THE EXPLANATION OF FLY-BELTS. 11 



statements refer ; since, owing to the opening-up and colonisa- 

 tion of many parts of Africa and the enormous diminution in 

 the numbers of big game, as will be shown in the next section, it 

 by no means follows that the limits of a Fly-belt are the same 

 to-day as they were even a few years ago ; while in certain cases 

 the belt itself may have ceased to exist. 



The limitation of the Tsetse to " belts " is not so remarkable 

 as might at first sight appear. There can be little doubt that it 

 is due to a characteristic of the Order to which these insects 

 belong, which, though frequently overlooked, is exhibited by the 

 majority of species of Diptera, and has attracted special attention 

 in the case of the Tsetse owing to their blood-thirsty nature and 

 the fatal consequences of their bites when inflicted upon domestic 

 animals. Although not " social" insects in the ordinary sense of 

 the term as applied to animal communities, Diptera, as a whole, 

 show a marked social tendency on the part of individuals of the 

 same species. That is to say, there is a tendency which causes 

 individuals of the same species to assemble in particular places in 

 more or less close proximity to one another, although without 

 exhibiting anything in the shape of mutual aid. Thus, in spite 

 of the fact that the distribution of most species of Diptera is 

 remarkably wide, it will be found in the majority of cases 

 that individuals of the same species do not occur everywhere 

 throughout its area, but are confined to particular spots; and 

 wherever they are met with they are usually to be found in 

 some numbers. Of course there are many exceptions, but the 

 generalisation nevertheless holds good. Again, although remark- 

 able instances of migration have been recorded in the case of 

 certain species of Diptera, they must be regarded as exceptional, 

 and it will be found that Diptera, as a rule, do not travel far 

 from the spot in which their larval existence was passed. The 

 result is that we find a larger or smaller number of individuals 

 of the same species collected within, perhaps, quite a small area, 

 outside which it may be impossible to find a single example. 

 This is well-known in the case of the midges and other allied 

 forms, which attract attention owing to the habit of the males 

 of collecting together and dancing in the air in a swarm at 

 certain seasons ; and similar phenomena are exhibited by the 

 males of certain species of Muscid<e (belonging to what is still 

 generally termed the Family Anthomyidjie). In England, again, 

 phenomena precisely analogous to " Fly-belts " are displayed on 

 a limited scale by some of the smaller forms of " daddy-long-legs " 



