96 WILLIAM F. FISKE — THE BIONOMICS OF GLOSSINA ; 



river with equal freedom. They freely crossed and recrossed the Nile at Foweira 

 (some 300 yards), and it was impossible to say that one sex was more given to 

 wandering than the other. 



Such movements as these are suggestive of no particular purpose other than to 

 satisfy the bodily wants of the individual ; they would seem to be incited by 

 desire for food, shade, shelter, or humidity ; by the fear of enemies or by the 

 immediate necessities associated with reproduction of the species. They appear to 

 be mere ordinary, everyday, goings and comings. There is, however, another 

 very different sort of movement from one place to another, characteristic of many 

 insects, including all the social and many of the solitary species. Amongst the 

 social, the two are very sharply differentiated. 



In the case of the honey bee, to exemplify this difference, large numbers of 

 workers issue daily from the hive ; they fly far afield after different kinds of bee 

 commodities, and invariably, barring accident, they return. These are the ordinary 

 goings and comings. One day the hive becomes the scene of unwonted activity. 

 The queen issues, followed by a full half, perhaps, of the workers, and the whole 

 swarm flies away without intention to return. This is a migratory or going move- 

 ment ; it is governed by instincts which ordinarily lie dormant in the individual, 

 and which are stirred into activity by extraneous influences that are quite different 

 from those inciting the ordinary goings and comings. These last mentioned, being 

 stimulated by the immediate needs of the individual or the colony, are regularly 

 or frequently recurrent, and may be repeated many times in the life of an 

 individual. The true goings, on the contrary, are usually undertaken only once 

 in the lifetime of an individual and frequently are not undertaken at all. They 

 are specialised movements and are essentially necessary to the welfare, not of the 

 individual, or colony, but of the species as an entity. The individual undertaking 

 them does so at a risk to itself, which may be very great. Among the ants and 

 termites, for example, probably not *00i per cent, of the migrating females 

 survive to form new colonies. It is deliberate sacrifice of individuals for the 

 good of the race or the species. 



Without exception, it is easy to differentiate between these two sorts of move- 

 ments amongst the social insects ; it is not so easy amongst the solitary insects. 

 Nevertheless, the writer strongly contends that this is not due to the absence of 

 specialised migrations, but to our imperfect perceptive abilities and incomplete 

 knowledge of the insects themselves. It is easy to follow the movements of 

 social species from inception to conclusion ; very difficult and finally impossible 

 to do this when solitary species are involved. Moreover, a great amount of 

 positive evidence may be adduced to support the contention, against very little 

 tending to controvert it. 



In short, the writer believes that every species of insect must be in some 

 manner adapted for dispersion. The contention is upheld to a certain extent by 

 analogy with plants. It is well known that in most plants some specialised 

 physical adaptation is provided for seed dispersion. Such adaptation may be 

 physical (as in the instance of the gypsy moth, the young larvae of which are 

 wind-borne) or it may be along the lines of instinctive specialisation. If it 

 appears to be absent, its existence will either be demonstrated by research, or it 

 will be found that the insect has such habits as to render the execution of true 



