161 



then cut through with needles till nothing remains externally connecting 

 the anterior and posterior surfaces of the head except the thin soft 

 integument immediately behind the proboscis. The proboscis is then 

 seized with forceps and a needle is placed on the last segment of the 

 abdomen. A slow pull on the proboscis causes the glands to be with- 

 drawn as before. This method has been found to fail so frequently, 

 however, that it cannot be recommended as a working dissection. With 

 several species of Tabanus this latter method of dissecting the salivary 

 glands has been found to be invariably successful." 



CHAPTER XVIII. 



COLOUR CONVENTIONS TO INDICATE THE DISTRIBUTION OF 

 TSETSE-FLIES. 



The system described and illustrated below was proposed ten years 

 ago by Dr. J. J. Simpson (143), and has since been generally adopted in 

 maps of our Tropical African Colonies and Protectorates showing the 

 ascertained occurrence of different species of Glossina. In introducing 

 his scheme of conventional colours and symbols, Dr. Simpson wrote as 

 follows : 



" The necessity for graphic representation of the distribution of 

 insects and the diseases which are transmitted by them is now so 

 evident as to need no special demonstration. But it is further 

 obviously important that some uniform method should be adopted in 

 the preparation of such maps, in order to make them intercomparable, 

 and the object of the present notes is to suggest a comprehensive 

 system of notation for the whole genus Glossina a system which may 

 be readily adapted for use in other genera or families of blood-sucking 

 insects/' 



" Symbols. To achieve their aim these ought not to be excessive in 

 number and should be sufficiently distinctive to be easily recognisable 

 from one another at a glance. After a series of trials the following have 

 been considered most satisfactory : (1) a solid circle, (2) a solid square, 

 (3) a circle enclosing a cross, (4) a square enclosing a cross, (5) a solid 

 triangle, (6) a hollow triangle, (7) a hollow circle, and (8) a hollow 

 square. Modifications of these could easily be multiplied [see below]. 

 In the present system it is not necessary to adopt any more, as only 

 in one group, namely the palpalis-group of Glossina, in which there are 

 six species, do we require more than four symbols. 



" Coloiirs Red has already been assigned to sleeping sickness, and 

 it seems most advisable to retain its use for this disease. By means of 

 the symbols already given it is easily possible to represent other 

 diseases, if necessary, by the same colour." 



" The following colours have been selected after a series of tests as 

 being sufficiently distinct to avoid unnecessary confusion and have been 

 assigned to the various groups as follows : 



palpalis-group blue ; 

 morsitans-group green ; 

 fusca-group brown ; 

 brevipalpis-group orange . 



" Yellow has been purposely avoided as it is never very distinct, 

 while in artificial light it is hardly recognisable." 



(5979) L 



