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This first-ever extensive review of the construction behaviour of orb webs, of webs secondarily derived from orbs, and of non-orbs shows that the evidence favouring monophyly over convergent evolution of orbs is stronger than previously appreciated. The two major orb-weaving groups, Uloboridae and Araneoidea, share 31 construction behaviour traits, 20 of which are likely to be both derived and to have feasible alternatives, making convergence an unlikely explanation. Convergence in two lineages seems unlikely, and convergence in five different lineages, as proposed in some recent molecular studies of phylogeny, is even less credible. A further set of seven shared responses in orb design to experimentally constrained spaces also supports orb monophyly. Finally, a ‘control’ case of confirmed convergence on similar ‘pseudo-orbs’ in a taxonomically distant group also supports this argument, as it shows a low frequency of behavioural similarities. I argue that the omission of behavioural data from recent molecular studies of orb web evolution represents a failure of the analytic techniques, not the data, and increases the risk of making mistakes. In general, phylogenetic studies that aim to understand the evolution of particular phenotypes can benefit from including careful study of the phenotypes themselves.

Arthropod behaviour is usually explained through ‘hard-wired’ motor routines and learning abilities that have been favoured by natural selection. We describe observations in which two arthropods solved rare and perhaps completely novel problems, and consider four possible explanations for their behaviours: (i) the behaviour was a pre-programmed motor routine evolved to solve this particular problem, or evolved for other functions but pre-programmed to be recruited for this function under certain conditions; (ii) it was learned previously; (iii) it resulted by chance; or (iv) it was the result of insightful behaviour. Pre-programmed solutions can be favoured by natural selection if they provide solutions to common or crucial problems. Given the apparent rarity of the problems that these animals solved, the solutions they employed are unlikely to represent innate behaviour. Learning and random chance seem unlikely, although we cannot rule them out completely. Possibly these animals employed some degree of insight.

A recent theoretical model predicts that the allometries of structures under sexual selection vary according to their functions in sexual interactions. Structures that are specialized for exclusive...

In previous studies, the “cocoon” webs built by spiders of the genera Cyclosa Menge, 1866 and Allocyclosa Levi, 1999 under the influence of Polysphincta spp. wasps resembled molting webs built by unparasitized spiders; this behavioral manipulation was associated with increased concentrations of the molting hormone ecdysone. The present study documents an additional aspect of the cocoon webs of Allocyclosa bifurca (McCook, 1887) built under the influence of the wasp Polysphincta gutfreundi Gauld, 1991. Molting webs were more likely to have an associated barrier web than were prey capture orbs; and cocoon webs were even more likely to have barrier webs. The similarity between molting and cocoon webs accords with previous indications of ecdysone use by the wasps. The accentuation of molting web traits in the cocoon webs (also seen in other species) implies that the wasps manipulate the spiders using mechanisms other than simply replicating hormonal stimuli involved in normal molting.

Studies of web evolution in spiders generally focus on the overall designs of webs in the field. As has been typical for dictynids and several other cribellate families with “irregular” webs, this study detected few discernable patterns in the field regarding the spatial organization of the highly variable, three-dimensional and largely aerial webs of the dictynid Dictyna meditata Gertsch, 1936. Nevertheless, there were three consistent sub-unit designs in the additions that spiders made to their webs in captivity, and in webs that they built from scratch in captivity: “silk ladders”, with a cribellum line that zig-zagged between a pair of approximately parallel non-sticky lines; “twig ladders”, with a cribellum line that zig-zagged between a non-sticky line and the substrate; and long non-sticky lines that each supported a long, slightly looped cribellum line. I suggest, using examples from dictynids and other families with long-lived, geometrically irregular webs, that this pattern of using consistent behavior patterns to add geometrically regular “modules”, is widespread and ancient, but has often been missed due to damage and additions to webs in the field, and to lack of direct behavioral observations. Recent attempts to link web evolution to studies of spider phylogeny could benefit from a change of emphasis, focusing on the additions that spiders make to their webs, rather than on the currently common but necessarily vague characterizations of overall web designs seen in the field.

