Ten ant species (Hymenoptera: Formicidae) from three subfamilies feed on honeydew excreted by nymphs of the leafhopper Balclutha punctata (Hemiptera: Cicadellidae). This relationship is facultative for the partners. Preliminary field observations suggest a mutualistic interaction that represents an intermediate stage in the spectrum of formicid-hemipteran mutualisms. Behavioural adaptations of the leafhopper (no escape reaction) and the ants (no predation of leafhoppers, protection from spiders) are evident, but characters of advanced facultative mutualism such as physical contact between the partners or ant-triggered excretion of honeydew were not observed.
A permanent snow cover for several months is typical for large parts of Norway, Sweden and Finland. Snow layers thicker than about 20 cm insulate the soil surface and stabilize the ground temperature close to 0°C. Many ground-living invertebrates are active at this temperature in the subnivean air space. From this "base camp", some invertebrates migrate upwards to use the snow as a substrate. The intranivean fauna consists of springtails (Collembola) and mites (Acari) that are small enough to move within the narrow pores between snow crystals. The supranivean fauna consists of various invertebrates that are active on the snow surface. Some of them are Collembola that have migrated through the snow layers. However, most of them are larger insects and spiders which migrate between the subnivean and supranivean habitats following air channels which are naturally created along tree stems, bushes etc. penetrating the snow. Likewise, certain Chironomidae and Plecoptera, hatching from winter-open rivers and brooks, are active on the snow surface. The supranivean arthropod fauna has the following characteristics: 1. It is a weather dependent assemblage of species, coming and going with changes in air temperature, cloud cover, and wind. Below ca. -6°C animals are absent, but at temperatures around or above zero, many groups can be simultaneously active on snow. 2. The snow surface fauna shows phenological changes throughout the winter, as certain species and groups are mainly active during certain months. 3. Some invertebrates are highly specialized and take advantage of the snow surface as an arena in their life cycle. Examples are Hypogastrura socialis (Collembola), and the two wingless insects Chionea sp. (Diptera: Limoniidae) and Boreus sp. (Mecoptera). They use the smooth snow surface for efficient migration. Chionea sp. and Boreus sp. lay their eggs during the snow-covered period, while H. socialis migrates to create new colonies. The cold tolerant spider Bolephthyphantes index is unique in constructing webs in small depressions on the snow, to catch migrating Collembola. Various adaptations for using the snow as a substrate are discussed. Besides physiological and morphological adaptations, snow surface arthropods show special behavioural adaptations. Most conspicuous is the ability of several Collembola species to navigate during migration, using the position of the sun for orientation. Furthermore, in Collembola and Mecoptera, jumping as an original mechanism to escape predators has independently evolved into a migrating mechanism. An evolutionary potential exists for more invertebrate groups to take advantage of snow as a substrate in their life cycle. For instance, several more cold tolerant spiders might evolve the ability to catch migrating Collembola on snow.
Circadian rhythms play an essential role in the adaptation of organisms to the environment and may show species-specific or sex-specific differences even within a closely related taxonomic group. Although spiders (Araneae) are sexually dimorphic in several morphological and behavioural features, there are very few studies on the sex-specific differences in their biological rhythms. This study evaluated the circadian rhythm in the locomotor activity of two agrobiont hunting species of spider, Carrhotus xanthogramma (Latreille, 1819) (Salticidae) and Philodromus cespitum (Walckenaer, 1802) (Philodromidae), under natural photoperiod conditions. Particular attention was paid to possible differences between the sexes in both species. We found that C. xanthogramma is a strictly diurnal species with a mean activity peak in the morning in both sexes and the females are more active than males. The locomotor activity rhythm of males was richer in ultradian (shorter than a day but longer than an hour) components, although the relative power of these components was negligible compared to the main, 24-h period component. In accordance with these results, the diel pattern of locomotor activity of C. xanthogramma can be described by a unimodal cosine curve. In contrast to C. xanthogramma, both sexes of Ph. cespitum showed cathemeral activity (i.e., activity occur within both the light and dark portions of the daily cycle) and females and males follow quite different activity schedules: females were most active at night, shortly before nautical dawn, whereas males were most active early in the morning. Unlike C. xanthogramma, Ph. cespitum has more ultradian components, with higher relative power especially in females, where besides the 24-h circadian component there is a particularly strong 12-h ultradian period. Based on these factors, females of Ph. cespitum show a bimodal and males a unimodal pattern.
