ABSTRACT
The singular success of the social insects in creating for themselves a peculiar medium has exposed them to the invasion of a host of greedy guests and parasites, which it is now our task to consider. Nearly all of these intruders are Arthropods, and the great majority are Hexapods belonging to the most diverse families and orders. Why so many alien species should come to live in the nests or trophoporic fields surrounding them is obvious when we think of the many attractions and opportunities afforded by the social medium. In the first place, the galleries and chambers of the nests furnish very convenient hiding places and are, moreover, owing to the congestion of their population and especially during cold weather, of a somewhat higher temperature than the surroundings. Secondly, the refuse, the weak and dying or even the able-bodied workers, and particularly the inert and defenceless larvæ and pupæ represent an abundant supply of food. And thirdly, since the social insects have a more or less pronounced tendency to repel aggressive enemies of all kinds, all small organisms that can elude the hostility of the owners of the nests may share in the general protection. The disadvantages are less obvious, but they are real nevertheless, because the workers, even of the same nest, often vary considerably in their toleration of intruders. And even when most of the workers of a colony are willing to tolerate or adopt a strange insect, the latter may be eventually ejected or destroyed if there be present only a few persistently hostile and intolerant workers, since they must sooner or later come in contact with the intruder. Hence unanimous toleration or adoption on the part of the colony has to be secured by any insect if it is to become a regular guest, and this is more difficult than would be supposed. It is therefore surprising to find so great a number of satellites in the nests of all the various groups of social insects. We can most naturally divide all these creatures into two groups, namely, other social insects (social parasites), and sporadic, solitary, non-social species. The latter I shall discuss here and devote the next lecture to a consideration of the various social intruders.
