306 JAS. J. SIMPSON—ENTOMOLOGICAL 
mountains ; within the angle thus formed lies the Lake Chad system. These 
watersheds are indicated on the map. 
West of the first lies the Niger system, which has its westerly and southerly 
limits in another watershed which practically forms the boundary between 
Northern and Southern Nigeria along the Borgu, Ilorin and Kabba 
frontiers, and separates it from the smaller rivers, such as the Ogun in Southern 
Nigeria. 
South of the second watershed lies the Benue system, which is bounded on the 
south by a spur of the Kameruns; this spur abuts into Southern Nigeria 
in the newly opened up Sonkwala country and separates the Benue from 
the Cross River, which flows through the Eastern Province of Southern 
Nigeria. 
As has already been mentioned, the Bauchi Plateau, lying near the middle 
of the Protectorate, is the central watershed, and from it rivers radiate in all 
directions, but eventually join one or other of the two main systems already 
described. 
The largest river running into Lake Chad is the Yo, which flows in a north- 
easterly direction and receives the waters of the Shidya, Delime, Katagum, and 
Hadeija Rivers, which flow from the Bauchi Plateau itself and the watershed 
running north from it, while from the watershed that passes eastwards come the 
Maiduguri and Yedseram Rivers. All the rivers running south from this last 
watershed drain into the Benue, which rises in the Kamerun mountains, and flows 
slightly south of east. The largest of these is the Gongola, which rises in the 
Bauchi Plateau, runs north-east and then south, and joins the Benue near Yola. 
From the south drain the Taraba, Katsena, etc. The Benue enters the Niger at 
Lokoja. Quite as erratic in its course as the Gongola is the Sokoto River, 
which rises near Katsena, flows north-west then south, and eventually joins the 
Kebbi, which in its turn enters the Niger near Ilo. Other important tributaries 
of the Niger on the north bank are the Malenda, Kontagora, Kaduna, Bako and 
Garara, while on the south there are none of any great size. 
Such, then, are the main physical features of the Protectorate, and a knowledge 
of the relative positions and directions assumed both by the watersheds and the 
various rivers serves to elucidate many interesting but otherwise obscure facts in 
insect distribution. This can most easily be understood by a study of .the 
accompanying map. 
(c.) Vegetation, 
It is very difficult to describe in any general way the nature of the vegetation 
in Northern Nigeria, as there are no definite zones of afforestation, and the 
different types found are modified by local conditions. 
Generally speaking, however, the monsoon forest type is predominant in the 
south ; and in the north the thorn forest, though here the savannah forest or open 
