54 ENTOMOLOGY FOR MEDICAL OFFICERS 



retards growth, and in northern latitudes some larvae both 

 of Anopheles and Culex remain unchanged throughout the 

 winter. 



The pupa (Fig. i6) is of the naked or "obtected" kind; 

 it also is formed for an aquatic life, and though, of course, it 

 does not feed, it is active. In shape it is something like a 

 tiny lobster deprived of appendages and carrying its tail bent. 

 The head and thorax form one large mass, and through the 

 chitinous skin the eyes and wings can be seen, as well as the 

 folded appendages ; from the back of the mass a pair of ear- 

 like breathing-trumpets projects ; these have open ends, and 

 are lined by short interlacing hairs which prevent water from 

 entering. The curved abdomen consists of 9 segments, 

 and ends in a pair of large blade-like fins ; on the dorsum 



FiG. 16 Pupa of a Cw?ca;. 



of the 1st segment there is a pair of large fan-like tufts of 

 hairs, the use of which is to catch the surface film and to 

 buoy the pupa in such a position that the air-trumpets may 

 act. The duration of the pupal stage is, as a rule, short — 

 about two days in hot weather, but a week or more in colder 

 weather. 



When the adult is about to emerge, the pupa straightens 

 out and the skin of the cephalothorax bursts along the back ; 

 the head of the adult then appears, followed gradually by the 

 body and appendages. After delivery the insect rests for a 

 few moments on its old pupal envelope until its wings are 

 hardened and fit for flight. 



Any account of the three principal diseases — filariasis, 

 malaria, and yellow fever — which the great discoveries of 

 Manson, Ross, Grassi, and the distinguished members of 

 the American Yellow Fever Commission have shown to be 



