may be ancestral in one case, and that blue structural colors were ancestral to orange and green pigmentations. Of the polymorphic species surveyed, 13% had more than two morphs, 4% had multiple heteromorphs but no andromorph, and 7% of ‘monomorphic’ congeners were functionally polymorphic because developmental variants mate. Such female signal variation lies beyond the scope of simple male mimicry, but nevertheless should exacerbate a males problem in searching for mates. Andromorphs were the majority morph in at least some populations of 17% of the species for which data were available. Andromorph frequencies of Enallagma species were generally higher than in Ischnura species, as expected if Ischnura andromorphs have higher signal apparency. Andromorph frequency varied significantly across habitats and species, as expected if per capita harassment and signal apparency vary among habitats. Quantification of signal apparency and per capita harassment across populations and among species is required to more rigorously test the extent to which variation in signal crypsis can explain observed variation in morph frequencies.

The conspicuousness of colour cues in male pond damselflies depends on ambient light and visual system

The colours of male coenagrionid damselflies have been interpreted by some as intraspecific signals that reduce intrasexual harassment by advertising the unprofitability of pursuing conspecific males as potential mates. As visual cues, male colours should be conspicuous to other males under the specific light environments where males search for females. We tested this prediction by using spectroradiometry and two models of damselfly colour vision to determine the chromatic and achromatic contrast of males from six species of Enallagma damselflies with pond backgrounds under the ambient light conditions when each species was most active. The males of five species were active at the time when their colour was most conspicuous against aquatic vegetation. Three blue species were most active and attained their highest levels of contrast during midday, while species that became active in late afternoon or evening reflected longer wavelengths and increased in brightness contrast under low sun angles. A sixth species, Enallagma pictum, departed from this pattern. We propose that colour may serve as a signal of both sexual and species identity among males.

The utilization of patchy thermal microhabitats by the ectothermic insect predator, Cicindela sexguttata

Abstract.1. Tiger beetles (Cicindela) of open habitats have served as model ectotherms in studies of the dependence of activity and habitat utilization on temperature. Potential departures from the cicindelid model were investigated in Cicindela sexguttata, a species inhabiting forests where thermal resources are patchy and ephemeral.

Preferred Body Temperature, Metabolic Physiology, and Water Balance of Adult Cicindela longilabris: A Comparison of Populations from Boreal Habitats and Climatic Refugia

Preferred body temperatures, water-loss rates (WLR), and standard metabolic rates (SMR) were measured for adult tiger beetles (Cicindela longilabris) from disjunct populations in Arizona, Colorado, Maine, and Wisconsin. The mean body temperatures of foraging C. longilabris did not vary significantly among the four populations. The mean body temperatures of basking (29.7° C), foraging (34.1° C), and stilting (36.2° C) C. longilabris were lower than those recorded for most Cicindela species. Mean WLR of live beetles in dry air at 30° C (21.2-25.2 μg cm⁻² h⁻¹ mmHg⁻¹) was not significantly different among populations and was similar to WLR of other upland cicindelids. The SMR measured at 15°, 25°, and 30° C for freshly captured beetles varied among populations but converged after all beetles were acclimated at 25° C. Differences in SMR among populations after acclimation could not be discerned; however, the pooled SMR for C. longilabris were higher than SMR for other Cicindela species over the same temperature range. The results indicate that C. longilabris is adapted to cooler climates than are experienced by most cicindelids and are consistent with the hypothesis that C. longilabris occupies climatic refugia at lower latitudes.

Structural Colors of Tiger Beetles and Their Role in Heat Transfer through the Integument

The thermal significance of interference (metallic) colors in insect cuticles was investigated by comparing the reflectivities and heat gain of cuticles from brightly metallic and achromatic morphs of two polymorphic tiger beetle species (genus Cicindela). The cuticular basis of color differences was determined using transmission electron microscopy. The reflectivities of metallic blue-green cuticle from Cicindela horni were 6.9% higher than black cuticle from the same species at wavelengths between 400 and 700 nm, but differences between 250 and 2,000 nm were only 1.2%. In contrast, the white elytra of C. formosa were 21% more reflective in the visible and 35% more reflective in the near-infrared than metallic red elytra from the same species. When elytra were heated in pairs within a closed chamber by visible light from a tungstenhalogen lamp, no differences were observed in the temperature excesses of iridescent green and black cuticles (elytra) of C. horni (mean difference = 0.01 C). In comparisons of white and metallic red elytra of C formosa, the metallic red elytra attained significantly higher temperature excesses (mean difference = 2.2 C). Our results confirm that metallic colorations in the elytra of tiger beetles are not effective in reducing heat gain from solar radiation, but that beetles with white elytra likely experience significantly lower body temperatures than either black or iridescent beetles under comparable environmental conditions.

