Cave spiders choose optimal environmental factors with respect to the generated entropy when laying their cocoon

Abstract

The choice of a suitable area to spiders where to lay eggs is promoted in terms of Darwinian fitness. Despite its importance, the underlying factors behind this key decision are generally poorly understood. Here, we designed a multidisciplinary study based both on in-field data and laboratory experiments focusing on the European cave spider Meta menardi (Araneae, Tetragnathidae) and aiming at understanding the selective forces driving the female in the choice of the depositional area. Our in-field data analysis demonstrated a major role of air velocity and distance from the cave entrance within a particular cave in driving the female choice. This has been interpreted using a model based on the Entropy Generation Minimization - EGM - method, without invoking best fit parameters and thanks to independent lab experiments, thus demonstrating that the female chooses the depositional area according to minimal level of thermo-fluid-dynamic irreversibility. This methodology may pave the way to a novel approach in understanding evolutionary strategies for other living organisms.

Figure 1. FESEM characterization of the Meta menardi spider cocoon.

(a) A cocoon of the European cave spider Meta menardi, photo by Francesco Tomasinelli (2009). Scale bar: 5 mm. (b) The upper part and the stalk of the cocoon. Scale bar: 1 mm. (c, d) The walls of the cocoon at different magnifications, scale bars correspond to 1 mm and 500 μm, respectively. The scissors and the black dotted lines indicate cutting of the cocoon wall, which was removed for internal inspection of the cocoon structure and eggs, which are indicated by black arrows.

Figure 2. Summary of the work flow.

(a) in-field 1-year monitoring and measurements of airflow velocity, distance from cave entrance, presence of the cocoons and final cocoon collecting procedure; (b) laboratory experiments for characterizing the cocoon thermal properties; (c) theoretical modeling based on the EGM method using as input data only the properties measured in phase (b) and direct comparison with observations in phase (a). A relevant agreement is demonstrated without best fit parameters.

Figure 3. The binomial Generalized Linear Mixed Model (GLMM).

Probability surface shows the relation among mean airflow velocity, distance from the cave entrance and probability of presence of the cocoon of Meta menardi, obtained by the binomial GLMM applied to the in-field collected data.

Figure 4. Experimental results from laboratory tests comparing different setups: naked with BH (big heater), naked with SH (small heater), BH within cocoon, SH within cocoon.

(a) Transmittance as a function of the airflow velocity U_air. Points refer to experimental data, while lines denote best-fitting curves. (b) Nusselt number as function of Reynolds number. Points refer to experimental data, while continuous lines indicate best fitting curves. (c) Thermal and mass transport transmittances Tr and K as a function of the airflow velocity based on the laboratory experiments. (d) The characteristic drying time τ as function of the airflow velocity U_air. Triangles are the experimental points, while continuous line is the corresponding best fitting curve.

Figure 5. Entropy isocontours.

(a) Isocontours of the total generated entropy according to equation (15) corresponding to: ϕ = 0.65, D_c = 1.8 cm, T_a = 15°C, D_s = 6 mm, U_spider = 0.4 m/s, N_spider = 250 (see ref. 28). (b) Cocoon deposition probability by equation (16) is reported with a reference entropy of S₀ = . In-field experimental data are also reported with disks and circles denoting presence and absence of cocoon, respectively.