Most lines in the sheets of four linyphiid species bore numerous small droplets. Droplet-bearing lines of Linyphia simplicata (F. O. Pickard-Cambridge, 1902) and Neriene coosa (Gertsch, 1951) adhered weakly to smooth, dry surfaces, contradicting previous statements that such lines in linyphiid webs are not sticky. Droplets at the intersections of lines tended to be larger than nearby droplets on the same lines, and were more widely separated from adjacent droplets than were other, nearby droplets on the same lines. A self-assembly hypothesis to explain these findings, that larger droplets accumulated at intersections as a result of lines in the sheet scraping against each other, was supported by confirming that larger intersection droplets were more widely separated from adjacent droplets than were smaller intersection droplets. These observations suggest a new, dynamic view of linyphiid sheet webs, in which the distribution of sticky material is adjusted advantageously immediately after lines are produced, due to the upward dabbing movements of the spider's abdomen that press the sheet lines against each other during sheet construction, and perhaps also to other movements of newly laid lines against each other. Larger droplets maybe advantageous in binding lines together, and in adhering to prey.

Male courtship signals often stimulate female sense organs whose sensitivities originally evolved under natural selection. In classic female choice models of sexual selection, females can benefit f...

Orb web construction was originally thought to be highly stereotyped, but adaptive flexibility is now well established in several aspects. This study reviews published data on one behavioural cue and presents new data on flexibility in experimentally modified and control webs of Zosis geniculata and Uloborus diversus. By occasionally ignoring this cue temporarily, spiders gained access to otherwise inaccessible portions of their webs. I discuss three hypotheses concerning the mechanism that resulted in this flexibility. Several types of evidence argue against the hypothesis that the adjustments were pre-programmed: substantial variation in the contexts when adjustments occurred; substantial variation in details of the adjustments; and rarity of the contexts that require adjustments in nature. Lack of plausible links between behavioural decisions and payoffs from prey capture argue against a second, learning hypothesis. By elimination, this flexibility may require a third type of explanation that includes more elaborate cognitive processes.

The brains of smaller animals are smaller than those of their larger relatives, but it is not clear whether their adaptive behavioral flexibility is more limited. Previous interspecific comparisons found that aspects of web construction behavior of very small orb weaving spiders (0.005 mg) were no less precise than those of much larger related orb weavers (30 mg), but the behaviors tested were relatively simple. Here we perform a more sensitive intraspecific test involving the multiple behavioral adjustments of orb web designs made by Leucauge argyra to confinement in very small spaces. Web adjustments of spiderlings as small as ~0.1 mg were compared to previously published observations of ~80 mg conspecific adults. Spiderlings in constrained spaces made all of the complex adjustments made by adults in at least seven independent web design variables, and their adjustments were no less precise. Rough estimates based on previously published data on total brain volumes and the mean diameters of neuron cell bodies suggested that spiderlings and adult females of Leucauge may have similar numbers of neurons, due to spiderlings having smaller neurons and a greater percentage of body tissues dedicated to the brain. We speculate that this neural similarity may explain why L. argyra spiderlings showed no behavioral deficits compared with adults. Universidad de Costa Rica/[833-C0-251]/UCR/Costa Rica UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Biología

Small swarms of male Leseha vespaSmith & Nishida, 2019 flew during the morning hours searching for the much less abundant females on the upper surfaces of protruding leaves of various plant species at a site where a large population of the host fern Phlebodium pseudoaureum had been recently decimated (probably by L. vespa larvae). Males showed no sign of aggressive behavior toward other males, and only poor abilities to locate females resting on leaves. Although >100 male-female interactions were observed, they seldom led to copulation, as females were generally unreceptive. The males probably used visual cues to choose leaves, but the short-range cues that they used to encounter females there were not clear. Individually marked males returned to the same site for up to 11 days.

The webs of hahniid spiders are poorly known. Those of Neoantistea riparia (Keyserling, 1887) all included sheets near the surface of the ground, but were more complex and variable than the simple sheets mentioned in previous accounts. Additional components included: sparse tangles of variable size above the sheet; lines below the sheet; small, dense tangles near some edges of the sheet; and a temporary feeding chamber below the sheet built near a recently captured prey. Webs were close to the surface of damp ground and, as a result, spent long periods coated with droplets of water. These droplets may aid in prey capture, as water accumulated on struggling prey and may have hindered their escape. In contrast, droplets seldom adhered to the spiders.

Many spectacular cases of biological diversity are associated with sexual selection, and structures under sexual selection often show positive static allometry: they are disproportionately ...

