Gail E. Stratton

Sexual selection, male morphology, and the efficacy of courtship signalling in two wolf spiders (Araneae: Lycosidae)

Abstract Males of the brush-legged wolf spider, Schizocosa ocreata (Araneae: Lycosidae), possess a conspicuous male secondary sexual character: dark pigmentation and tufts of bristles on the tibiae of their forelegs. We tested several hypotheses relating to the role of this conspicuous trait in sexual selection. Triad mating experiments suggest that the tufts do not play an obvious role in the operation of sexual selection by either male competition or female choice, as there were no significant differences in the mating success of intact and experimentally shaved males. However, females mated more often with males that initiated courtship first, suggesting that capture of a female’s attention by male signalling may play a critical role. In behavioral experiments that paired a single male with a female in arenas that allowed both visual and vibratory signal transmission during courtship, female receptivity did not vary significantly with the presence or absence of tufts. However, experiments that isolated the visual component of communication (by eliminating vibratory communication) revealed a significant effect of the presence of tufts: females showed receptivity less often to males with tufts removed. Female response to visual signals was much greater in S. ocreata than in its sibling congener, Schizocosa rovneri, which lacks male tufts. We hypothesize that the tufts serve to increase the efficacy of visual displays of S. ocreata, as vibratory communication is constrained by the complex leaf litter habitat of some populations. Such environmental constraints may make visual signalling over distance a critical factor for effective courtship communication, which may in turn strongly influence male fitness.

THE INHERITANCE OF COURTSHIP BEHAVIOR AND ITS ROLE AS A REPRODUCTIVE ISOLATING MECHANISM IN TWO SPECIES OF SCHIZOCOSA WOLF SPIDERS (ARANEAE; LYCOSIDAE)

The courtship behaviors of two morphologically similar spider species, Schizocosa ocreata and S. rovneri, are distinctive and prevent interbreeding. We used “forced” copulation between these species to investigate the mode of inheritance of the courtship behavior and to determine whether postmating isolating mechanisms exist. F1 hybrids proved to be behaviorally sterile, but they were capable of producing viable offspring when forced to interbreed. Analysis of the courtship behaviors of F1, F2, and backcross progeny showed that the inheritance of some aspects of these behaviors is consistent with models involving single autosomal loci. The inheritance of secondary sexual characteristics in the males is also investigated. The genes for courtship behavior and secondary sexual characteristics do not assort independently.

EVOLUTION OF ORNAMENTATION AND COURTSHIP BEHAVIOR IN SCHIZOCOSA: INSIGHTS FROM A PHYLOGENY BASED ON MORPHOLOGY (ARANEAE, LYCOSIDAE)

Abstract A phylogenetic analysis for the North American Schizocosa species was undertaken by scoring 49 morphological characters for 31 taxa representing all of the Nearctic species of Schizocosa plus individuals that are hybrids between S. ocreata and S. rovneri. Rabidosa rabida, Allocosa georgicola and Gladicosa pulchra were used as outgroups. Three clades are recognized: a large clade from eastern North America (Clade A) within which is nested the S. ocreata clade; Clade B, which includes the widespread S. avida and the western S. mccooki, and a smaller, third clade, Clade C. Sexual ornamentation occurs on the first legs of mature males of several species within the Schizocosa and takes the form of pigmentation and or bristles primarily on the tibia of leg I; there is at least one species with bristles in each of the three main clades. Mapping the occurrence of male ornamentation on the preferred phylogeny suggests that ornamentation evolved 5 or 6 separate times and was subsequently lost 2 or 3 times. The ornamentation is concentrated in the S. ocreata clade, a clade defined by a finger like projection on the paleal process of the male pedipalp. Courtship behavior is known for 20 of the 31 taxa. All species studied utilize chemical communication and seismic signals for communication; some species also have distinct visual signals. Seismic signals are produced by palpal drumming (as is seen in several species within Clade B), or by stridulation (seen in Clade A). Visual signals consisting of movements of the first pair of legs are common in species that are distinctly ornamented. This study provides the first phylogenetic study of a North American genus of wolf spider and provides morphometric comparisons of the North American species in Schizocosa.

