Due to their widespread use as model systems and their reputation as living fossils, horseshoe crabs (Xiphosura) have been studied intensively by physiologists and paleontologists. The close phylogenetic relationship between horseshoe crabs and arachnids might also have been expected to inspire studies of xiphosurans by comparative arachnologists, but surprisingly few have been undertaken. Here, the first exhaustive survey of muscular anatomy of the Atlantic horseshoe crab is conducted as part of an on-going study of the evolutionary morphology and phylogeny of arachnids. Dissections of adult and immature individuals established 113 muscle groups comprising over 750 individual muscles, with several being recognized or correctly described for the first time. New insights into skeletomuscular evolution and phylogeny of arachnids were derived primarily from the axial muscle system. Specifically, it is argued that Limulus retains a box-truss axial muscle system like that of plesiomorphic members of other arthropod groups, that this is also a plesiomorphic condition for Chelicerata, and that arachnids are united by the loss of one component of this system, the anterior oblique muscles. Combined with comparative morphological and molecular evidence from previous studies, this study adds greater weight to the widely held view that, among extant chelicerates, Xiphosura and Arachnida are monophyletic sister groups and counters recent speculation that scorpions are more closely related to xiphosurans than to spiders, whipscorpions and other arachnids.

Diplocentrus lindo new species, from west Texas, USA, central Nuevo Léon and northern Coahuila, México is described. This description is based on the morphological examination of 199 specimens from nine Texas counties and the Mexican states of Coahuila and Nuevo Léon. This species represents the third Diplocentrus known from the state of Texas, has a wider distribution than D. diablo and D. whitei, and it exhibits a marked range in adult size.

In this study we present a redescription of Scytodes championi, S. romitii and S. guttipes. Seven species are newly described: S. panamensis from Panama; S. vaurieorum and S. chiquimula from Guatemala; S. cogu and S. armata from Costa Rica; S. tegucigalpa and S. zamorano from Honduras. Four of these were described as variations of S. championi in a recent revision of the species of Central America. New records are presented for S. championi, S. romitii, S. guttipes, S. gertschi and S. cubensis.

The Chinese psechrid spiders of the genera Fecenia and Psechrus are reviewed. The species Fecenia hainanensis is newly synonymized with F. cylindrata. The species P. mimus is considered a nomen dubium. The species P. senoculata is regarded as a valid species. The male is newly described for P. tingpingensis. Three new species are described: P. jinggangensis new species, P. rani new species, and P. taiwanensis new species. In all, nine psechrid species are recognized from China. The spinnerets, trichobothria, and tarsal organ morphology of P. tingpingensis are presented. A key to Chinese Psechrus species is also provided.

A comparison of the biology and karyotypes of Zodarion germanicum and Zodarion rubidum (Araneae, Zodariidae) which occur in central Europe was carried out. Surprisingly, these species were found to differ in a number of characters such as pattern of activity, reproduction and karyotypes. Zodarion germanicum was observed to be diurnal, whereas Z. rubidum is nocturnal. Courtship and mating were markedly longer and more complex in Z. germanicum than in Z. rubidum. Females of Z. germanicum produced only one or two successive egg sacs including 17 eggs on average which they would guard, while females of Z. rubidum produced up to 5 egg sacs each having 4 eggs that they abandoned. The two species differ from each other also in number of chromosomes and the sex chromosome system. Results suggest these species belong to distant evolutionary lineages within the genus Zodarion.

On the evening that it will kill its host, the orb-weaving spider Plesiometa argyra, the larva of the ichneumonid wasp Hymenoepimecis argyraphaga induces the spider to perform highly stereotyped construction behavior and build an otherwise unique “cocoon web” that is particularly well-designed to support the wasp larva's cocoon. Cocoon web construction behavior is nearly identical with the early steps in one subroutine of normal orb construction, and is repeated over and over. Usually all other normal orb construction behavior patterns are completely or nearly completely repressed. Experimental removal of the larva one or a few hours before cocoon construction would normally occur is sometimes followed by nearly normal cocoon web construction, and sometimes by construction of other highly altered web designs. The mechanism by which the larva induces these changes in the spider's behavior is thus apparently a fast-acting chemical, with effects that are manifested gradually. Partial recovery of orb designs sometimes occurred several days later.

Populations of four species of Pardosa, P. fuscula, P. groenlandica, P. hyperborea and P. moesta, were sampled during summer 1997 on the west coast of the Island of Newfoundland, Canada. Measurements of carapace width indicated that all four species fit a biennial life-cycle model where new individuals join the population in summer, live through the following winter, grow throughout the next year, live through the next winter, and then mature, breed and die in the year following their second winter. All species showed only one defined recruitment of new spiderlings during the sampling period, but at least two species may have extended periods of recruitment and some individuals may have an extended life-cycle.

