A comparison of morphology and web geometry between hypogean and epigean species of Metellina orb spiders (family Tetragnathidae)

Abstract

Studies on the behaviour of subterranean animals are rare, mainly due to the problems with collecting data in these inaccessible habitats. Web-building cave spiders, however, leave a semi-permanent record of their foraging behaviour, which can relatively easily be recorded. In this study, we compare size, leg lengths and web characteristics between hypogean populations of Metellina merianae with its close wood-inhabiting relative M. mengei. We confirm previous observations that M. merianae does not show any obvious morphological and behavioural adaptions to a subterranean life-style, although individuals of the cave species were significantly larger and had webs with relatively fewer radii and capture spiral turns than M. mengei. We were, however, not able to determine if these findings indicate a transition towards behavioural adaptation to caves or if they are a result of behavioural flexibility in response to the different humidity and temperature between caves and woodland. Finally, we did not find any effect of cave characteristics on either the number of radii or the area of the M. merianae web.

Unique behavioural modifications in the web structure of the cave orb spider Meta menardi (Araneae, Tetragnathidae)

“…However, the number of capture spiral turns (17.0 ± 4.3) was considerably greater than recorded for M. menardi in this study (6.0 ± 1.7), suggesting that the mesh height would be lower. A similar number of radii (16.9 ± 3.0) and spiral turns (13.0 ± 3.0) were observed in the facultative cave spider M. merianae , but this species does not show any reduction of frame threads and only a few mooring threads connect the web to the cave wall 41 . Interestingly, the webs of M. menardi were more symmetrical than the webs of the two surface-dwelling species.…”

“…The anapid Anapisona simoni shows more dramatic changes in frame, radial and capture threads that are somewhat similar to the responses observed in this study 69 , but anapid webs are very different from tetragnathid webs as they are more three-dimensional and lacking the non-sticky scaffolding spiral. Overall, comparisons between strongly constrained spiders in the laboratory and cave spiders in the wild have only limited validity as natural caves typically offers a range of potential web sites as demonstrated by the normal cave orb webs of the sympatric Metellina merianae 41 . Finally, it is also possible that the observed changes occur as a direct response to the low food availability in the subterranean habitats.…”

“…However, in our comparative study, M. menardi, contrary to our hypothesis, did not have the longest relative leg length for either leg I or III, although it did have statistically significantly longer relative leg lengths than its closest surface relative, Metellina mengei. The facultative cave spider Metellina merianae has relative leg lengths in between these two species 41 , so more detailed studies are needed to determine if some degree of limb elongation might be present www.nature.com/scientificreports/ in cave orb spiders. The large overall size of M. menardi could potentially also be viewed as an adaptation to the subterranean environment as an increased body size is a frequent morphological adaptation seen in cave arachnids 3 .…”

“…To determine differences in morphology (total length, cephalothorax width, and patella-tibia length of leg I and leg III) between M. menardi and its surface-dwelling relatives, we performed a principal component analysis with the prcomp() function in R and used the ggplot2 package 78 to generate ordination plots. However, in order to investigate potential limb elongation in M. menardi , we followed Hesselberg and Simonsen 41 to control for spider size by using relative leg length (patella-tibia lengths of leg I and III divided by cephalothorax width) as the response variables in linear mixed models where species was the fixed factor with location (transect number and cave ID) as a random factor. To ensure normality, the relative leg length of leg I in the comparison of all species was transformed with the natural logarithm.…”

Negative allometry of orb web size in spiders and the implications for the evolution of giant webs

“…Previous studies comparing different orb-weaver species living in the same local areas have predictably found that species with larger body sizes have larger capture areas within their webs (LeGuelte 1966;Carrel & Deyrup 2019;Hesselberg & Simonsen 2019). Many studies have found that larger individuals of a species tend to build larger webs, using either age categories (Kuntner et al 2008) or direct measurements of size of the spiders (Alam & Butt 2016).…”