Stegodyphus sarasinorum, , Karsh, 1891

Drisya-Mohan, Ovatt Mohanan, Kavyamol, Pallath & Sudhikumar, Ambalaparambil Vasu, 2019, Effect of Kleptoparasitic Ants on the Foraging Behavior of a Social Spider (Karsch, 1891)., Zoological Studies 58 (3), pp. 1-11 : 2-5

Kleptoparasites of S. sarasinorum

The nests of the spiders were examined in its habitat at different locations of Christ College campus; covering both winter and summer seasons. S. sarasinorum thrives on shrubs and the lower branches of tall plants. Kleptoparasites were collected by hand as follows. The specimens were collected by leading them into glass tubes containing alcohol with the help of a brush dipped in alcohol. The collected spiders and nonspider kleptoparasites were preserved in 70% alcohol.

The collected specimens were then taken to the laboratory and examined under Magnus MSZ TR stereo microscope for taxonomic identification. Kleptoparasitic spiders were identified with the help of available literature ( Sebastian and Peter 2009; WSC 2018) and kleptoparasitic ants were identified using the catalogue of ants ( Bolton et al. 2007) and AntWeb online database (2017, http://www.antweb.org). The specimens are housed in the reference collection maintained at the Centre for Animal Taxonomy and Ecology, Department of Zoology, Christ College, Irinjalakuda, Kerala, India (ID – CATE0306).

Effect of exposure of ants on web building behavior of S. sarasinorum

S. sarasinorum was collected from Christ College campus. Adult and subadult females were used for this study. Spiders (two groups, 30 spiders each) were introduced into the 45 × 30 × 30 cm enclosures ( Fig. 2B ) made up of nylon mesh and wooden reapers (Wooden sticks used for making the frame of the enclosure). Twigs and wet cotton were placed in the enclosure which facilitates web building and maintains moisture content in the enclosure respectively. Grasshoppers (Chortopaga viridifasciata (0.22 ± 0.008 g), Oxya hyla intricata (0.13 ± 0.007 g), and Neorthacris simulans (0.28 ± 0.014 g )) were collected from the field and grown in an artificial terrarium (60 × 30 × 30 cm) and they were used as prey for the spiders. Two similarsized grasshoppers were given to each group daily in order to homogenize the feeding status (We observed the influence of feeding status and exposure of ants). After 5 days, the first group received two grasshoppers (“fed”) while the second group received no prey (“fasted”) over the next 4 days.

These two groups were again subdivided into two sub-groups (15 spiders each). The facultative kleptoparasitic ants, Oecophylla smaragdina (K) were collected from Christ College campus and 10 individuals each were exposed into one of the fasted and fed sub-groups. Some ants moved on the web and some others moved on the enclosure ( Fig. 2C ). After 1 hour, ants were removed and all webs both in the K exposed and K unexposed groups were destroyed manually. The experimental setup with spiders alone was kept undisturbed for the next four days except during daily web size measurements. The day 1 measurement was done 24 hrs after the web was destroyed. The process was not invasive as the measurements were taken by opening the lid gently without disturbing the web. For analyzing the surface area of the rebuilt web, we used 30 cm ruler, 180° protractor and dividers to measure the edges and angles and approximated it in to many polygons (Rectangular or triangular structure) to calculate the surface area. The dimensions of the surface area of the three replicates of the experiment were measured on 4 consecutive days.

Statistical analyses

Welch Two independent Sample t-test was performed to compare the means of the web size built on the first day in the K exposed and K unexposed groups. Two-way ANOVA was conducted for analyzing the significance of the size of the web in the K exposed and K unexposed groups on any of the experimental days and also to compare the relationship between the web sizes built by the fed (K exposed vs K unexposed) and fasted (K exposed vs K unexposed) spider groups on the same. Significance level 95% has been used to indicate the level of significance in the result. Statistical tests were done using the software R (R Development Core Team 2018). Daily changes in web rebuilding size were measured and analyzed.

Effect of exposure of ants on prey capture ability of S. sarasinorum

We collected a second set of spiders for this experiment. The spiders were prepared as before except for the following differences. Spiders were assigned in to two groups (30 spiders each) and fed with two similar sized grasshoppers per week for two weeks until acclimated (The spiders were fed weekly as the influence of ant exposure rather than previous feeding on prey capturing ability was taken into consideration). Spider mass was calculated using electronic balance (Readability- 0.001 g) to get the equal-sized groups. The ant O. smaragdina (K) was introduced in to the web of one group. After that, each of the weighed grasshoppers

(A)

(C)

was introduced to both groups (In K exposed group; grasshoppers were introduced one hour after K exposure). The grasshoppers were placed on to the web at a distance of 10 cm from the nest and awaiting its capture. Sometimes in both groups spiders were attracted towards the grasshopper and sometimes the grasshoppers escape from the vicinity of spiders. So we replaced the new grasshopper for the unsuccessful prey capture. We noted the handling time (The time started from the first attack of the spider towards its prey until its immobilization) in three different times of the day (9 am, 1 pm and 5 pm). The difference in the time of the attack in the two groups was calculated. Then the time of prey ingestion was noted in the two groups. The leftover of the grasshopper was taken back exactly after 4 hours and was weighed. The difference between the initial and final weights was calculated (In gram) as the amount which is consumed by the spiders. Three replicates of this experiment were done.

(B)

Statistical analyses

The Spearman’s rank correlation was run to access the relationship between prey handling time of each group with the times of the day (9 am, 1 pm and 5 pm). Wilcoxon rank sum test (‘W’ is the test statistic) was used to analyze the mean ranks of the handling time of two sample populations (K exposed and K unexposed), which whether differed or not. Two-way ANOVA was done in R to find if there was any significant difference in prey ingestion rate of the two groups at different times of the day.

Figures

Fig. 2. (A) An individual colony of S. sarasinorum. (B) Experimental set-up for the study. (C) Spiders (Host) in captivity- Ant (Kleptoparasite- O. smaragdina) exposed.