Imperfect knowledge of ancestral behaviour often hampers tracing behavioural evolution. This limitation is reduced in orb weaving spiders, because spider orb web construction behaviour and the cues used by modern orb-weavers are well-studied and highly conserved. Several species in orb-weaving families build non-orb webs that are clearly derived from orbs, allowing transitions from ancestral to modern behaviours to be described with high confidence. Three major patterns of general evolutionary significance were found in 69 phylogenetically independent transitions in 15 groups in 8 families: ancestral traits were often maintained as units; the most frequent of the eight different types of ancestral trait change was transfer of an ancestral behaviour to a new context; and ‘new’ traits that had no clear homology with ancestral traits were also common. Changes occurred in all major stages of orb construction. This may be the most extensive summary of evolutionary transitions in behaviour yet compiled.

Funnel webs are common and widespread taxonomically, but little is known about how they are built or details of their structure. Aglaoctenus castaneus (Mello-Leitao, 1942) (Lycosidae) builds horizontal, densely meshed funnel webs of non-adhesive silk, with a tangle of lines above. Web construction behavior was unique in that the spider frequently laid swaths of lines rather than simple drag lines, both to float bands of fine lines on the breeze as bridges to distant objects and to fill in the sheet. Spiders utilized special spinneret movements to widen the swaths of lines that they laid on sheets. These movements have not been seen in web construction by other araneomorphs, but are were similar to and perhaps evolutionarily derived from those used during prey wrapping by many other species. Observations, made with a compound microscope, of the construction behavior of the agelenid Melpomene sp. O.P. Cambridge 1898, and of lines and attachments in sheets of these species and another funnel web spider, the zoropsid Tengella radiata (Kulczynski, 1909) demonstrated the possibly general nature of including obstacles in the web. This probably disadvantageous behavior may be related to constraints in selecting web sites imposed by the need for sheltered retreats, or to the spider's inability to remove preliminary lines. The observation also showed the importance of further improvements in the discriminations made between “sheet” and “brushed” webs in recent discussions of spider web evolution.

Spiders are effectively blind with respect to the lines in their own webs. Species in four orb-weaving families solved the problem of finding lines by tapping with their anterior legs, like a blind man with his cane, and then “following” these anterior legs with more posterior legs, which grasp lines that the anterior legs are already holding. Following behavior occurs during several stages of orb construction, and probably economizes on searching time and effort. The movements of following legs are finely adjusted to the morphological details of the grasping structures on their tarsi (the middle claw and its associated serrate accessory setae): the small searching movements made by following legs have consistent orientations: legs I and II move prolaterally, while legs III and IV moved retrolaterally. These orientations are appropriate to bring the asymmetrically placed middle claw and accessory setae into contact with the line. Spiders solved the additional problem of grasping lines that are more or less parallel to the long axis of the leg by using a previously unrecognized movement; they rotate the distal portion of the leg on its longitudinal axis, orienting the middle claw so that it is more or less perpendicular to the line. As an orb-weaver moves across her web, she probably constantly adjusts the rotation of each leg to align its middle claw perpendicular to the lines that it grasps.

Traditional views of copulation and sperm transfer supposed that females are passive participants. Recent discoveries suggest, however, that females actively influence the chances that a copulation will result in fertilization of their eggs, and that they sometimes signal to males during copulation in order to elicit male responses. This paper concerns two apparent female signaling behaviors, wing vibration and body shaking, in the tsetse fly Glossina pallidipes, a species in which the male squeezes the female’s abdomen rhythmically with his powerful genitalia. Vibration was associated with male squeezes at several levels of analysis. Its coordination with male behavior suggests that vibration functions as a signal that induces the male to interrupt squeezes, but that does not forcefully dislodge the male. The female tended to vibrate her wings soon after the male began a squeeze; and when the female vibrated her wings during a squeeze, the squeeze tended to be shorter. Female body shaking was usually elicited by especially powerful squeezes. Previous studies showed that stimuli from male structures that squeeze the female probably function to induce her to ovulate, to facilitate movement of sperm into her spermathecae, and to reject the sexual advances of additional males. This study is one of the first to document an exchange of signals between male and female insects during copulation, and extends the new field of research on copulatory dialogues.

