An example of an invader having no such merit, but harmless, and that has become tolerated through necessity, is, in my opinion, the little white wood-louse before mentioned. It apparently has now lost the power of rolling itself into a ball, but when it first began to penetrate into the galleries of _Formica flava_—our little yellow ant—it may very well have had it, and this would have rendered it impervious to attack, whilst its weight and round scaly surface would have made the task of removing it almost an impossible one. Thus, perforce, it stayed where it wished to stay, penetrating, perhaps, deeper and deeper into the labyrinth of galleries, as successive generations of cave animals have retreated farther and farther from the light of day, until at length, finding the wherewithal to live, it became wholly subterranean in its habits, lost the power of doing what it never required to do—namely, of rolling itself into a ball—and, through the absence of all sunlight, lost, too, its colouring matter, and became of its present dead, bleached white. Whether its eyesight, if it ever had any, is also gone, I do not know; but it can hardly, under present conditions, have any use for it, whereas its antennæ are constantly moving, and seem to be of extreme delicacy. I could never observe—for I have kept nests of _Formica flava_—the smallest sign of any kind of relations between these wood-lice and their hosts; and if any scavenger work is done by the former, from which the latter derive benefit, I believe that this is merely incidental, and that the ants know nothing about it. But they have got accustomed to the wood-lice being there, and put up with them because they cannot help it. It must be remembered, in regard to _Paussidæ_, that the family is represented by some two hundred species, all of which pass the greater part of their lives with ants. In regard to this Mr. Kirby remarks: “The observations made upon the family are so contradictory that the discrepancies can only be accounted for by supposing that different species have very different habits. Possibly some species may perform various useful services to the ants, while others are hostile; or they may be so useful that the ants are willing to pay toll of a certain number of their offspring, in return.”[11] This last, however, does not seem very well to accord with recorded observations as to ants attacking any individual _Paussus_ whom they may chance to see devouring their larvæ, nor with the latter refusing to bombard, under these circumstances, even when in danger of their lives. It is impossible to imagine a hostile _Paussus_ not bombarding, in such a case, unless, indeed, we suppose it to have first lost its hostility, and then again to have become hostile, without, however, regaining the power of using its natural weapon. But this is a state of affairs hardly to be conceived.