Roadside verges in densely populated areas are often a significant addition to the total semi-natural area and as such may contribute to the conservation of biodiversity. Furthermore, they can enhance the ecological cohesion of a region, especially when the existing nature reserves are small and/or highly fragmented. We investigated the occurrence of ground beetles and spiders in six highway verges with grey hair-grass vegetation in the Veluwe region, The Netherlands. Total species number in the verges was similar to the values found in nearby nature reserves with comparable vegetation, but the ground beetles tended to be more abundant in the reserves. Many stenotopic species were present in the verges, confirming the significant conservation value of this habitat. However, compared with the nature reserves, there were fewer species of stenotopic ground beetles and the stenotopic spiders were less abundant in the verges. From our knowledge of the biology and ecology of the species captured it seems likely that this is attributable to differences in "habitat quality". In the verges, species preferring bare sand are scarcer, and the weighted mean body length of ground beetles is shorter than in nature reserves. This suggests that the sward in the road verges is too dense and the suitable vegetation too patchy to sustain some of the stenotopic species. Habitat analyses support this contention: Compared with the nature reserves, the verges had proportionally less bare sand, more herb and tree cover and the vegetation patches were smaller. The strategy suggested to promote stenotopic species entails removing encroaching trees and shrubs from the verges in order to expand the nutrient-poor zone and (re-)create pioneer conditions. Subsequent management should aim at further improving the road verges as extensions of nutrient-poor habitat.
Pitfall traps are widely used in terrestrial ecology to capture ground-dwelling arthropods. In order to determine the effect of covers placed over pitfall traps on their efficiency at capturing of spiders, carabid beetles and ants, four types of pitfall trap were sunk into the ground at dry grassland sites: three pitfall traps were not covered, three were covered with white, three with green and three with black plastic covers. The total catch was 9,364 spiders, 4,352 carabid beetles and 4,157 ants. The distribution of species of spiders and carabid beetles, and the total catch of spiders, carabid beetles and ants did not differ significantly among the four types of trap. Therefore, covers do not appear to affect the capture efficiency of pitfall traps.
The relationship between web design and prey capture in orb-web spiders was examined by correlating the mean mesh height with the mean prey length per species taken from existing literature (15 species) and new data (Larinioides sclopetarius and Argiope keyserlingi). Pooling the data from all species, the results revealed no significant relationship. Analysing the data from L. sclopetarius and A. keyserlingi separately, no overall significant relationship was found. However, the analyses of the separate observation days showed that mesh height correlated significantly with prey length on one of the five observation days for A. keyserlingi, but not for L. sclopetarius. Consequently, the spacing of the sticky spiral in the orb-web can have a significant effect on the length of the captured prey under certain circumstances, which are discussed in the present paper.
Pitfall trapping is one of the most commonly used methods of sampling ground-dwelling arthropods. There are many studies on the effect of design of pitfall traps on their capture efficiency but, so far, none on the influence of the colour of the pitfall trap. In this study the abundances of seven invertebrate groups (Apidae, Araneae, Carabidae, Diptera, Formicidae, Isopoda, Vespinae) caught in pitfall traps of different colours (white, yellow, green, brown) at a dense and dry grassland site in Northwestern Germany were determined. White and yellow pitfall traps caught by far the highest numbers of individuals of Apidae, Araneae, Carabidae, Diptera and Formicidae. Isopoda were most abundant in the catches of green and brown traps. Differences among numbers caught were significant for Apidae, Araneae, Carabidae and Diptera. The effect of colour on the catches did not differ significantly between sites. Knowledge of the variation in the catching efficiency of differently coloured pitfall traps is important for designing invertebrate surveys, both from a scientific and ethical point of view.
