Rieta Gols

Adult female spiders lay batches of eggs in silken egg sacs, and after hatching, the spiderlings live for transient periods in their mother's web before dispersing. Sibling cannibalism is frequently observed among spiderlings of many species under conditions of food deprivation. Here, we conducted assays in small Petri dishes with different densities of newly hatched (second instar) spiderlings of the false widow spider, Steatoda grossa , using a split‐clutch design. Prey (freshly killed fruit flies) availability was manipulated both numerically and temporally. Offspring from 10 different females were separated as siblings into densities of two, four, or eight spiderlings per Petri dish and these were provided with either 0 flies (starvation control), two flies, four flies, or eight flies that were replenished weekly or every 3 weeks. A further control was conducted with solitary spiderlings in Petri dishes deprived of flies. The number of surviving spiderlings per Petri dish was counted every 3 days until only one remained (or until death of the solitary spiderling). Our results show that the rate of cannibalism was lower with increasing spiderling density and when fresh flies were replenished more frequently, whereas the number of flies that were provided did not affect cannibalism. In S. grossa , juvenile cannibalism occurs primarily under conditions of extreme food limitation, although in synanthropic habitats where the spider is abundant, it may be an adaptive strategy owing to the potential scarcity of prey. Under certain conditions, cannibalism in spiderlings is adaptive by eliminating competitors and providing nutrient‐rich food.

Ecosystem engineers are species that manipulate the physical state of ecosystems and thereby affect the behaviour and ecology of other species. Mature larvae of the parsnip webworm, Depressaria radiella Goeze, chew holes in the hollow stems of Heracleum sphondylium L. into which they pupate. The stems are separated into several compartments that are separated by filamentous membranes. Holes excavated by webworm larvae attract several beneficial species of arthropods that use them for shelter in autumn, including the common earwig ( Forficula auricularia L.) and the common rough woodlouse ( Porcellio scaber Latreille). If artificially made holes mimic the engineering effect of D. radiella , they could be made to attract (local) communities of beneficial arthropods and perhaps facilitate overwintering habitat. We conducted a field experiment to determine whether artificial holes perforated into stem compartments of H. sphondylium mimic the natural situation with D. radiella . At five sites near the city of Leiden, the Netherlands, H. sphondylium plants were exposed to different treatments: a single hole perforated in the first, second or third stem compartment, or in all three compartments. After 3 weeks, arthropod numbers were counted inside and around hogweed stems. The arthropod community in the stems differed from that surrounding the stems; the latter consisted mainly of woodlice and wolf spiders, whereas in the stems, in addition to woodlice, many earwigs were found and no wolf spiders. Both artificial and webworm‐excavated holes that were present at one site were used by woodlice and earwigs. The position of the holes along the stem did not affect the number of arthropods found in that segment, although the arthropods exhibited a tendency to move up the stems. The results show that artificial holes mimic webworm‐excavated holes in that both attract the same species of beneficial arthropods.