The tendency for the genital morphology of animals to diverge more rapidly than other traits is one of the most pervasive evolutionary patterns in animal form. Current controversy regarding explanations of this pattern stems in part from the difficulty of observing the behaviour of male genitalia during copulation. This limitation is reduced in tipuloid crane flies, because most of the male's elaborate, divergent genital structures remain outside the female during copulation. Observations of genital behaviour during copulation in 45 species in 21 genera and subgenera, the most extensive sample of genital behaviour in any comparable group of animals, show a combination of trends that fits better with the stimulation version of the cryptic female choice hypothesis than with any of the other hypotheses commonly cited to explain rapid divergent genital evolution: sustained, rhythmic male genital movements such as brushing and tapping; frequent female facilitation of stimulation; lack of consistent morphological coevolution between corresponding male and female structures; lack of forceful male manipulations of females; lack of female "defenses" that impede male stimulation; and lack of direct male interference with other males' sperm. Tipuloids are atypical among Diptera in generally lacking rhythmic, forceful genital thrusting and squeezing.

One of the most pervasive trends in animal evolution is for genitalia to diverge more rapidly than other body structures. Observing the behaviour of the genitalia during copulation is crucial to discriminate among the various hypotheses proposed to explain this pattern, but such behavioural studies are surprisingly rare. Here, I test predictions of several hypotheses to explain genital divergence in syrphid flies, which have diverse, often species-specific male genitalia with puzzling morphological designs. I observed the behaviour of male and female genital structures during copulation in the microdontine syrphid fly Menidon falcatus in the field. Copulation behaviour of M. falcatus included several types of repeated rhythmic genital squeezing, and single pairs coupled and uncoupled up to 17 times in a single mating. The species-specific male genital surstyli appear designed to function as clasping devices, but their behaviour and their morphological mesh with the female suggest further stimulation functions. The genital behaviour and male-female morphological interactions that I observed clearly contradict predictions of several hypotheses previously proposed to explain rapid divergent genital evolution, including mechanical lock-and-key, sperm competition and male-female mechanical conflict of interest. There was little behavioural evidence of male-female antagonism; females actively cooperated at some stages of copulation. Instead, several morphological and behavioural traits of male genitalia seem designed to stimulate the female. Sustained, rhythmically repetitious genital movements suggest that rhythmic male genital behaviour functions to reduce the female's habituation or neural adaptation to stimulation. Future observations of genital behaviour, combined with data from the abundant, often exquisitely detailed but underutilized taxonomic literature, may illuminate major evolutionary puzzles in genital evolution and sexual selection in general. Genital behaviour is an exciting frontier of the classic, historically successful biological tradition of functional morphology. (c) 2023 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Sticky spiral construction behavior in orb-weaving spiders is strikingly flexible: 10 to 16 different cues influence the hundreds of decisions that a spider makes while building the sticky spiral in an orb. Several variables that are influenced by radial tensions, including the frequencies and amplitudes of vibrations and the extensibility and tensions of the radii, have been repeatedly mentioned as possible cues guiding sticky spiral placement. Nevertheless, previous studies have suggested that radius tensions do not affect sticky spiral spacing. These tests were complicated, however, by the tight correlations between variables due to the highly regular geometry of orbs, possible context-dependent variation in the use of cues, and the intrinsic difficulty of demonstrating the absence of an effect. Greater certainty regarding tension-dependent cues is crucial for understanding the cues guiding orb construction, so this study extends these experiments, examining the effects of both increasing and decreasing radius tension, and of different sequences and magnitudes of changes. The results confirm previous findings: changes in radius tensions did not influence sticky spiral spacing decisions in consistent ways and generally failed to produce statistically significant effects. I argue that the likely reason why spiders do not use tension-dependent variables to guide sticky spiral placement is that such cues are unreliable because the spider's own weight substantially alters the tensions on nearby web lines in complex and variable ways.

Psychological mechanisms involved in perceiving and processing sensory information likely influence how sexual selection acts on mate choice signals. Two such mechanisms are habituation and neural adaptation. Both are nearly universal, and doubtless affect female perception and analysis of male mate choice signals, thus potentially affecting male reproductive success. Habituation may be especially important because it affects the female's attention. Both likely favor greater variation in mate choice signals, contrary to many typological models of mate choice based on "the" male display behavior of a species. They may favor variation in the sites on the female's body that are stimulated by male somatosensory stimuli, as well as signal intensity and rates and patterns of repetition. Both habituation and neural adaptation are advantageous under natural selection and could provide original, naturally selected biases in female responses that lead to Fisherian runaway evolution under sexual selection by female choice. The multiplicity of mechanisms affecting habituation and neural adaption may help explain the general evolutionary trend for male mate choice signals to diverge rapidly. Avoidance of female habituation and sensory adaptation may explain the previously unremarked but widespread trend in vertebrates and arthropods for male genitalia to make rhythmic, repetitive movements during copulation.