Pitfall traps are widely used for sampling ground-dwelling arthropods. Their sampling efficiency is affected by several factors, e.g. material, size and modification of parts of the trap and sampling design. Pitfall trap sampling is also affected by the accumulation of plant litter in the traps, rain fall and by-catches of small vertebrates, which may cause a bias in the catch by obstructing traps or attracting certain insects. A roof that prevents rain and plant litter entering a trap, prevents dilution of the preservative and escape of arthropods. The main goal of present study was to compare the effect of four types of differently combined funnel and roof pitfall traps on the capture efficiency of epigeal arthropods. We found that a funnel and/or a roof had no effect on spider catches. Total abundance of large carabids and thus the total abundance of ground beetles was lower in funnel pitfall traps without a roof than in other types of traps. However, funnel pitfall traps with roofs collected significantly more carabid beetles, especially individuals of those species that are large or good fliers. We conclude that funnel pitfall traps with roofs have no negative effects on capture efficiency of ground beetles and spiders, therefore application of this sampling technique is strongly recommended.
The relative occurrence and seasonal abundance of aphidophagous predators (Insecta and Arachnida) were determined in alfalfa fields at two locations in the northeast Iberian Peninsula, using D-Vac and sweep-net sampling techniques. Among the insects, Heteroptera, Dermaptera, Neuroptera, Coleoptera and Diptera were found. The relative abundance of predators was similar at the two locations. Polyphagous predators were much more abundant than aphid specific species. Heteroptera were the most abundant order of insect predators: members of the Nabidae, Anthocoridae and Miridae were very common. Coleoptera were also abundant, represented mainly by the family Staphylinidae. Coccinellidae were less abundant and their occurrence coincided with maximum aphid presence. There was a low occurrence of other aphid-specific predators, i.e. Diptera: Syrphidae and Neuroptera: Chrysopidae. Arachnida varied between localities (from 35% to 65%). Over a five-year period, numerical responses by Orius spp., Nabis provencalis and Coccinellidae to aphids were found using polynomial regression models; however the abundance of predators was weakly explained by aphid abundance. Alfalfa, maize and winter cereals share similar aphid predator groups and these crops partially coincide in time, enabling the movement of predators among crops. The persistence of several predatory groups (Anthocoridae, Nabidae, Staphylinidae, Araneae) during most of the season suggests that alfalfa plays a major role as a reservoir of aphid predators.
The absolute population density of adult Pardosa agrestis (Westring, 1862), the dominant epigeic spider species in many arable lands in Central Europe, was quantified in two alfalfa fields using a multiple mark-recapture method. The resulting density estimates are presented together with catch data from simultaneously performed suction sampling and pitfall trapping. Two week long mark-recapture surveys were conducted in August 1995 and 1996 using grids of 11 × 11 live-catching pitfall traps covering a square area of 400 m2 in the first, and 900 m2 in the second survey. The trap checking and marking procedure, using individual codes, was done daily. Over 5,000 spiders were marked in the two surveys. The number of spiders caught varied greatly between days. The recapture rate was also variable, ranging between 5-19%. Recaptured animals moved considerable daily distances, marked animals left the area of the trapping grid within a few days. Population sizes were estimated for 3 day time windows during which period the population could be considered closed. Spider densities were calculated from the estimated abundances and the capture area, which consisted of the grid area plus a boundary strip calculated from the estimated movement ranges of spiders. The resulting densities were close to 2 males and 1 female per m2 in the first, and 4.5 males and 4.5 females per m2 in the second experimental site. Suction sampling caught very few adult individuals and gave unsatisfactory data for statistical comparison. Live catching pitfall trap catches did not correlate with the derived mark-recapture density estimates across the short time windows, but for the entire trapping sessions the catches were consistent with the density estimates.