MALARIA EXPEDITION TO NIGERIA 235 
Ova. — Besides the characteristic manner in which Anopheles ova are deposited 
on the surface of water, they can also be distinguished from Culex ova in other ways. 
The ovum of A. costalis is roughly ovoid in shape ; its length is about 0-48 milli- 
metre, its total breadth o - i6 millimetre. The anterior end is the broader; the 
superior surface is slightly concave antero-posteriorly ; this surface is broad anteriorly, 
narrower posteriorly, and constricted in the centre, presenting a shape somewhat 
resembling the sole of a boot ; it is limited on all sides by a striated border. The 
inferior surface, that in contact with the water, is convex. 
The chitin of the lower surface is beautifully marked with hexagonal figures, 
which are well seen in an empty egg case. The upper surface has no such markings, 
but at each end has five very small rounded bosses of transparent chitin. The dis- 
tinguishing feature of the Anopheles ovum, pointed out by Ross, 2 is the presence of 
two lateral wings of transparent cuticle. These have a width of about 0*02 millimet re, 
and are about 0-3 millimetre long. They are attached one on either side of the 
ovum throughout the greater part of its length. This cuticular structure has a wide, 
oval area of attachment, extending below in a semi-circular manner on to the inferior 
surface, but limited above by the serrated edge of the upper surface. It is hollowed 
out on its upper surface. The free outer edge is serrated, the whole surface present- 
ing a milled appearance. Slight maceration of the ovum separates this structure from 
the egg case. 
The larva lies with its head situated at the broader end of the egg case. 
The top of the anterior end of the egg case is broken off in a spiral manner to allow 
of the escape of the larva. 
It was noted that when mosquitoes of the genus Culex are forced to lay their 
eggs on a small surface, e.g., on water in a narrow test tube, the eggs are arranged 
in a pattern very similar to that of Anopheles ova. 
When freshly laid, the eggs are whitish in colour, becoming black in the 
course of an hour. 
Actual countings of the number of ova deposited at one laying gave the 
following numbers : 138, 145, 233, and 179. 
Larvae. — Anopheles larvae were occasionally found infected with a parasite 
{Brachionus) which caused them to present a fluffy appearance and hindered their 
growth, so that they continued in the larval stage for an abnormally long period, 
and at length died. 
The numbers of males and females which hatched out from a number of 
larvae were counted on several occasions —396 larvae produced 185 males and 21 1 
females. 
Habits of Adult Anopheles. — In the preceding paragraph it is seen that the 
numbers of male and female Anopheles which hatch out from a bate of larvae are 