Male genitalia have been hypothesized to function as courtship devices during copulation, but it is difficult to use behavioral observations to test this hypothesis because male genitalia are usually hidden inside the female during copulation. In tipuloid flies, however, nearly all of the male’s complex genital structures remain outside the female. Copulation behavior and genital morphology in Tipula (Triplicitipula) colei and T. (Lunatipula) translucida suggest that some male genital structures function to stimulate the female: male structures that contact the female bear tufts or dense arrays of modified setae on precisely the surfaces that contact the female; contact involves repeated, stereotyped rhythmic movements that include brushing, vibrating, scraping, and tapping; the movements are appropriately designed to utilize the morphology of the modified setae to stimulate the female; and the movements have little or no other perceptible mechanical effects on the female. The female structures contacted by these male genital movements fail to show the defensive designs predicted by the theories of genital evolution that are based on morphological species isolation or male–female morphological conflicts of interest; also unexplained by the conflict of interest hypothesis are female movements that seem designed to increase rather than avoid stimulation by the male.

The flexible execution of highly repeated, discrete, and easily recorded tasks makes orb web construction an attractive model for a new generation of general questions concerning complex, flexible behavior by small, apparently simple animals. Accumulated data make it profitable to focus attention on one type of decision that is repeated over and over in each orb: where to attach the sticky spiral to each radius that it crosses. Spiders use at least ten cues to make this decision. Combining this progress with new neurobiological techniques and concepts makes it possible to address new questions concerning higher levels of behavioral organization, including behavioral imprecision (errors), attention and lack of attention, mental maps, open-ended versus rigidly flexible behavioral rules, and the effects of psychoactive drugs. I discuss these and other promising lines for future research. Some data suggest that at least rudimentary higher- level analytical processes occur in orb weavers.

Orb-web construction behaviour, a classic example of 'innate' behaviour, is highly flexible. Determining which cues guide construction behaviour is complicated by the strong correlations between some variables, and the difficulty of manipulating of some web variables in biologically realistic ways. This study utilized a new experimental technique, adding sticky lines to webs during construction, to examine cues that guide sticky spiral spacing. Deflections and subsequent reductions in spacing produced by experiments that involved relatively long radii resembled the reduced spacing pattern on short radii in control webs; radius length was thus not the cue eliciting reduced spacing in control webs. An alternative, that smaller inter-radius distances elicited reduced spacing, was supported by experimentally removing radii to increase the distances between radii. The new technique also elicited variation in responses that suggested that spiders may use mental cues such as memories or expectations of line positions to guide sticky spiral placement.

A new bridge between studies of sexual selection and the massive literature on Caenorhabditis elegans behaviourand nervous system properties promise to provide important new insights into both fields. This paper shows that mate choice likely occurs in hermaphrodite C. elegans on the basis of stimulation from the male genital spicules, making it possible to apply the toolkit of extensive background knowledge of C. elegans and powerful modern techniques to test in unprecedented detail the leading hypotheses regarding one of the most sweeping trends in all of animal evolution, the especially rapid divergence of genital morphology. The recognition that sexual selection by mate choice may also occur in other contexts in C. elegans suggests additional payoffs from exploring previously unrecognized possibilities that female-active hermaphrodite reproductive behaviours are triggered by male stimulation. These facultative behaviours include attracting males, fleeing from or otherwise resisting males, opening the vulva to allow intromission, guiding sperm migration, avoiding rapid oviposition following copulation that results in sperm loss, expelling recently received sperm, and increasing feeding rates following copulation.

Males of the agaonid wasp Heterandrium fallax have wing spots that they display during aggressive encounters near the ostioles of syconia of the fig Ficus pertusa, apparently in competition for the opportunity to copulate with emerging females. In accord with predictions of the “functional allometry hypothesis” for the allometry of structures that are under sexual selection as threat devices and that do not function as weapons, larger males tended to have proportionally larger wing spots. Some smaller males lacked spots, and smaller males were less likely to be collected near female emergence sites or to show antennal damage that probably results from male fights.

