24 The Structure and Special Physiology of Insects 
to be innervated by long nerves running from the thorax. The thoracic 
ganglia may fuse to form one, and in extreme cases all the abdominal and 
thoracic ganglia may be fused into one large mid- 
thoracic center. 
In tracing the development of the nervous 
system during the ontogeny of one of the special¬ 
ized insects, the changes from generalized condi¬ 
tion, i.e., presence of numerous distinct ganglia 
segmentally disposed, shown in the newly hatched 
Fig, 49. 
Fig. 48. —Part of sympathetic nervous system of larva of harlequin-fly, Chironomus 
dorsalis, oes., oesophagus; }.g., frontal ganglion; r.n ., recurrent nerve; d.v., dorsal 
vessel; w 4 , nerve passing from brain to frontal ganglion (Newport’s fourth nerve); 
br., brain; rn., point of division of recurrent nerve; tr., tracheae; pg., paired ganglia; 
d.v.n., nerve of dorsal vessel; d.v.g., ganglia of dorsal vessel; g.n., gastric nerve 
to cardiac chamber. The course of the recurrent nerve beneath the dorsal vessel is 
dotted. (After Miall and Hammond; greatly magnified.) 
Fig. 49. —Stages in the development of the nervous system of the honey-bee, Apis melli- 
fica; 1 showing the ventral nerve-cord in the youngest larval stage, and 7 the system 
in the adult. (After Brandt; much enlarged.) 
larva, to specialized condition, i.e., extreme concentration and cephalization, 
that is, migration forward and fusion of the ganglia, shown in the adult, 
are readily followed (Figs. 49 and 50). 
The special senses of insects and the sense-organs are of particular inter¬ 
est because of the marked unusualness of the character of the specialization 
of both the organs and senses, as compared with the more familiar condi¬ 
tions of the corresponding organs and functions of our body. The world 
is known to animals only by the impressions made by it on the sense-organs, 
