Eco. Env. & Cons. 26 (November Suppl. Issue) : 2020; pp. (S91-S97) Copyright@ EM International ISSN 0971–765X A study on web asymmetry and prey capture in Argiope pulchella Thorell, 1881 (Araneae: Araneidae) Sangeeta Das*, Jatin Kalita and Nilutpal Mahanta Department of Zoology, Gauhati University, Guwahati 781014, Assam, India (Received 10 April, 2020; Accepted 9 May, 2020) ABSTRACT For a sit and wait forager like Argiope spiders, the web is their ultimate foraging strategy, therefore investment in web building is crucial for their survival. For understanding this foraging strategy, studying of web architecture and web symmetry become essential as any change in web can severely affect the prey capture success. The present study focused on studying the web asymmetry in Argiope pulchella Thorell, 1881, from juvenile to the adult stage. We observed a deviation from the original circular orb web as the spider grows. Adult webs had higher vertical asymmetry in comparison to the juvenile webs. The reason for such asymmetry was tested by correlating it with the number of prey captured in the upper and lower halves of the webs. The study showed significant relationship between the lower web area and the number of prey captured in the webs. This study concludes that web transition takes place from symmetrical to asymmetrical as the spider matures and larger lower web parts in adult spiders aids in higher prey capture success. Key words: Web asymmetry, Orb-web spiders, Prey capture, Argiope pulchella, Web area, Capture area. Introduction ous investigators have tried to explain the web variation and web asymmetry occurring across the Argiope pulchella Thorell, 1881 of family Araneidae is developmental stages of orb web spiders (Eberhard a downward facing spider known to build the clas- et al., 2008). The best theory explained so far is the sical vertical orb webs. Araneids are known as ‘sit biogenetic law that states- the ontogenetic changes and wait’ foragers owing to its specialized foraging that occur during an individual’s development oc- strategy in which it resides at the central portion of cur in similar order that take place in lineages the web, known as hub from where it usually at- (Hesselberg, 2010). In other words, the web charac- tacks its prey (Zschokke, 1999). Several proximate teristics of juveniles would resemble the phyloge- factors are reported to affect the architecture of ver- netic ancestral taxa but as the juveniles grow to tical orb webs such as nutritional and reproductive more mature stages, more derived features are accu- status, prey and predator species present in the sur- mulated to the basic developmental plan (Eberhard rounding environment (Eberhard, 1975; et al., 2008; Hesselberg, 2010). Since the ultimate for- Herberstein, 2011; Robinson and Robinson, 1972), aging strategy of orb web spiders is to build a web space available for web construction (Adams, 2000; as a trap for its prey, therefore the success of prey Harmer and Herberstein, 2009) and, gravity capture should be directly dependent on captive (Herberstein and Heiling, 1999; Nakata and ability of the orb webs (Herberstein and Heiling, Zschokke, 2010; Zschokke and Nakata, 2010). Vari- 1999). Web asymmetry is simply the deviation of the Assistant Professor, Assam Don Bosco University S92 Eco. Env. & Cons. 26 (November Suppl. Issue) : 2020 web from circularity and may result from behav- Since female webs were larger, occurred in higher ioral adaptation to prey capture. Asymmetrical na- numbers and could be easily found both in breeding ture of orb webs has been reported in many species and non- breeding season; for adult and sub adult but was never studied for A. pulchella which is a stages, only female webs studied. Each web was di- common synanthropic species of India (ap Rhisiart vided into two halves – upper and lower and two and Vollrath, 1994; Das et al., 2018; Masters and sides- left and right with reference to the observer. Moffat, 1983; Zschokke and Nakata, 2015). The Each half had two quadrants, i.e. upper half with present study attempts to analyze the web variation quadrant 1 on the left, quadrant 2 on the right and across the developmental stages of A.pulchella and lower half with quadrant 3 on left whereas quadrant to verify the relationship between web asymmetry 4 on the right side (Figure 2). Prey items intercepted and amount of prey captured by the web. on the web were classified into 3 categories based of their size– 1) small sized prey: 0 to 5 mm, 2) me- Materials and Methods dium sized prey:>5 mm but<15 mm and, 3) large sized prey: >15 mm.Prey captured in each of these The study was conducted in Kamrup metropolitan quadrants was counted to quantify which quadrant district, Assam, India from November 2014 to June of the web has higher prey capture rates. 