Abstract. Photographs of the webs of approximately 113 species in 52 genera show that the web architecture of linyphioid spiders (Linyphiidae and Pimoidae) present many variations on a single basic pattern. Nearly all species built webs with a more or less horizontal, continuous sheet with an open space just below the sheet. However, the details in the designs showed great diversity; we recognized >50 web traits, including positions relative to the ground or large objects; sheet shapes and orientations; secondary sheets; “slime” on the sheet; patterns and densities of lines in sheets; visible droplets on sticky lines; upward and downward directed dimples in the sheet with tensor lines; primary and secondary frame lines; retreats; and the presence, location, and designs of associated tangles. This survey probably substantially underestimates both intraspecific and intrageneric diversity of web forms. Intrageneric comparisons in just over 20 genera documented varying degrees of intrageneric variation; some genera show striking differences. Several web designs were widely distributed: sheets with dense tangles above commonly had arrays of lines attached to the sheet's upper surface; downward dimples in sheets commonly occurred at sites where the sheet curved upward, but upward-directed dimples were rare and small; and sheets built next to the surface of the ground almost always lacked extensive tangles above them. New web patterns not previously reported for linyphiid webs include: sandwich webs (pairs of closely spaced, otherwise naked sheets); extensive vertical sheets next to tree trunks; tubular retreats at the edges of the sheet where the spider rests; trough-like sheets just above the upper surface of a curled leaf; sizeable, apparently sticky droplets densely covering many lines that were apparently placed in pairs in the sheet; long sheet lines that converge at one corner; runways to a sheltered site where the spider waits beyond the sheet's edge; and apparent skeleton lines in the sheet (probably from early stages of construction). Patterns of lines within the sheet may reflect patterns of movement during sheet construction behavior. We propose that some webs on the substrate function to extend the spider's sensory field rather than detain prey and that some tangles below and perhaps some above the sheet may function to defend the spider from enemies. A suite of linyphiid traits, including leg, chelicera, and spinneret morphology; details of web design, such as numerous small upward-directed dimples in dome-shaped sheets but fewer, large downward-directed dimples in cup-shaped sheets; and attack behavior, appears to function to increase the speed with which spiders attack prey. The functions of many architectural features remain obscure.

Several traits of adult female Allocyclosa bifurca, including the forms and the colours of the dorsal and the ventral sides of the abdomen that match bumpy and smooth surfaces and associated colours of the two sides of the egg sac, the colours and positions of the spider's legs and the spiders' prey wrapping behaviour and their orientation of prey packages while they feed all increase the match with the overall shapes and colours of the egg sacs, visually camouflaging the spider at the hub of its orb. This camouflage is partially broken in two contexts: apparent attempts to avoid overheating in direct sunlight, and feeding. Neither a predatory bird nor a parasitoid wasp showed signs of being fooled by the camouflage.

Possible functions of several species-specific, sexually dimorphic male structures and of the male genitalia of Lytta eucera were deduced from observations of behaviour in the field and captivity, and were used to test theories of sexual selection. The male rubbed and tapped on the female’s antennae with sexually dimorphic segments of his antennae, and on her elytra with brushes of setae on his hind tarsi. He forcefully grasped the female’s prothorax and at least occasionally perforated the female’s prothoracic membranes with his modified middle tibiae, and her relatively uniform, membranous vaginal lining with his strong aedeagal teeth. His aedeagal teeth snagged her vaginal lining, and his gonostyli usually pressed against a featureless external female intersegmental membrane. None of these male structures was used as a weapon or in threat displays. No female structure fitted tightly in a “lock-and-key” manner with any of the male structures, nor was any female structure capable of selectively impeding their use, thus ruling out some hypotheses explaining their species-specificity in males. Female resistance to males, including occasional violent “tantrum” displays, was energetic, persistent, and highly effective; the functional significance of this resistance is unclear. If females distinguish the stimuli produced by species-specific traits of male genitalia, as supposed by some hypotheses, they likely use higher-level analyses in the central nervous system rather than the locations of the particular sense organs that are stimulated.

