http://link.springer.com/content/pdf/10.1007/978-3-642-67868-4_4.pdf

Spatial Vision in Arthropods

Fecundity characteristics, phenology, and behavior of insect flower-visitors were stud- ied for 7 early flowering woodland herbs: Claytonia virginica, Dentaria laciniata, Dicentra canaden- sis, Dicentra cucullaria, Erythronium albidum, Isopyrum biternatum, and Sanguinaria canadensis. Sanguinaria canadensis is facultatively autogamous, the Dicentras are obligate outcrossers, and the remainder are self-compatible, at least within a stem. All are insect pollinated except sometimes S. canadensis. The numbers of ovules per flower and flowers per stem tended to be inversely correlated, and large-seeded species (S. canadensis, E. albidum, I. biternatum) had lower numbers of potential seeds per stem than did small-seeded species. Flowering of all species typically occurred during the first prolonged period of weather suitable for pollinator activity and ceased by the time the canopy closed. Annual differences in flowering times were associated with differences in average temperatures (i.e., early blooming in a warm, early spring), but cumulative degree-hours or degree-days of air or soil temperatures were not well corre- lated with flowering times. Other constraints on flowering phenology are discussed, including the predictability of suitable conditions, a proposed "fail-safe" mechanism that may assure flowering before canopy closure even if temperatures are abnormally low, and the importance of nontemperature factors in defining suitable conditions. Flowering time was not very finely tuned to the temperature regime and pollinator activity; flowers blooming during the flowering peak often had low seed pro- duction and the fertilization rate of most species was low. Evidence that seed production may have been pollinator limited for several species was obtained by comparing the success of hand pollination and of natural pollination, rarity of certain specialized pollinators, and estimates of the abortion rates of fertilized ovules. We suggest that flowering in early spring is a high-risk option in terms of insect- mediated sexual reproduction. Certain flower-visiting insects favored D. laciniata out of proportion to its abundance, but no effect on seed set of other species was detectable. Honeybees were abundant and active flower visitors with the potential for disrupting ecological/evolutionary relationshps between native insects and flowers.

Flowering Ecology of Some Spring Woodland Herbs

Freshly emerged flower visitors exhibit colour preferences prior to individual experience with flowers. The understanding of innate colour preferences in flower visitors requires a detailed analysis, as, on the one hand, colour is a multiple-signal stimulus, and, on the other hand, flower visits include a sequence of behavioural reactions each of which can be driven by a preferential behaviour. Behavioural reactions, such as the distant approach, the close-range orientation, the landing, and the extension of mouthparts can be triggered by colour stimuli. The physiological limitations of spectral sensitivity, the neuro-sensory filters, and the animals' different abilities to make use of visual information such as brightness perception, wavelength-specific behaviour and colour vision shape colour preferences. Besides these receiverbased factors, there are restrictions of flower colouration due to sender-based factors such as the absorption properties of floral pigments and the dual function of flower colours triggering both innate and learned behaviour. Recordings of the spectral reflection of coloured objects, which trigger innate colour preferences, provide an objective measure of the colour stimuli. Weighting the spectral reflection of coloured objects by the spectral composition of the ambient light and the spectral sensitivity of the flower visitors' photoreceptors allows the calculation of the effective stimuli. Perceptual dimensions are known for only a few taxa of flower visitors.

Innate colour preferences of flower visitors

Genetically conditioned mechanisms which restrict or limit gene exchange between coexisting species or population systems permit the preservation of their genetic integrity, and afford evolutionary independence In the absence of genetically controlled isolation, coexisting populations would fuse into a single variable population The fact that biotically sympatric populations which differ in adaptive mode almost invariably maintain their identity is prima facie evidence for their genetic isolation Genetic isolation is evident in some form in essentially every genus where it has been sought The degree of isolation need not be a function of genetic divergence or taxonomic distance, although in most instances a positive correlation may be found

