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Numerous studies have focused on the influence of the social environment and the interactions between individuals on the collective decision-making of groups. They showed, for example, that attraction between individuals is at the origin of an amplification of individual preferences. These preferences may concern various environmental cues such as biomolecules that convey information about the environment such as vanillin, which, for some insects, is an attractant. In this study, we analysed how the social context of the cockroaches of the species Periplaneta americana modifies preferences when individuals are offered two shelters, of which one is vanillin scented. One of the principal results of our study is that isolated individuals stay longer and more frequently in a vanillin-scented shelter, while groups choose more frequently the unscented one. Moreover, the proportion of sheltered insects is larger when the group selects the unscented shelter. Our experimental results and theoretical model suggest that the individual preference is not inverted when insects are in a group but, rather, the response to vanillin decreases the attraction between individuals. As a result, aggregation is favoured in the unscented shelter, leading therefore to a collective inversion.

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Positive thigmotactic behavior is associated with the ability to hide from predators and is important to explain aggregation and collective patterns in various animals. For example, this behavior has been observed in woodlice, domiciliary cockroaches, ants, and fish. Lately, research on different species is focused on the importance of animal personality for ecological and evolutionary processes, individual fitness and group cohesion. In fact, it is generally expected to find some degree of interindividual consistent differences for a behavior, unless specific circumstances, like predator attacks, hide the presence of personalities. In this research, we analyzed the individual thigmotactic preference of domiciliary cockroaches (Periplaneta americana (Linnaeus, 1758) (Blattodea: Blattidae)) and how it affected the fleeing behavior of isolated individuals inside a shelter after receiving a light stimulus. We notably highlight how isolated individuals show different consistent preferences regarding their position in the shelter, which is due to the individual thigmotaxis level, before the fleeing behavior. During the fleeing itself, cockroaches nearer to the wall, and therefore with more positive thigmotaxis, showed slower reaction lantencies to the stimulus. We propose that thigmotaxis homogenizes the interindividual differences among individuals and is important to explain the individual and collective fleeing behavior.

Aggression toward competitors is a useful measure of resource ownership and defense in animals, but aggressive behavior is costly. Therefore, it is predicted that animals will display aggression only when the expected benefit to individual fitness exceeds the expected cost. In ants, when conspecific individuals belonging to different colonies encounter each other, fighting occurs, seemingly facultatively. However, the context that influences the expression of ants' aggressive behavior, especially in the field, is still largely unknown. We investigated the plasticity of aggressiveness toward non-nestmates in Diacamma sp. from Japan. Our field experiment clearly showed that the same foragers that were aggressive toward non-nestmates in the vicinity of their nest changed to be non-aggressive at greater distances from the nest. Furthermore, the size of the colony to which the foragers belonged weakly but significantly affected their aggressiveness: foragers belonging to larger colonies behaved more aggressively toward non-nestmates. We discuss the possible adaptive significance of the observed facultative aggression between conspecific non-nestmates. Digital video images related to the article are available at http:// www.momo-p.com/showd etail-e.php?movie id=momo1 90618 ds01a and http://www.momo-p.com/showd etail-e.php?movie id=momo1 90618 ds02a .

Group-level properties, such as collective movements or decisions, can be considered an outcome of the interplay between individual behavior and social interactions. However, the respective influences of individual preferences and social interactions are not evident. In this research, we study the implications of behavioral variability on the migration dynamics of a group of gregarious insects (Periplaneta americana) subjected to two different disturbance regimes (one without disturbances and another one with high frequency of disturbances). The results indicate that individuals presented consistent behavior during the nighttime (active phase of cockroaches) in both conditions. Moreover, we used a modeling approach to test the role of personality during the migration process. The model considers identical individuals (no personality) without memory and no direct inter-attraction between individuals. The agreement between theoretical and experimental results shows that behavioral variability play a secondary role during migration dynamics. Our results showing individual personality during the nighttime (spontaneous decision to forage) but not during the emigration process (induced by environmental disturbances) highlight the plasticity of personality traits.

The original article has been corrected.