Lost in the crowd or hidden in the grass: signal apparency of female polymorphic damselflies in alternative habitats

Animals must locate prey and mates in noisy sensory environments. Species that rely on visual cues, and which are prey of visual predators, consequently face trade-offs. Additionally, within species, sexual conflict over mating may impose pressures to avoid both predators and mates. Many studies have attempted to explain female-specific polymorphisms in damselflies, but without considering their actual conspicuousness under natural conditions. Using models of colour perception for damselflies and birds, we assessed the detectability of female coloration to conspecific males and potential predators. Alternative colour morphs reduce female apparency either through signal similarity with conspecific males (i.e. mimicry) or by matching the noise of the visual background. The colours of male-mimicking andromorphs that reduce their apparency among groups of males at breeding sites render them highly detectable to males as well as visual predators in offshore vegetation, where females occur when not reproducing. By presenting tethered female damselflies to free-flying males amidst vegetation, we demonstrated that, among flying females, males were able to detect andromorphs more easily than the more cryptic heteromorphs. Thus, when male density is low, cryptic heteromorphs may experience less harassment than andromorphs, suggesting a scenario of disruptive selection on female coloration driven by males as well as predators. Greater attention is warranted not only to the predation risks of female signals, but also to the effect of variation in the visual environments on encounters between males and unreceptive females.

Age-Specific Behavior and Habitat Selection of Adult Male Damselflies, Calopteryx maculata (Odonata: Calopterygidae)

We compared the age, movement, and time–activity budgets of male Calopteryx maculata damselflies occupying off-stream tree-fall gaps with those at stream sites within a 10-ha woodland. All males collected at off-stream sites were younger than males collected at stream sites—as indicated by their significantly higher wing transmittance. Thirty-three percent of teneral males marked at off-stream gaps moved to stream sites within 4 days (mean distance = 140 m), while mature males marked at stream sites never left the stream. In contrast to stream site males, off-stream males spent significantly more time capturing prey and never engaged in aggressive interactions with other males. Behavioral differences were not due to variations in the operative body temperature. However, malaise trapping revealed a greater frequency of suitable prey in forest light gaps. Our findings support the idea that teneral male Calopteryx leave their emergence sites along the stream for off-stream light gaps to forage without interference and build the energy reserves necessary to attain and hold streamside territories.

Coloration and its thermal consequences in the New Zealand tiger beetle Neocicindela perhispida

Abstract 1. 1.|The thermal consequences associated with background matching were studied in two subspecies of the coastal New Zealand tiger beetle Neocicindela perhispida . 2. 2.|The dark N.p. campbelli , which inhabits hot black ironsand beaches, experiences higher body temperatures because of increased absorptive and convective heat loads. 3. 3.|During midday, it must shuttle frequently between sun and shade to prevent body temperatures from exceeding optimal levels, thus reducing its foraging time. 4. 4.|The light N.p. giveni , which inhabits white quartz sand beaches, is active for longer periods during midday because of cooler habitat temperatures and its highly reflective elytral surface.

Diversity and Habitats of a Prairie Assemblage of Odonata at Lostwood National Wildlife Refuge, North Dakota

Abstract An inventory of the dragonflies and damselflies (Insecta: Odonata) at the Lostwood National Wildlife Refuge (LNWR) was conducted during the summer of 2003. Adult censuses and larval sampling at 32 wetland sites produced 10 dragonfly and 14 damselfly species that were resident in the refuge. In 2006, two additional species were added. The odonate fauna of LWNR consisted primarily of widespread, common species that are adapted to fishless lentic communities and tolerant of alkaline and impermanent water regimes. Enallagma annexum (Hagen), Enallagma boreale Selys, Lestes disjunctus Selys, Lestes congener Hagen, Sympetrum costiferum (Hagen), and Sympetrum internum Montgomery were the most abundant odonates at the refuge. The odonate communities of semipermanent, oligosaline ponds were the most diverse and included species of Aeshna, Anax, Libellula, Leucorrhinia, Sympetrum, Lestes, Coenagrion, Enallagma, Ischnura, and Nehalennia. Large polysaline lakes were inhabited only by Ischnura damula Calvert and four species of Enallagma. Seasonal ponds that remained flooded until mid-July produced large numbers of Sympetrum internum, Lestes disjunctus, and Lestes unguiculatus. Spring-fed bogs and fens supported several species that were rare at the refuge including Sympetrum danae (Sulzer), Sympetrum semicinctum Say, and Amphiagrion abbreviatum (Selys). Long-term monitoring of odonate diversity and abundance may be useful in tracking the effects of climate change in the prairie pothole region but must take into account yearly fluctuations due to variation in winter and summer precipitation.