184 LIVERPOOL BIOLOGICAL SOCIETY. 
ganglia shift bodily into the thorax—the distance is, how- 
ever, only a very short one. During the pupal period the 
three pairs of thoracic ganglia and these from the abdo- 
men grow considerably and fuse together to form one con- 
tinuous mass, though its composite character is readily 
recognised in sections. The gangha of the eighth segment - 
gradually shift forwards, the connectives being absorbed, 
so that before the end of the four days of pupal life the 
eighth pair have fused with the seventh. At the moment 
of escape of the imago the composite mass thus formed 
shifts suddenly into the eighth segment in the female but 
remains in the seventh in the male. 
The ganglia of the head increase enormously in size so 
as to almost fill the head. At certain places, especially 
at the margins of the eyes and at the bases of the antenne, 
ereat thickenings of the epidermis are continuous with the 
ganglia, and the new cells of the gangha would thus appear 
to be derived directly from the epidermis. The “ brain” 
is also continuous, with the deeper cells in the basal joint 
of each antenna, and this mass in turn with the epidermis 
at the line of insertion of the antenna into the head. 
The sense-organs of the larva are antenne and hairs or 
sete, and especially some at the end of the respiratory 
siphon. Those of the pupa are sete and ocelli. Those 
of the imago are eyes, ocelli, maxillary palps, labelle (the 
modified palps (?) of the labium), the antenne with the 
ear, and the hairs upon the antenne and the maxillary 
palps; possibly also the halteres. 
I shall not describe most of these, but shall record just 
so much as appears to me to be of special interest. Of 
the setee of the pupa, one pair on the first abdominal seg- 
ment seem to be the organs by means of which the animal 
detects movements of the surface of the water. Hach 
consists of a triangular basal plate attached to the soft 
