Living in groups is widespread and found in insects, spiders, birds and mammals, among other animals. Individuals that live in groups obtain benefits such as predator avoidance, foraging efficiency, and enhanced reproductive success. However, group living has Forsythin associated costs: compared to solitary individuals or small groups, individuals in large groups can incur costs such as increased competition for resources. Moreover, group-living animals are faced with the potential risk of accumulating pathogens that can spread more easily between group members. Therefore, group living can not only be costly in terms of competition between individuals but also in terms of pathogen defense and disease contagion. One important cost derived from contagious diseases is the activation and use of immune responses. Immunity can be costly because of toxic byproducts of immune reactions or because it requires resources that are spent at the expense of other functions. To decrease these costs, some group-living animals modulate their investment in immune response according to the risk of infection. Under crowded conditions, some insects show more active immune system compared to organisms living in low densities, this might allow them to be more resistant than individuals kept solitarily. Such density-dependent activation of immune responses can be interpreted as an adaptive strategy to decrease the costs associated with the maintenance and activation of immune defenses. A simpler strategy to deal with microorganisms is to avoid contagion, either via behavioural Saikosaponin-C avoidance of infected individuals or places, hygienic behaviour in the nest or via chemical avoidance with antimicrobials on the skin or cuticle. Despite incurring some cost, both behavioural and chemical protections can reduce the cost of activating the immune system once the pathogen has infected the host. The subsocial crab spider Diaea ergandros lives in nests built from Eucalyptus leaves. Nests contain up to 70 spiderlings that are usually the offspring of a single female. These nests persist several months and all spiders of the group communally enlarge the nest by a aching more leaves. The inside of these nests can be quite sealed, moldy and contain food debris, and therefore favors the development of pathogens with the risk of pathogenesis being elevated at increased conspecific density. Furthermore, infections can be particularly dangerous because group members are close relatives and there is thus low genetic variability that could result in more susceptible groups. Previous experimental research on D. ergandros shows that individuals in large groups build larger and more protective nests and survive be er in the presence of a predator compared to small groups or singly kept spiders. However, the influence of pathogen pressure on large spider groups might be higher but has not been explored yet. The main aim of this study was to investigate whether 
D. ergandros spiders have developed density-dependent polyphenism pathogen defenses. Antifungal cuticular response in both natural nests and artificial nests of varying density was measured. Costs involved in the maintenance of cuticular antifungal activity were examined by measuring spiders’ lipid body reserves.