A. D. Imms 
111 
the late larva and, as development progresses, they become as it were 
drawn closer together. 
In its general structure, the brain consists of an outer sheath of 
ganglion cells, which are thickly massed together in the optic and 
antennal lobes, and an inner central area consisting for the most part of 
nerve fibres. Paired nerves are given off supplying the antennae and 
larval eyes, together with a frontal nerve, which expands into a frontal 
ganglion lying anterior to the brain and just above the dorsal pharyn¬ 
geal muscles. The antennal nerve passes obliquely outwards, and 
enters the imaginal antennal bud where it terminates (Fig. 20). Ac¬ 
cording to Miall and Hammond a similar condition is found in the 
Chironomus larva and in their book on that Dipteron (1900, p. 130) 
they remark that the imaginal antenna encloses the larval antennary 
nerve. 
The ventral nerve chain comprises the sub-oesophageal ganglion, 
three thoracic ganglia, and a series of eight ganglia in the abdomen. 
The sub-oesophageal ganglion innervates the mandibles, the maxillae, 
and the labium, and is to be regarded as a complex ganglionic mass 
exhibiting no traces of the primitive ganglia of which it is composed, 
but its paired structure is noticeable in sections (Fig. 32). Histologi¬ 
cally, it consists of an external investment of ganglion cells en¬ 
closing a central medullary mass of nerve fibres, which is directly 
prolonged outwards into the lateral nerves and connectives. The 
ganglion cells are principally congregated along its anterior, ventral, and 
lateral borders and are much less numerous on its dorsal aspect. A 
structureless coat of connective tissues invests the exterior of the 
ganglion. Posteriorly, a pair of stout nerve cords are given off which 
pass through the occipital foramen and unite with the first thoracic 
ganglion. The remaining ventral ganglia have a very similar histo¬ 
logical structure, and consist of an outer layer of ganglion cells which 
enclose an inner core of nerve fibres, and the whole is invested by 
a sheath or epineurium. The connectives, though they are in reality 
double, appear to consist of a single cord. The thoracic ganglia are 
situated close together, the intervening connectives being very short, 
and the abdominal ganglia are placed anteriorly in their respective 
segments near to the line of junction with the segment in front 
(Fig. 22). In the eighth segment the ganglion is larger than those of 
the preceding segments, it is somewhat triangular in form, and it gives 
off pairs of nerves which supply both the eighth and last segments. It 
is therefore to be regarded as a fusion of the two primitive ganglia of 
