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

Thomas Hesselberg,Daniel Simonsen

doi:10.3897/subtbiol.31.36222

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.

Highlights

The unique adaptations to the dark and nutrient poor subterranean habitat have fascinated biologists since the dawn of speleobiology more than 200 years ago
To control for differences in spider size, the following variables were used in linear mixed models (LMMs) as the response variable: relative patella-tibia length of leg I and III, relative web area, number of spiral turns per cm2 web and number of radii per cm2 web with species as predictor variable and location as a random effect
The differences in morphology and web parameters between M. merianae and M. mengei were visualised with a PCA ordination plots (Fig. 1), which showed that the two species differed substantially in size with the M. merianae about 1 1⁄2 times larger than M. mengei (Fig. 1A; Table 1)

Summary

Introduction

The unique adaptations to the dark and nutrient poor subterranean habitat have fascinated biologists since the dawn of speleobiology more than 200 years ago. Mainly being found within the first 20 m of caves and not showing any troglomorphic adaptations, cave orb spiders in the genus Meta are one of the few cave organisms, where unique behavioural adaptations to subterranean life have been observed (Novak et al 2010; Hesselberg et al 2019) These include modified orb webs with missing frames and radii attaching directly to the substrate (Yoshida and Shinkai 1993; Simonsen and Hesselberg unpublished), possibly off-web foraging (Eckert and Moritz 1992; Smithers 2005a) and a life history which includes one or two instars spent outside of the subterranean environment (Smithers 2005b; Mammola and Isaia 2014)

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Conclusion