Arachnologische Mitteilungen 58

Prey capture in the social spider Stegodyphus sarasinorum 99 in one month.The type (order) and size of the prey remnants were noted and identified to the order level with the help of taxonomic keys. We selected two natural types of prey based on their size; a beetle (Coleoptera) and grasshopper (Ortho- ptera). Time of activity of spiders. The test periods were chosen by observing and recording the activity of spiders in the field at different times of the day (8 am to 5 pm at each hour). Obser- vations were made during 5 days chosen randomly at the be- ginning of the test. We noted the different activity of spiders including web weaving, prey capture, feeding, etc. Close ob- servation of the spiders in the field revealed increased weaving and prey capturing/feeding activity at 8.00–8.30 am, while a decrease in these activities was found at around 11.00–11.30 am. From these observations, two periods were chosen: active (8.00–8.30 am) and passive (11.00–11.30 am). Size of the colony. At the end of the experimental period, all spiders were collected and carefully counted. The average numbers of individuals were 85 per colony (range 20 to 130). Efficiency, predation and cooperation of S. sarasinorum . Grasshoppers were captured with a sweep net (Mean Length = 30 mm, SD = 0.366, n = 72), and beetles with a light trap (Mean Length = 20 mm, SD = 0.311, n = 72). Of the 144 tests, 72 tests were carried out during the inactive period and the remaining 72 tests were conducted during the active pe- riod. The test was conducted in 9 colonies over 8 days either with an equal amount of grasshoppers or beetles. We placed larger prey (grasshoppers) and smaller prey (beetles) 15 cm away from the nest entrance and observed the spider-hunting behaviour. The main events of prey capture, the number of spiders recruited, recruitment time and prey immobilization time were recorded. Statistical analysis. A Wilcoxon rank sum test (‘W’ is the test statistic) was performed to compare the frequencies of capture for the two prey types (grasshoppers and beetles) in the nine colonies, and also for analyzing immobilization time and recruitment time of two prey types during two diffe- rent periods. The Spearman’s rank correlation coefficient was computed to access the relationship between immobilization time and numbers of recruited spiders for subduing the two different prey types. A significance level of 95% was used to indicate the level of significance in the results. Statistical tests were done using the software R (R Core Team 2018). Results Natural prey of S. sarasinorum . From the nests of all colonies sampled, remnants of 120 insects were collected, identified (to insect order) and measured (Tab. 1). The median size of the prey was 10 mm and the largest prey item reached 50 mm in length. Coleoptera (40%) and Orthoptera (22%) were the most common prey types, followed by Hymenoptera (18%), Hemiptera (15%) and Isoptera (5%). We collected 19 prey in the process of being eaten (median size = 20 mm; the largest size = 40 mm in length). Coleoptera was the most numerous (63% of total captures).The data show that S. sarasinorum cat- ches prey ranging from 10 to 50 mm and a large proportion are Coleoptera and Orthoptera (Tab. 1). Efficiency, predation and cooperation of S. sarasinorum . During the active period (8.00–8.30 am) most of the spi- ders were occupied outside the nest and some of them fed on prey. But in the passive period (11.00–11.30 am) the number of spiders present outside the nest was less. Whenever prey was placed in the web it created vibrations in the silky th- reads. These vibrations allow the spider to localize the prey and move asynchronously from the nest towards the prey and entangle the prey by biting different parts of its body. After immobilization, some spiders fed on the prey and some stayed in different parts of the web while others moved into the nest. It was found that the frequency of reaction to prey did not differ in the 9 cases considered (W = 43.5, p-value = 0.821). Throughout our experiment, we found that the spiders’ con- sumption of a prey item was influenced by vibrations made by the prey and not solely by the differences in the size of gras- shoppers and beetles.The immobilization time was higher for grasshoppers than for beetles both in the active (W = 1296, p-value = 2.652 e -13 ) and passive (W = 1296, p-value = 2.716 e -13 ) periods (Tab. 2 & Fig. 2). Similarly, recruitment time was longer for grasshoppers than for beetles both in active (W = 1296, p-value = 1.58 e -14 ) and passive (W = 1296, p-value =1.619 e 14 ) periods (Tab. 2).The spiders always reacted faster to beetles than to grasshoppers (Fig. 2). In the passive periods, there was a tendency towards a positive correlation between the immobilization time and a number of recruited spiders to subdue grasshoppers (Spearman’s rank correlation, r s = 0.288, p-value = 0.087). In the case of beetles, the correlation between immobilization time and the number of recruited spiders, although numerically negative, did not significantly differ from no-correlation (r s = -0.119, p-value = 0.487). Si- milarly, in the active periods, both in the case of grasshoppers and beetles immobilization time and number of recruited spiders was not significantly correlated (r = -0.160, p-value = 0.3499; r = -0.064, p-value = 0.706) (Tab. 2). Tab. 2: Medians of the three different parameters used to explain spider cooperation in prey capture; data presented in relation to spider activity periods and prey types; in bracket: superior and inferior quartiles of the data Period Inactive Active Prey types Grasshopper Beetle Immobilization time (minutes) 20 (4–60) 10 (2–30) 30 (12–60) 6 (2–15) No. of spiders recruited 8 (3–18) 9 (6–30) 12 (6–30) 7 (3–16) Recruitment time (minutes) 8.5 (1–40) 3.5 (1–30) 21.5 (5–40) 1 (1–3) Tab. 1: Number of prey sampled from the webs of S. sarasinorum (prey remnants sampled and fresh prey captured); in brackets: percentage of each order in relation to a total number of prey sampled Coleoptera Orthoptera Hymenoptera Hemiptera Isoptera Total Remnants of the prey 48 (40%) 26 (22%) 22 (18%) 18 (15%) 6 (5%) 120 Fresh prey 12 (63%)  5 (26%)  2 (11%) 0 0  19