"Primary'' webs of uloborids have large numbers of very fine lines and usually lack sticky cribellum silk. This paper reviews their taxonomic distribution (19 species in 5 genera) and the ontogenetic stages in which primary webs are built (spiderlings newly emerged from the egg sac, older juveniles, mature males, and normal and senile females), expands the knowledge of construction behavior, and describes several previously unnoticed design details. Primary webs differ from typical uloborid orbs in several ways: large numbers of fine radial and non-radial lines; facultative hub removal and replacement; usually closely spaced temporary spiral loops; and lines beyond the frame lines. Construction of supplemental radii in primary webs is distinctive in several respects: break and reel construction; tendencies to lay successive radii either on opposite sides of the web or close together in the same sector; high frequencies of aborted trips from the hub to the frame; production of multiple lines during a single trip from the hub to the frame and back; long pauses during the production of single radii; and variation in the sequences in which radial lines are added to a given sector. Some aspects of primary web construction resemble araneoid rather than typical uloborid behavior. The relation between primary webs and the evolution of orb webs, and the mechanism that spiders use to produce abundant non-radial lines despite making only radial movements during web construction remain uncertain. We speculate that primary webs are favored when spiders are unable to afford the costs of producing cribellate silk for a typical orb.

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.

Abstract Linyphiid webs have often been characterized as dense horizontal sheets suspended in aerial tangles. Recent observations indicate that several web traits vary considerably between species, but nearly all information comes from small samples of webs, and little attention has been given to intra-specific variation. This paper documents the intra-web and intra-specific variation in several web traits in Linyphia simplicata, and makes brief comparisons with the webs of Neriene coosa. The objective is to provide perspective for other comparisons of the web designs in linyphiid taxa that are based on small samples. Some traits were relatively uniform and did not differ between the species; others, especially some measures of the tangles above and below sheets, were more variable but nevertheless differed significantly between the species. Two traits not previously documented in linyphiid webs occurred on both species: a central area of the sheet with larger holes; and lower densities of lines near the outer edges but higher, uniform densities in the central areas of sheets. A third new trait, objects projecting through sheets, occurred only in N. coosa.

Web designs have long been used to characterize spider taxa and to deduce the relations between them; but systematic documentation of the amount of variation in webs within and between taxonomic groups is rare. This study, based on previously published observations and new observations of 15 species in the family Uloboridae, including two genera, Octonoba Opell, 1979 and Siratoba Opell, 1979, whose webs were previously undocumented, reviews the taxonomic distribution and variation in 22 orb web traits in at least 43 species in 11 genera in uloborids. These traits appear to occur in all orb-weaving genera in which reasonable samples are available, though only small samples are available for many species. Larger samples of the webs of three species of Uloborus Latreille, 1806, two of Hyptiotes Walckenaer, 1837, and one each of Zosis Walckenaer, 1841, Siratoba, Octonoba, Waitkera Opell, 1979 and Philoponella Mello-Leitao, 1917, revealed greater intra-specific consistencies in some traits than others. Hub traits were especially consistent. Variations in three traits may represent adjustments to the size of the space in which the orb is built. "Primary'' webs, which combine orb and sheet-web traits, are built by spiderlings newly emerged from the egg sac and by adult males in at least five genera of orb-weaving uloborids and may be unique to this family. Preliminary comparisons between uloborid and araneoid orbs suggest that uloborid orbs may also differ from araneoid orbs in combining several other traits.

Spider webs in general and orb webs in particular are delicate, ephemeral structures that are frequently damaged in nature. Some orb weavers respond to damage by quickly "shoring up" their webs with non-sticky dragline silk. This study of how Micrathena duodecimspinosa (O. Pickard-Cambridge, 1890) shores up damaged frame lines shows that repairs were largely based on a single, repeated pattern of attachments. These movements are shared with the distantly related family Pholcidae, suggesting a possibly ancient origin. Spiders tended to initiate repairs at the lower edge of a damaged sector, probably to reduce the damage produced when the spider's own weight caused sticky lines in slack portions of the web to sag into and adhere to each other. Repairs of lateral frames recuperated capture area more successfully than did those of upper frames, probably because damage caused by the spider's own weight during repair was reduced.

Webs of Dictyna bellans Chamberlin, 1919 in captivity included several characteristics seen in other dictynid webs, including a fine-meshed tubular retreat of non-sticky lines with multiple exits, "runways'' of dense, fine non-sticky lines that were continuous with the floor of the retreat, and "ladders'' of cribellum silk that zig-zagged between more or less parallel non-sticky lines. They also included two traits not previously reported for dictynids: especially tightly spaced cribellum lines with tight zig-zags that were on and very close to the substrate at the edges of the web near the retreat; and extensive coiling of cribellum silk on a few long, elevated non-sticky lines. The webs of D. bellans argue against the supposition made in some studies of web evolution that webs built especially near the substrate lack complex organization. They also indicate that the degree of coiling of cribellum lines varies among dictynids and in other cribellate spiders, and may represent a useful axis of comparison that reflects adaptations for prey retention.

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.