Synonymy among three species of linyphid spiders of the genus Frontinella F. O. Pickard-Cambridge 1902 is established based on field association between males and females, mating records, and morphological data from recently collected specimens. It is concluded that F. caudata Gertsch & Davis 1946 and F. lepidula Gertsch & Davis 1946 are both junior synonyms of F. tibialis F. O. Pickard-Cambridge 1902. A redescription of this species is included.

Experiments were conducted to determine the effects of male-male agonistic encounters on changes in monoamine neurotransmitter concentrations in the supraesophageal ganglion (brain) of the tarantula, Aphonopelma hentzi. Serotonin levels were significantly reduced 30 min after fighting in both dominant (66.5 ± 9.1 SE nmol/mg protein) and subordinate (42.8 ± 7.6) animals as compared to isolated controls (89.7 ± 13.2), and these differences persisted for up to 24 h. A similar decrease was found for octopamine concentrations in dominant (43.7 ± 7.7) and subordinate (31.2 ± 4.9) spiders when compared to controls (56.9 ± 5.8). In addition, serotonin and octopamine levels were significantly lower in subordinate vs. dominant spiders. Agonistic interactions had no effect on the concentrations of dopamine, norepinephrine, and epinephrine. In isolated control spiders, serotonin (89.7 ± 13.2 SE nmol/mg protein) was present in highest concentration in the brain, followed by octopamine (56.9 ± 5.8 nmol/mg), dopamine (22.4 ± 3.8 pmol/mg), norepinephrine (15.3 ± 4.7 pmol/mg), and epinephrine (0.57 ± 0.2 pmol/mg). The results indicate that following agonistic encounters, monoamine concentrations in the brain decrease to different levels in winners and losers. This is the first demonstration that the establishment of social status causes changes in brain monoamines in spiders.

Based largely on 668 one-hour samples collected during a survey of spiders in 16 major habitats of the Great Smoky Mountains National Park, habitat distributions, life history patterns, and other natural history traits are described for 14 species in the related theridiid genera Theridion, Rugathodes, and Wamba. Two to eight of these species were found in each of the 16 habitats. Among-habitat differences in the kinds and relative abundance of these species suggest that they may be good predictors of habitat. Richness, diversity, and evenness of this species assemblage are highest in middle to low elevation habitats. Rugathodes aurantius and R. sexpunctatus, two boreal sister species, are abundant in the highest elevation habitats, but differ sharply in microhabitat and habitat preference. Theridion frondeum is much more common in high elevation habitats than is its sister species, T. albidum, which is virtually limited to middle and low elevation habitats. Theridion lyricum is most common in dry, pine-dominated forests. The three most common species (R. aurantius, R. sexpunctatus, and T. frondeum) have a simple annual life cycle of five or six instars and similar phenologies: they mate during late spring (R. aurantius and R. sexpunctatus) and early summer (T. frondeum) and over-winter in antepenultimate and/or penultimate instars. Female-biased sex ratios were observed in juvenile cohorts of these species. Rugathodes aurantius, its natural history previously unknown, places its webs on the undersides of broad-leafed herbs close to the ground and captures small flying insects. Adult females engineer partly folded leaf retreats, carry the egg sac when disturbed, help their instar II spiderlings exit the egg sac, and then share the retreat with these spiderlings for at least a few days. Rapid early development (about two weeks from oviposition to emergence from the egg sac), the presence of females with egg sacs throughout the summer, and smaller clutch sizes in late summer suggest that a typical R. aurantius female produces more than one clutch.

In sub-social spiders, restricted dispersal of young (i.e., natal philopatry) and the potential for inbreeding could contribute to within-population subdivision, thus resulting in a population structure similar to that found in social congeners. In this context, we analyzed the origin and mode of individual distribution patterns and their contribution to within-population structure in juveniles of the sub-social spider Stegodyphus lineatus. We investigated the distribution of juveniles for four months after leaving the maternal nest using allozyme genetic markers. We found that isolated groups of juveniles consisted predominantly of siblings, whereas larger aggregations of individuals showed mixing of different juvenile sibling groups. However, even within such aggregations, sibling groups could be identified. Within the population at large, a heterozygote deficit and an uneven distribution of alleles were found. This was caused by limited movement of juveniles and males away from the natal site. Thus, the within-population (intrademic) structure could be partitioned into two components, resulting from kin-groups and population subdivision into demes. We compare this type of population structure with that found in non-social and social species, and discuss whether it provides conditions that could favor the evolution of sociality.

Eilica pomposa new species, from Buenos Aires, Argentina, and the male of E. uniformis (Schiapelli & Gerschman 1942) are described for the first time. New records from Argentina for E. uniformis, E. modesta and E. trilineata are included.

Field observations of Homalometa chiriqui (Araneae, Tetragnathidae), a common habitant in coffee plantations in Chiapas, México, provide biological information on this poorly known species. Collection data revealed several generations per year. The web architecture and microhabitat selection of young juveniles differ from those of older juveniles and adult females. Females deposit their eggs inside the retreat forming a straight cylindrical egg-rod.