INSECTS AND MANKIND 431 



grows down towards the margin of the water, and it was the 

 island population of the Victoria Nyanza that suffered most 

 severely in the Uganda outbreak of sleeping-sickness. 

 Nothing short of the removal of the remnant of the lakeside 

 population from the regions serving as breeding-haunts 

 for the flies succeeded in staying the epidemic. Attention 

 is now given to clearing the vegetation so as to make localities 

 unsuitable for breeding, and attempts have also been made 

 to provide areas with well-aired loose soil so as to attract 

 female Glossina to deposit their larvae there ; then the 

 entomologist searches the ground diligently for the discovery 

 and destruction of pupae. It is remarkable that a form of 

 sleeping-sickness prevalent in Nyassaland is due to another 

 species of Tr}'panosoma, T. rhodesiense, the alternative 

 insect host of which is not G. palpalis but G. tnorsitans, the 

 tsetse-fly which transmits the parasite of nagana to cattle 

 and horses. 



The tsetse-flies are entirely confined to the tropical 

 regions of Africa, but there are few parts of the world in 

 which some blood-sucking insects are not possible agents in 

 the spread of human disease, and most of these belong to 

 the Diptera or two- winged flies. Perhaps of all the various 

 families of this order whose members may be deadly to 

 mankind, the Culicidae, which comprise the typical gnats or 

 mosquitoes, are the most important because the minute 

 parasites harboured by various types of such insects are the 

 causative organs of serious diseases, some of which are 

 among the most fatal known to medical science. Of these 

 the species of Plasmodium, Haemosporidia which in human 

 blood give rise to the various types of malaria or ague, have 

 attracted especial attention. The life-cycle of one of 

 these Plasmodia in its alternating mosquito and bird hosts 

 has been described in the preceding chapter (pp. 413-414), 

 and the parasites of human malaria have a closely similar 

 life-history, undergoing asexual reproduction in human 

 blood by multiple division, and passing in the digestive 

 tract of the mosquito, through a sexual phase which results 

 in the production (Fig. 84, b) of a relatively large cyst 



