Even cockroaches get lonely.

The research highlights section of the July 29 Nature points out an interesting study from Lihoreau et al. suggesting why, when you find cockroaches in your home, "there's never just one."

Even cockroaches develop psychological problems if they are denied a normal social life. Animals reared in solitude are less likely to explore new environments or search for food, are more timid when approaching other cockroaches and are less able to spot the signs of a good mate. The effects of solitary confinement parallel those of 'isolation syndrome' described in a variety of vertebrates, and the suggestion is that it may develop when any group-living species is denied company.

Here is the abstract from the Lihoreau et al study:

Social isolation has dramatic consequences on the development of individuals of many vertebrate species, and it induces a set of behavioural disturbances rending them unable to process environmental as well as social stimuli appropriately. We hypothesized that isolation syndrome is a ubiquitous trait of social life that can be observed in a wide array of species, including invertebrates. Here we report that gregarious cockroaches (Blattella germanica) reared in isolation showed (i) stronger exploration-avoidance, (ii) reduced foraging activity, (iii) reduced willingness to interact socially, and (iv) reduced ability to assess mating partner quality than conspecifics reared in groups. We demonstrate the occurrence of a behavioural syndrome induced by social isolation, similar to syndromes described in vertebrates, revealing the importance of social interactions and group-living in this non-eusocial insect species. We suggest that investigating social isolation effects on individual development should provide interesting results to assess social cohesion of species and thus constitute an additional tool for comparative studies focusing on the evolution of social life.