Geographical variation in male courtship behaviour and sexual isolation in wolf spiders of the genusSchizocosa

We surveyed 12 populations of the wolf spider Schizocosa crassipes (Walckenaer) and S. nr. crassipes in Tennessee, Mississippi, Louisiana and Florida, in the U.S. to determine the extent of variation in male courtship behaviour when observed in standard laboratory conditions. We observed variation in both the frequency of occurrence and the sequence of the four principal male courtship behaviours and, when compared statistically, most of these behaviours differed between populations. Although there was no clinal pattern in the frequency of courtship behaviour, we observed geographically consistent patterns in the sequence in which male courtship behaviours are displayed. We conducted two subsequent studies to determine whether sexual isolation among selected populations existed. In the first study, we performed male-female reciprocal crosses of brush-legged spiders (S. nr. crassipes) between two populations from the Yazoo-Mississippi Delta physiographical region and one population from the Loess Bluff region of Mississippi and measured mating success, number of matings that produced egg sacs, and the number of egg sacs that hatched young. The results of that experiment suggest that sexual isolation occurs among populations in different physiographical regions but not between populations within regions of the state. In a second crossing study, we paired the brush-legged spiders from one of the Mississippi physiographical regions (Yazoo-Mississippi Delta) with Schizocosa ocreata (Hentz) from Ohio. The results of that cross suggest that the Mississippi Delta population is more similar to the northern S. ocreata populations than to S. crassipes. We discuss the results of the crossing experiments in terms of what was revealed about geographical patterns in male courtship behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Substrate-dependent signalling success in the wolf spider, Schizocosa retrorsa

Signals used in communication are often hypothesized to be optimally designed for their signalling environment. Here, we explore the importance of signalling substrate on seismic signal efficacy and reproductive behaviour in the wolf spider, Schizocosa retrorsa: a species found on multiple signalling substrates (pine litter and/or red clay or sand). In this multimodal signalling species, simultaneous with conspicuous visual displays, males produce percussive seismic signals via an impulse mechanism which tends to excite a substrate evenly across a wide band of frequencies. We first quantified the transmission characteristics of this broadband percussive signal by playing recorded signals back across three naturally occurring substrates, two of which represent substrates upon which S. retrorsa is commonly found: leaf litter, pine litter and red clay (the latter two exemplify their natural habitat). The substrates varied in their transmission characteristics with respect to both attenuation (higher on red clay) and filtering. Next, we compared copulation success, courtship behaviour and microhabitat choice among these same substrates. Copulation frequency was higher on the natural substrates of pine litter and red clay as compared with leaf litter. Males took longer to initiate courtship on leaf litter, but once initiated, courtship behaviour did not vary across substrates and we were not able to discern any choice with respect to the first, or the most common, substrate chosen. Our results show that while S. retrorsas percussive signals may not be matched to the specific properties of any one substrate, copulation success was substrate dependent and we discuss potential explanations for this substrate-dependent signalling success.

WATER SURFACE LOCOMOTION BY SPIDERS: DISTINCT GAITS IN DIVERSE FAMILIES

Abstract Pisaurids such as Dolomedes triton (Walckenaer 1837) are well known as inhabitants of ponds and streams and are adept at locomotion on the water surface. In a broad survey of water surface locomotion in spiders, we have found that most taxa do not use specialized gaits under these circumstances. However, some tetragnathids, araneids, and salticids (three families that are outside of the superfamily Lycosoidea to which the pisaurids belong) resemble D. triton to the extent that they do use specialized gaits when on the water surface. Of these, the tetragnathids are particularly accomplished at water surface locomotion, achieving velocities that exceed those of D. triton when it rows, but not when it gallops.

