Onychophora lead a cryptic lifestyle as predators in decaying logs and leaf litter. Hence, very little is known about their behaviour. They are generally assumed to possess only a limited behavioural repertoire, even though their surprisingly evolved brain suggests otherwise. Our studies on the Australian species Euperipatoides rowelli show for the first time that onychophorans are indeed capable of complex behaviour. Euperipatoides rowelli individuals form aggregations of up to 15 females, males and young. These aggregations are not random assemblages, but close social groups organized in a hierarchy based on female dominance. Food is hunted collectively, but the dominant female of a group feeds alone, before other females, males and young. Hierarchy within a group is established by aggressive-dominant and passive-subordinate behaviours, the latter leading to tolerance of body contact and aggregation. Euperipatoides rowelli from foreign groups, i.e. from different logs, are met with intense aggression, and individuals rarely aggregate. The reasons for this aggression are not clear, but we suggest that its origins lie in kin recognition. The evolution of social behaviour within the Onychophora is discussed with respect to their ecology, population genetics and phylogeny.

Embryos of four ovoviviparous species of Australian onychophorans were examined to establish the process of their development, their reproductive cycle and to estimate the gestation period. The process of development is the same in all four species with, in the early embryo, segment halves separated by broad bands of extra-embryonic ectoderm and the embryos growing by the posterior addition of segments. Seven developmental stages (I–VII) can be clearly identified from external morphology. Analysis of the developmental stages of the embryos present in females collected throughout a calendar year has allowed estimation of the time taken for each developmental stage and demonstrated that there is an annual reproductive cycle with a total gestation period of c. 12 months. Stages I and VII are prolonged, the latter related to the late completion of midgut development. The simultaneous presence of stage I and stage VII embryos at certain times of year indicates an overlap of generations within the uteri, which is more extensive for some species than others. As the stage VII embryos of one cohort complete their development, enlarged oocytes are released from the ovaries to become fertilized as they pass the seminal receptacles and enter the uteri to start their embryonic development to form the next cohort. Cephalofovea clandestina, Phallocephale tallagandensis and Ruhbergia bifalcata have head structures in males, Euperipatoides rowelli does not. The presence of embryos at all times of year in the uteri of mature females has implications for mating and sperm storage that are discussed.

Life-history details of the live-bearing Peripatoides novaezealandiae (Hutton) were obtained by measuring and dissecting field collected animals. As in other studies, the sexes were found to have different demographics. Sex-specific weight frequency distributions and regression of weight vs length suggest sex differences in patterns of growth and/or mortality. Mature males were smaller than mature females; females apparently undergo a further period of growth between maturity and the onset of embryo production. The sex ratio of the whole sample was female biased; the sex ratio in the smallest size class was 1:1, and then oscillated from female- to male-, and then back to female-biased as size increased. Embryos were unable to be sexed. The sex ratios in individuals of mature (i.e. mating) size, and of reproductive size, were both 1:1. Females, especially but not exclusively before the onset of embryo production, had widely varying numbers of sperm in their haemolymph, indicating dermal-haemocoelic sperm transfer. These data also suggest that females are able to mate throughout life. Numbers of sperm in spermathecae of females were negatively correlated with body size. Fecundity increased with size in both sexes, somewhat more so in females. Embryo weight was correlated with maternal weight only at the beginning of the embryo's development. Rotting logs containing small populations contained a disproportionate number of large females.

The remarkable diversity in chromosome number within Planipapillus Reid 1996, the most speciose genus of peripatopsid onychophorans, centred in the alpine region of south-eastern Australia is documented. Robertsonian, whole-arm rearrangements account for the twofold range of diploid numbers in Planipapillus. Ooperipatellus Ruhberg 1985, another clade of oviparous onychophorans, shows a very different pattern, with no karyotypic diversity among species from Australia and New Zealand. Rate constancy in chromosomal evolution among peripatopsid genera would indicate an ancient radiation in Planipapillus, with extant species representing relictual survivors of more recent Pleistocene climatic upheavals. Conversely, if the rearrangements in Planipapillus are the result of recent and rapid karyotypic restructuring, the karyotypic and species diversity within the genus may be attributed to recent population fragmentation and isolation resulting from the repeated glaciation and warming cycles of the Pleistocene rather than stemming from a more ancient radiation. Data from other animal groups support a model of accelerated rates of Robertsonian centric fusions concordant with a recent radiation in Planipapillus. Karyotype change may be an important factor in the generation and maintenance of Planipapillus diversity.

