NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS— SNODGRASS / 



tute the sensory neuropiles. This is attested by the works of vom 

 Rath (1896), Kenyon (1896), Jonescu (1909), Haller (1910), 

 Zawarzin (1924), and Orlov (1924). 



THE VENTRAL NERVOUS SYSTEM 



The lateral cords of the neural ridges of the insect embryo ex- 

 tend from the first segment of the head to the eleventh segment 

 of the abdomen. Each forms a lateral mass of ganglionic cells in 

 each segment, and intersegmental connecting commissures of fibrous 

 tissue between them. Then the ganglionic masses of each segment 

 become united by transverse commissural fibers, forming, in most 

 cases, a compact double ganglion in the segment. The first three 



Fig. 3. — Diagrammatic cross-section of an abdominal ventral 

 ganglion (based on diagram by Zawarzin, 1924 a). 



a. b, motor cytons ; c, cytons of transverse connectives ; d. cytons of 

 dorso-ventral connectives; c, f, g, li, i, cytons of longitudinal commis- 

 sural connectives; ;', k, I, ni, 11, p, dorsal and ventral fibrous tracts of 

 the longitudinal commissures. 



primitive pairs of ganglia in the head consolidate to form the brain, 

 the next three unite to forin the siihasophagcal ganglion; the last 

 four body ganglia consolidate to form the eighth abdominal ganglion 

 of the adult, the others may remain separate, or they may unite 

 in various combinations. After the formation of the alimentary 

 canal, the brain comes to occupy a dorsal position in the head above 

 the oesophagus, but in its origin it is a part of the ventral nerve cord. 

 The body ganglia. — In a fully-formed ganglion of the thorax 

 or abdomen the principal cellular elements are arranged peripherally 

 (fig. 3, a-i), mostly in the lateral and dorso-lateral parts, while the 

 central and ventral regions are occtipied by fibrous tissue, consisting 

 of connective and commissural fibers and the branching roots of 

 sensory nerves and collaterals from motor axons. This fibrous core 

 of the ganglion is known as the medullary tissue, or the punctate sub- 



