THE EARLY STAGES OF CERTAIN WEST AFRICAN MOSQUITOS. 139 



Within the siphon tube a number of complicated structures can be seen. 

 Running from end to end of the tube and extending for some distance into the 

 abdomen there is an axial rod of chitin, a narrow flat structure with its edges directed 

 towards the dorsal and ventral aspects of the siphon, and with a hinge or joint at 

 its lower end. The two tracheal trunks pass upwards inside the siphon to the right 

 and left of this axial rod : in the apical third they lose their spiral structure and 

 unite with the rod so as to form the " common hollow space " described by Raschke. 

 This arrangement is usual in mosquito larvae, but in the case of Mansonioides africanus 

 the margins of the space do not form simple valves, but are highly specialised so as 

 to enable the larva to penetrate the roots of Pistia stratiotes and to attach itself 

 firmly to the plant. Laterally the outer walls of the tracheal tubes are continued as 

 highly chitinised plates, which rapidly taper to points. On the ventral aspect the 

 axial rod is continued as a stout, highly chitinised structure, on which a trough- 

 shaped body composed of a mesial and two lateral parts is jointed, each of the 

 lateral pieces bearing at its distal end a triple hook of great strength. A delicate 

 flexible membrane connects the bases of these hooks with the edge of the siphon. 

 On the ventral margin of the " common hollow space " a highly chitinised trough- 

 shaped structure with a saw-like dorsal keel is hinged. Howard, Dyar and Knab 

 state that m Mansonia the " outer portion of the tube is . . . furnished with 

 serrations," but in Mansonioides africanus the saw appears to be attached to the 

 margin of the " common hollow space " and to be separate from the tube, and for 

 these reasons we conclude that it probably represents a modified valve. These 

 structures may perhaps be correlated with the primitive elements in the closing 

 mechanism above the spiracles of Anopheline larvae, corresponding respectively to 

 ' the lateral flaps, the posterior plate, and the anterior fan-shaped plate. 



The saw-like structure has about fourteen teeth and appears to be attached a little 

 above its lower end to the dorsal margin of the " common hollow space." Its lower 

 end divides, a strong median process passing down into the siphon tube and two 

 knob-like bodies projecting dorsally so as to lie between but in close apposition 

 to the bases of the two stout curved bristles situated at the junction of the apical 

 third of the siphon with the basal portion. These bristles have a delicate fringe or 

 membrane on their lower surfaces, which suggests that they may have a sensory 

 function and may be of service to the larva when feeling its way between the rootlets 

 in search of a suitable point at which to attach itself. The larvae of Mansonioides 

 africanus after the first instar attach themselves to the tap-roots of Pistia plants ; 

 they swim about in the water until they encounter the rootlets and then, tail fore- 

 most, follow these inwards to the root itself. In this manoeuvre vision can be of 

 little assistance to them ; but if the large bristles are sensory, as we suggest they may 

 be, they might help the larvae to reach the root, and, once arrived, might transmit 

 a stimulus enjoining the larvae to attach themselves. 



The actual process by which the larvae penetrate the tissues of the root, appreciate 

 the fact when they have reached the air- containing tubes, and fix themselves firmly, 

 must be a matter of surmise, but as extremely powerful muscles are attached to the 

 axial rod, it may be conjectured that this structure plays an important part both by 

 approximating the terminal elements, so as to form a sharply pointed cone for thrusting 

 into the root, and by putting into action the saw-like structure attached to it, which, 

 (C394) b2 



