Behavioural Flexibility in neotropical orb-weavers

Objectives and methodology

The overall aim of the project was to investigate the consequences of miniaturisation in orb-weavers for their ability to adjust web-
building behaviour and web-geometry adaptively to spatial constraints, which is a common problem in nature. The aim was split into two specific objectives.

1) Web-geometry. To determine if size has any influence on the degree to which web-geometry is affected spatial constraints.

2) Web-building behaviour. To determine if there are specific differences in web-building behaviour between different sized spiders in their behavioural adjustment to spatial constraints.

Methodology

Spiders: Early instar spiderlings and adult females of three species were used throughout this study. The advantage of using different developmental stages is that they control completely for phylogenetic differences and relatively well for web designs, since web-geometry does not change during ontogeny. Spiders were collected in the wild and given minimum a week of acclimatisation to the laboratory environment, where they readily build webs in square frames. The species used will be: (i) Nephila clavipes, (ii) Eustala illicita, (iii) Cyclosa caroli. Surveys of web size and shape and of the percentage of spiders with one or more missing legs will be carried out in the wild. About 20-30 spiders from each species and developmental stage will be used for each treatment.

Web-building behaviour and web-geometry will be compared between spiders subjected to the following four different spatial frames. (i) Standard sized square frames (with adequate space to build the species specific web) (ii) Square frames half the size of the original, (iii) horizontally oblong rectangular frames, a third as high as the original frames and (iv) vertically oblong rectangular frames, a third as wide as the original frames.

Problems encountered

I had no prior experience in working with spiders in the tropics and it proved more difficult than expected to locate the individuals required for the experiments. Several of the proposed species, furthermore, turned out to be unsuitable for laboratory experiments as they refused to build webs in captivity. Unfortunately, it proved impossible to get the smallest of the proposed species Anapisiona simonii to build two-dimensional orb-webs in the laboratory. Since no other suitable small orb-weaver was encountered it was decided to abandon an direct interspecific comparison of behavioural flexibility in relation to (brain) size. Instead it was decided to focus on three species (Nephila clavipes, Eustala illicita and Cyclosa caroli) and make intra-specific comparisons of web-accuracy between different age and (brain) size classes and to compare behavioural flexibility in adapting their web-shapes to spatial constraints between species that differ in the length of their anchor threads as observed in the wild.

In view of the enlargement of the research to include early and mid-sized juveniles, as well as experiencing some problems regarding getting spiders to build under the lighting conditions necessary for video recording, it was decided not to pursue research on the actual web-building behaviour itself, and instead focusing on getting a good sample size of photographs of webs instead.

Research conducted

Morphological data of leg length, total size and carapace width were collected for all species and age-classes used in the study.

During identification of orb spiders suitable for laboratory studies, a very suitable and interesting spider (Eustala illicita) was found in connection with ant-acacia trees. Together with a student from the University of Costa Rica on a short-term STRI fellowship (E. Triana) the following research project was undertaken to describe the web and the natural history of this spider. A great number of spiders build orb-webs and although the overall structure is the result of fixed behavioural patterns, much small scale inter- and intra-specific variation is nonetheless evident. Thus in order to fully understand the orb-web and the web-building behaviour in these spiders, we need to study substantial samples of many different species of orb-weavers. However, to date only a few species have been rigorously studied both in the field and in the laboratory. Here, we investigate the ecology, behaviour and the orb-web of the neotropical spider Eustala illicita and suggest it as suitable for further studies based on i) the ease at which it can be located in abundant numbers in the field, ii) its willingness to build webs in the laboratory, iii) the plasticity of its behaviour and iv) its interesting ecology in the form of interactions with the swollen-thorn acacias and their ant mutualists. We give an introduction to its natural history followed by a detailed description of orb-webs built in the field and in the laboratory, which we compare to other orb-spiders.

A study on ontogenetic changes in orb-web architecture was conducted on early instar juveniles, mid-sized juveniles and adult females of Nephila clavipes and Eustala illicita in order to test if juveniles with smaller absolute brain sizes built less optimal webs. Detailed web architecture was extracted from photoghraphs of laboratory webs with a comparison of over all shape and form from both laboratory wens and webs measured in the field. The analazing of data from this study is not yet concluded, but preliminary analyses suggest that early instar webs are not less optimal, but that they seem to follow the biogenetic law, which states that individual development in web-building behaviour occurs in the same order as evolutionary changes in the lineage.

A study on the spiders’ ability to adapt their orb-webs to different frame-sizes was conducted on the following species; Nephila clavipes, Eustala illicita and Cyclosa caroli. Detailed web architecture and shape was extracted from photoghraphs of laboratory webs. The analazing of data from this study is not yet concluded, but preliminary analyses suggest that the species with the shortest anchor-threads in nature (Eustala illicita) proved to be significantly better at adapting its webs to different frame-sizes that did the two other species with medium-long anchor threads. Data on anchor thread length to web-size was collected for a range of species in the wild. Note that species with long anchor threads (Micrathena sp. and Leucauge sp.) did not build webs in the laboratory at all.

Links
Smithsonian Tropical Research Institute
Smithsonian Postdoctoral Fellowships
Parque Natural Metropolitano, Panama

Publications
Hesselberg, T. (2014). The mechanism behind plasticity of web-building behavior in an orb spider facing spatial constraints. Journal of Arachnology 42: 311-314.
DOI: 10.1636/J14-05.1

Hesselberg, T. (2013). Web-building flexibility differs in two spatially constrained orb spiders. Journal of Insect Behavior 26, 283-303.
DOI: 10.1007/s10905-012-9335-7

Hesselberg,T. (2010). Notas sobre la capacidad de construir telas orbiculares en el laboratorio por arañas neotropicales. Puente Biológico 3, 101-109.

Hesselberg, T. (2010). Ontogenetic changes in web design in two orb-web spiders. Ethology 116, 535-545.
DOI: 10.1111/j.1439-0310.2010.01760.x

Hesselberg, T. and Triana, E. (2010). The web of the acacia orb-spider Eustala illicita (Araneae: Araneidae) with notes on its natural history. Journal of Arachnology, 38:21-26.
DOI: 10.1636/HI09-59.1

Further materials
A booklet containing illustrations of neotropicl Eustala genetalia by Herbert Levi – View PDF