While mating behaviour in onychophorans has been documented in only a few species, it would appear that male reproductive strategies include both dermal and vaginal insemination. To add to this diversity, the males of many newly described species of Australian onychophorans display elaborate head structures. In some species, these head structures have been shown to carry spermatophores. Here we describe Florelliceps stutchburyae gen. nov., sp. nov., a species possessing a unique head structure. An account of its unusual mating behaviour is provided in which the male's everted head structure was placed against the female's gonopore and held there by the claws of the female's reduced last pair of oncopods. After the individuals separated, the female's genital opening was covered by a spermatophore and histological sections showed that sperm had been released into her reproductive tract. A review of mating behaviour in onychophorans is also included with a discussion of sperm acquisition by the female and its correlation with her reproductive cycle.

We demonstrate that the males of the peripatopsid Euperipatoides rowelli secrete a pheromone from crural papillae, which acts as an attractant to both males and females of the species. Patterns of colonization of decomposing logs and differences in sex ratio between incipient vs established populations indicate that males are the initial dispersers and colonizers, finding suitable log habitats in an exploratory fashion, while females subsequently colonize logs. This results in a disproportionately high frequency of males in newly colonized logs, followed by a gradual increase in female proportion. We argue that females use the aggregating pheromone secreted by males to target appropriate rotting log microhabitats, resulting in a clumped distribution of females compared to a more random distribution for males among recently colonized logs. This mode of colonization reduces the time that animals, especially females, spend outside the safety of suitable logs, and the risk-taking strategy of male exploration may explain the marked sex ratio bias in favour of females in the population. Rapid and non-random dispersal may also account for the contradictory evidence of unexpectedly high rates of colonization of new logs, yet a paucity of animals in leaf litter samples. The possibility that pheromone trails play a part in the dispersal process is discussed.

The reproductive biology of the ovoviviparous peripatus Euperipatoides rowelli was investigated from field collections and laboratory cultures. The sexes have different demographics. The frequency distribution of individual weight is essentially L-shaped in females, but closer to normality for males: thus the sexes must exhibit different patterns of growth and/or mortality. Males are generally much smaller and rarer than females. The primary sex ratio seems to be 1:1 with equal investment in the sexes, while the tertiary ratio is highly female-biased. Logs with fewer individuals tend to be male-biased while well-populated logs tend to be female-biased. Males mature at 15–30% of the bodyweight of mature females. The weight frequency distribution of males without developed sperm in their tracts is strongly skewed to the lower weights, while that of males with sperm is more normally distributed, indicating that sperm production occurs as soon in life as possible. Males mature in their first year of life, if growth rates in culture may be extrapolated to the wild. In contrast to this rapid maturity in males, females may mature as late as their second or third years. Most mature females, and many prior to maturity, carry sperm in their spermathecae. After maturity, there is an approximately linear relationship between body mass and number of developing embryos. Reproduction in E. rowelli is significantly seasonal despite high individual variance, with a major bout of parturition in November–December (summer). A female can harbour one developed and one undeveloped batch of embryos in each uterus. Excesses of developed embryos in one uterus are counterbalanced by deficits of undeveloped ones, indicating that females can use their paired reproductive tracts independently. Individual females in culture can experience episodes of parturition approx. 6 months apart without re-mating, thus gestation may be 6 months or more. Sperm in spermathecae remain capable of vigorous swimming for at least 9.5 months.

The microenvironments of decaying logs support a large invertebrate fauna, but are vulnerable to the effects of logging and land-clearing. We present an analysis of the relationship between a number of environmental variables and the occurrence and abundance of the onychophoran Euperipatoides rowelli within decomposing logs in a forest in south-eastern Australia. Aspect of slope is strongly associated with most of the environmental variables and is a good predictor of both presence and abundance of animals in logs; south-easterly slopes show higher levels of log colonization, larger populations within logs and a larger median weight of both males and females than north-westerly facing slopes. Degradation state of logs, log length, presence of termites and shrub cover are also associated with the presence of onychophorans, while log volume shows the most marked association with abundance. Euperipatoides rowelli prefers wetter logs, although there is considerable overlap between relative water content of logs with and without onychophorans. Log degradation state, an important factor in the presence of animals, is associated with years since felling, and our data indicate that logs become capable of supporting large populations of E. rowelli after a minimum of 45 years of decay. Thus future logging practices have important implications for the maintenance of suitable log environments for this species, and this result may be generalized to other log-reliant fauna.

An onychophoran-like fossil animal, Paucipodia inermis gen. et sp. nov., is described from the Lower Cambrian Chengjiang lagerstatte in Yunnan, China. The animal was soft-bodied, and possessed six pairs of unjointed legs, each having two claws distally. The body was finely annulated, lacked trunk plates, and apparently terminated posteriorly with the last leg pair, without preserved evidence of a posterior trunk extension. Paucipodia widens the morphological range hitherto known in Cambrian lobopodians, notably regarding leg number and presence or absence of plates. Its morphology also provides support for the recently suggested anteroposterior orientation of Hallucigenia and Microdictyon.