Spitting Performance Parameters and Their Biomechanical Implications in the Spitting Spider, Scytodes thoracica

Abstract Spitting spiders Scytodes spp. subdue prey by entangling them at a distance with a mixture of silk, glue, and venom. Using high-speed videography and differential interference contrast microscopy, the performance parameters involved in spit ejection by Scytodes thoracica (Araneae, Scytodidae) were measured. These will ultimately need to be explained in biomechanical and fluid dynamic terms. It was found that the ejection of “spit” from the opening of the venom duct (near the proximal end of the fang) was orderly. It resulted in a pattern that scanned along a lateral-medial axis (due to fang oscillations) while traversing from ventral to dorsal (due to cheliceral elevation). Each lateral-to-medial sweep of a fang produced silk-borne beads of glue that were not present during each subsequent medial-to-lateral sweep. The ejection of “spit” was very rapid. A full scan (5–57 fang cycles, one upsweep of a chelicera) typically occupied less than 30 ms and involved fang oscillations at 278–1781 Hz. Ejection velocities were measured as high as 28.8 m/s. The “spit” was contractile. During the 0.2 s following ejection, silk shortened by 40–60% and the product of a full scan by both of the chelicerae could exert an aggregate contractile force of 0.1 – 0.3 mN. Based on these parameters, hypotheses are described concerning the biomechanical and fluid dynamic processes that could enable this kind of material ejection.

Communication in spiders

Abstract As in other animals, the exchange of signals between spiders serves many purposes and in recent years it has been shown that they have evolved a very complex system. This includes the exchange of information by acoustic/vibratory mechanisms; by visual means; and by pheromones. This article reviews these systems and the circumstances in which they are brought into play.

SCYTODES VS. SCHIZOCOSA: PREDATORY TECHNIQUES AND THEIR MORPHOLOGICAL CORRELATES

Abstract Wolf spiders (Lycosidae) typically subdue prey using their legs for capture and their fangs for the injection of venom. Spitting spiders (Scytodidae), in contrast, subdue prey by entangling them, at a distance, in a spitted mixture of silk, glue, and venom that immobilizes and may also kill them. We selected individuals of Schizocosa duplex (Lycosidae) and Scytodes sp. (Scytodidae) of approximately the same mass and carapace width to provide a quantitative assessment of their relative allocations of biomass to morphological features that might be expected to vary with prey-capture technique. As expected, the wolf spiders allocated significantly more to legs, chelicerae, and fangs, and significantly less to the venom glands, than did the spitting spiders. Further comparisons of the legs and chelicerae of the two species provided surprises. First, the legs of Scytodes were 42% longer than those of Schizocosa despite smaller overall allocation to the legs in Scytodes. And second, although the relative sizes of the chelicerae differ greatly, the shapes of the chelicerae of Schizocosa and Scytodes were not significantly different despite the radically different tasks those structures must fulfill.

Mechanics and Energetics of Excavation by Burrowing Wolf Spiders, Geolycosa spp.

Abstract Burrowing wolf spiders, Geolycosa sp. (Araneae:Lycosidae), excavate vertical burrows and inhabit them throughout their lives or, in the case of males, until they mature and wander in search of mates. Three species: G. fatifera Kurata, G. missouriensis Banks, and G. rogersi Wallace were studied to understand how and at what expense the burrowing is accomplished. Normal and high-speed videography coupled with scanning electron microscopy revealed (a) that the convex surfaces of the two fangs, together, constitute the digging tool, (b) that boluses of soil are transported to the burrow entrance on the anterior surfaces of the chelicerae held there by the pedipalps, and (c) that each bolus is either incorporated into the growing turret or flung away, propelled by the forelegs. To elucidate the energetics of burrow construction, burrow volumes were calculated and then the costs associated with dislodging, elevating, and throwing the known volumes of soil were measured. A typical Geolycosa burrow, at a volume of 23.6 ± 2.0 ml and a depth of 13.2 ± 0.7 cm, required the removal of 918 boluses each weighing about 34 mg. The aggregate dislodging cost was close to 1.9 Joules in sand/sandy loam and 5.6 J in clayey subsoil, the work against gravity necessary to raise all of the boluses to the surface was about 0.13 J, and the aggregate cost of flinging the boluses was close to 0.014 J. Assuming that the ratio of external work to metabolic cost of external work is between 0.20 and 0.25 in spiders, the real cost of burrow construction would be between 8 J and 29 J, depending primarily on soil type. This is a small but not negligible cost when placed in the context of reproductive effort: a single Geolycosa egg, dozens to hundreds of which are produced in a clutch, contains about 10 J.