The Origin of Isolating Mechanisms in Flowering Plants

Various aspects of the pollination biology of the alpine flora of Kosciusko National Park, NSW, were examined from late December 1983 until the end of March 1984, including flowering phenology, corolla tube lengths, flower colour, ultraviolet reflectance patterns, visitation rates to the flowers and proboscis lengths of the flower-visiting insects. An average of 5.3 species flowered in each of 13, 2 m×2 m montane plots and 5.6 species in the 13 alpine plots. The maximum number in flower simultaneously averaged 4.1 species in the montane and 3.3 in the alpine plots; flowering peaked in mid-January, Corolla tube lengths of the flora averaged 1.73 mm.



The most common floral colour was white or predominantly white (40 species), followed by yellow (14 species). Only six of the 38 species (16%) examined had some type of reflectance pattern; the remaining species all absobed ultraviolet.



Flies appeared to be the major pollinators. The insects collected in the study area comprised 60 species of Diptera, 33 species of Hymenoptera, and several species each of Lepidoptera and Coleoptera. On average, 14.4% of flowers watched during 379 observation periods (10 min each) were visited. On average, each plant species was visited by 6.4 species of flies, 2.4 species of bees, wasps or sawflies, one species of butterfly or moth and 0.3 species of beetles. Visitation rates increased over the growing season, and were significantly affected by ambient temperature (positively), light levels (positively) and wind speed (negatively).



The maximum proboscis length for the 25 most common species of bees was 2.76 mm, but 18 of 51 species of flies had proboscis lengths longer than this. The mean proboscis length for all 25 species of bees was 1.68 mm, and for 51 species of flies was 2.31 mm. Proboscis lengths for flies were positively correlated with the average corolla length for the plant species they visited. For bees, however, the range in proboscis lengths was relatively small and did not show this pattern.



There appear to be significant differences between the plant-pollinator community of alpine Australia and other alpine areas where bumblebees are common pollinators. These differences include shorter proboscis and corolla tube lengths, and perhaps an increased diversity and significance of flies as pollinators.

Pollination biology in the Snowy Mountains of Australia: Comparisons with montane Colorado, USA

Flowers of 300 plant species representing 61 families were surveyed for their degree and pattern of reflection of near-ultraviolet (UV) light. Flowers were ranked in terms of four degrees of UV reflection and absorption. Yellow and violet flowers have the highest probability of reflecting UV, while white and green flowers generally reflect UV poorly. Pollination guides were generally nonreflective, independent of visible color. While independent of floral symmetry, UV reflection appears to be positively correlated with flower size. Although UV reflection probably occurs in all plant families, it appears to be more common in certain taxonomic groups. Like other floral colors, UV reflection and absorption seem to depend on both the physical structure and chemical composition of the petals.

Frequency of Reflection and Absorption of Ultraviolet Light in Flowering Plants

This study investigates the influence of host plants on the expression of floral variability in the hemiparasiteOrthocarpus densiflorus (Scrophulariaceae), to determine if interaction with host plants can facilitate the expression of normally hidden genes. Style extension inO. densiflorus is highly uniform in natural populations, with less than 1% of the individuals exhibiting an aberrant extended-style condition. Both seed treatment with giberellic acid and parasitic development with host plants significantly facilitates the expression of extended-style in progenies that show only minimal expression when grown autotrophically. Artificial selection experiments demonstrate the range of potential phenotypic variation to be under genetic control, and differentially influenced by the host environment. Although the biochemical basis of the host influence is unknown, some host plants produce stimuli that apparently disrupt buffering inO. densiflorus and expose normally unexpressed genetic variability to selection. If variation in other traits is similarly affected, the influence of host chemistry may provide parasitic plants with a greater than normal opportunity for rapid selectional change.

Larry D. Guldberg

Papers

Frequency of Reflection and Absorption of Ultraviolet Light in Flowering Plants

Host influence on floral variability inOrthocarpus densiflorus (Scrophulariaceae)