Among the beetles belonging to this migration, there is one of very considerable interest from a distributional point of view, for all the species of the genus--even the whole family to which the genus belongs--are what is known by zoologists as "Commensalists." These are animals habitually associating and living in close connection with others with which they are not tied by any family relations or kinship. Such a state of close and permanent friendship is called "commensalism." Now it appears as if the members of this family of beetles (_Clavigeridæ_) had of their own free will formed such a close connection with colonies of ants--sometimes with one species, sometimes another. They are the permanent guests of the ants, and in return they secrete a fluid which is apparently highly prized by them. All of the _Clavigers_ are provided with peculiar club-shaped antennæ, with which they ungraciously beat their hosts, when they are in want of food. According to some authorities, they even occasionally gnaw at the pupæ and larvæ of the ant with which they live. Such beetles naturally can only have extremely limited means of distribution, and they are comparable in that respect with the woodlice of the genus _Platyarthrus_, to which I have already had occasion to refer. All the species of _Claviger_ are confined to Europe, chiefly to the south, but one species, _Cl. testaceus_, has wandered farther north and occurs in the nest of the ant _Lasius flavus_ in the south of England, Ireland, and Scotland. Though none of the _Clavigers_ can be claimed as Oriental migrants, the centre of distribution of the genera belonging to the _Clavigeridæ_ is in Southern Asia, and it is probable that the ancestors of the European _Clavigers_ have spread westward from that region to Europe, eastward to Australia and Japan, and southward to Madagascar and South Africa. The genus _Hopatroides_, belonging to the same family as the so-called Spanish-fly (_Tenebrionidæ_), has twelve species in Western Asia and Greece. One only, _H. thoracicus_--an instance of discontinuous distribution--occurs in Andalusia. _Amphicoma_ is represented in Western Asia and the Balkan peninsula by fifteen species, while three others are met with in North-west Africa and Southern Spain. A genus of Dragon-fly, _Onychogomphus_, has in Europe a somewhat similar distribution to _Claviger_, but it has besides a very extensive foreign range. There are altogether thirty-five species; of these ten are Holarctic, twelve Oriental, five Mascarene, and eight Ethiopian. The centre of distribution is therefore in the Oriental region, and we may assume that in all probability the genus has originated there, the European species having travelled west with the Oriental migration at an early date of the Tertiary Era.