Rapid divergence in external genital structures occurs in nearly all animal groups that practice internal insemination; explaining this pattern is a major challenge in evolutionary biology. The hypothesis that species-specific differences in male genitalia evolved under sexual selection as courtship devices to influence cryptic female choice (CFC) has been slow to be accepted. Doubts may stem from its radical departure from previous ideas, observational difficulties because crucial events occur hidden within the female's body, and alternative hypotheses involving biologically important phenomena such as speciation, sperm competition, and male-female conflicts of interest. We assess the current status of the CFC hypothesis by reviewing data from two groups in which crucial predictions have been especially well-tested, Glossina tsetse flies and Roeseliana (formerly Metrioptera) roeselii bushcrickets. Eighteen CFC predictions have been confirmed in Glossina and 19 in Roeseliana. We found data justifying rejection of alternative hypotheses, but none that contradicted CFC predictions. The number and extent of tests confirming predictions of the CFC hypothesis in these species is greater than that for other generally accepted hypotheses regarding the functions of nongenital structures. By this criterion, it is reasonable to conclude that some genital structures in both groups likely involved sexual selection by CFC.

Sexually-selected traits often show positive static allometry, with large individuals bearing disproportionately large structures. But many other sexually-selected traits show isometry or even negative allometry, with trait size varying relatively little with body size. We recently proposed that the functions of these traits (as aggressive signals, weapons, courtship signals, and contact courtship devices) determine their allometries. Positive allometry is generally favored for aggressive signals because aggressive signals are selected to emphasize body size (and thus fighting prowess). In contrast, the biomechanics of force application in weapons only sometimes select for positive allometry; the content of courtship signals is even less often related to body size; and contact courtship devices are selected to be relatively invariant across body sizes. Here we summarize the arguments in favor of this “functional allometry” hypothesis and expand a comparative test of its predictions. Our results indicate that sexual traits have the allometric slopes predicted by our hypothesis, regardless of which body part bears the structure.

In male spiders, genitalia, sexual behaviour and secondary sex morphology tend to diverge rapidly across species, presumably as a result of sexual selection. In the three Leucauge species for which pre- and copulatory courtship behaviour is known, females clamp the male chelicerae prior to and during copulation. This brings the basal segment of the male’s chelicerae into contact with the anterior surface of the female’s chelicerae. The basal segment of male’s chelicerae has also morphological features such as sturdy, abundant setae which are thought to have evolved to stimulate females, as well as other morphological features whose specific function is yet unknown. We show here that in a fourth species, Leucauge sp., the female does not clamp the male’s chelicerae; as expected, this absence is associated with a lack of secondary sexual differences in the male chelicerae. UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Biología

Both males and females of the spider Leucauge mariana (Taczanowski 1881) contribute material to the plugs that often occlude the genital openings of females in the field. Males were sometimes unable to remove or penetrate these plugs, but overcame others using three different mechanical mechanisms: snag the plug and pull it off; break and penetrate through it; and break its adhesion to the epigynum by injecting material under it. They used their genitalia to accomplish these tasks, despite the fact that the genital bulb lacks muscles and innervation, thus limiting the male's ability to guide genital movements precisely. The effects of two male genital structures, the conductor tip and the conductor hook on sperm transfer and genital plug removal were tested by direct observations of their morphology and behavior, and by experimental removal of structures from one but not the other palp of the same male. Removal of the conductor tip reduced sperm transfer, while removal of both the hook and the co...

Spider orb webs are impressive for their apparently uniform geometric patterns. There are, however, consistent, substantial and taxonomically widespread periphery-to-hub differences in the distances between both adjacent radii and between sticky spiral lines. Radii in typical orbs were on average about 4–5 times farther apart at the outer edge than the inner edge of the area covered by sticky lines. Distances between sticky spiral loops were on average about two times larger near the outer edge than in more inner portions. This pattern in sticky spiral spacing was absent in the modified orbs of Nephila clavipes, in which distances between radii varied less. Thus, patterns in sticky spiral spacing may be related to inter-radial spacing; there is, however, probably no single explanation for all of the different patterns of sticky spiral spacing. The patterned differences in radius and sticky spiral spacing have important consequences for understanding orb function, because the lines in a prey's immediate vicinity largely determine whether it will be stopped and then retained, and elementary physics dictates that contact with more lines will tend to increase prey being stopped and retained. Rather than being a unit trap with a single set of prey capture properties, an orb has locally different trapping properties in different sectors. Abandoning the previous typological style of discussion of ‘the’ ability of a given design to stop and retain prey promises to lead to improved understanding of orb web designs. Published 2014. This article is a U.S. Government work and is in the public domain in the USA, Biological Journal of the Linnean Society, 2014, 111, 437–449.