LIFE HtSTORY AND SEASONAL PREVALENCE 125 



boats break up, and the empty shells form very pretty 

 microscopic objects, their surface being closely set with 

 minute spines, the form of which varies in different species, 

 and so might possibly be made to serve as a means of 

 identifying the species to which they belong. 



C. pipiens ova measure 0*9 mm. in length, C. nigrituliis 

 0'6, and according to Howard those of C. pungens (= C. 

 fatigans, Wiedemann) 0"7 in length and 0"16 in diameter. 



Anopheles eggs are quite different in form and are de- 

 posited separately. Sometimes, they lie on the surface of 

 the water in stars or groups of other forms, but as they 

 are in no way connected together, I regard this as quite 

 accidental, and of no moment. As regards the form of 

 the eg^, the following excellent description, extracted from 

 Dr. G. H. F. Nuttall and A. E. Shipley's paper " On the 

 Structures and Biology of Anopheles'' {maculipennis) in 

 the Journal of Hygiene, I., p. 49, may be quoted: — 



" When first deposited the eggs are white, but they soon 

 darken. Each ovum measures 0"7 to I'O mm. in length 

 and is at its greatest breadth about 0"16 broad. The egg 

 is boat-shaped and one end is slightly deeper and fuller 

 than the other. The surface which, were the egg a boat, 

 would be the upper, is flattened but slightly convex. It is 

 marked by minute reticulations (fig. 2). The under surface 

 of the boat is characterised by much larger and more 

 regular reticulations, which divide the surface into fairly 

 equal hexagonal areas. The rim {a) of the boat is thickened 

 and very regularly ribbed. Along the centre of each side, 

 extending over a space of rather more than one-third the 

 total length, this rim is much thickened, the ribbing is 

 more marked and the whole forms a very conspicuous and 

 characteristic feature of the egg. This thickening recalls 

 the rounded float which runs along the edge of a life-boat 

 (fig. 29 ; 7, 8, 9). It serves the same purpose, being com- 

 posed of air chambers and is used to keep the boat-shaped 

 egg with its flat surface uppermost. Howard refers to the 

 membrane we are about to describe as the ' clasping mem- 

 brane,' notes the reticulated surface exhibited by the eggs, as 

 also the presence of 5 — 7 minute dark circular spots at the 