2018. This area extends from 25° 59' 52.45" N to 26° Data analysis 15' 31.54" N latitude and from 91° 33' 08.04" E to 92° 10' 49.8" E longitude occupying an area of 1527.84 Total web area was calculated as per Koh and Li, km². It is a mosaic of hills, semi-urban, urban areas, 2002 - x a x b where, a is calculated as-[(upper ra- forests and wetlands.Field surveys were conducted dius + lower radius)/2] and b as - (left radius + right twice a week at 8:00-12:00 and 13:00-16:00 hours radius)/2 (Figure 2). Asymmetrical Index (A.I.) was along 15 fixed trails of one kilometer length and five meters width on both the sides. To equalize the ef- fort size, the survey duration was kept constant (Coddington et al., 1996). When a web was encoun- tered, 20-30 mins of field observations per web was performed in order to record the number of prey items captured at different sectors of the webs. Indi- rect evidence of prey capture such as counting dam- aged sectors of the web and prey already present in the hub were not considered in order to avoid faulty results. Three developmental stages based on size and morphology, viz. juvenile, sub-adult and adult Fig. 2. Segments of a typical orb web. 1, 2, 3 and 4 repre- were selected for the study. At the end of the obser- sent four quadrants of an orb web. Area covered vation period,individuals were photographed and by quadrant 1 and 2 forms the upper web area and webs were measured (Figure 1a, 1b and 1c). Analyz- by 3 and 4 forms the lower web area; vertical di- ing Digital Images Suite software was used for ana- ameter = upper radius+lower radius; horizontal lyzing the stage classes of A. pulchella and prey size. diameter = left radius+right radius. Fig. 1a. Juvenile Argiope pulchella Fig. 1b. Sub adult Argiope pulchella Fig. 1c. Adult female Argiope pulchella DAS ET AL S93 calculated for each web using web area (Nakata and metry at different life history stages. The study re- Zschokke, 2010; Zschokke and Nakata, 2015). We vealed both the symmetrical and asymmetrical na- calculated the upper and lower web area using the ture of webs in A. pulchella from juvenile to adult formula–[(x upper radius x b)/2] and [(x lower stage with higher proportion of negative A.I. (webs radius x b)/2], respectively. The left and right web with larger lower parts) than positive A.I. (webs area was calculated as-[( x left radius x a)/2] and with larger upper parts) in vertical asymmetrical [( x right radius x a)/2], respectively. For vertical webs (Figure 3). In case of horizontal asymmetry, web symmetry, asymmetrical index (A.I.) is calcu- the three stages showed more or less similar propor- lated as-(U-L)/(U+L), where, U = upper web area tions of positive and negative A.I. (Figure 4). In all and L = lower web area; for horizontal web symme- three developmental stages, majority of webs were try, A.I. is calculated as- (Lft-R)/(Lft+R), where, Lft asymmetrical but juvenile spiders were recorded to = left web area, R= right web area. Positive A.I. construct more symmetrical webs in comparison to value indicates asymmetrical web with larger upper adults and sub adults. Adults had the least sym- web half, negative A.I. value indicates asymmetrical metrical webs (Table 1). web with larger lower web half and zero A.I. value Prey density and size composition indicates perfect symmetrical web with both upper and lower equal halves. Prey densities were calcu- We studied the prey density and prey size compo- lated by counting the number of prey captured by sition in A. pulchella webs in order to find which the web per cm2 for each web half and size based area of web has higher capture success. Out of 111 prey composition was tabulated to check which prey size category contributed more towards the total prey captured by the webs. Linear regression analysis was performed in SPSS ver. 16.0 to study the relation between the size of web and number of prey captured by it. Since, A. pulchella is an upside- down oriented spider, therefore, upper and lower area of the web were only considered for the analy- sis to predict which half of the web had higher prey capture rates owing to vertical symmetry. Results Web variation Fig. 3. Box plot showing variation in vertical Asymmetri- A total 111 webs of A. pulchella, 38 juveniles, 23 sub- cal Index of webs of three different stages of A. adults and 50 adults, were studied and was found pulchella. Negative A.I. indicates that web has that from the juvenile to sub-adult and adult stages, larger lower part, positive A.I.