1176 
Journal of Agricultural Research 
Vol. XXVIII, No 12 
correlated with the differences in the environmental conditions of the two forms, 
yet some of these differences, such as the reduction in size and relative importance 
of the tritocerebrum and of the suboesophageal commissure, represent develop¬ 
ments in the direction of specialization. In other words, notwithstanding its 
relatively specialized form, the brain of the larva is appreciably nearer the 
primitive type, as represented in the lower orders of insects, than is that of the 
imago. 
VENTRAL NERVE CORD 
The ventral cord consists of 11 ganglia united by rather widely separated 
connectives (fig. 1, VNC). All of these ganglia, with the exception of the 
eleventh, or terminal ganglion, are situated in the anterior half of their respective 
segments. In form the ganglia are more or less lenticular, the first three, or 
thoracic, being much larger than the abdominal ganglia. The latter are subequal 
in size, with the exception of the eleventh or terminal ganglion, which is elongate 
in form and much larger than the others. Unlike the other ganglia, this one is 
situated in the middle of its segment, the eighth abdominal. This ganglion, as 
usual, is compound, consisting of three ganglia and the rudiment of a fourth 
(i 6 , 38 ), representing, respectively, the neuromeres of the 8th, 9th, 10th and 
11th abdominal segments. 
Each thoracic ganglion gives off two pairs of lateral nerves (fig. 1 and 4, A). 
The first pair arises from the antero-lateral margin of the ganglion close to the 
point where the latter joins the connectives. In the pro thorax these nerves run 
laterad in the anterior half of the segment, close to the hypodermis. In the 
mesothorax and metathorax these nerves also run laterad, giving off a branch 
to the ventral trunk muscles, and, continuing onward, skirt the anterior margin 
of the wing rudiments. At the antero-lateral border of the latter each of these 
again bifurcates, one branch going to the wing rudiment while the other passes 
dorsad and laterad to the viscera. 
The second of the two pairs of nerves given off arises from the lateral margin 
of the ganglion about midway of its length. The course and distribution of 
these nerves is the same in all three thoracic segments. Each passes laterad and 
slightly caudad to the leg rudiment (1 L~3L) of the corresponding segment, send¬ 
ing a branch to the base of the rudiment. It then continues laterad and is lost 
to view among the muscles and fat cells. 
Each of the abdominal ganglia, exclusive of the terminal ganglion, possesses 
but one pair of lateral nerves (fig. 1 and 4, B, LNv). These nerves run directly 
laterad, giving off branches to the ventral trunk muscles, as shown in Figure 
4, B, and become lost to sight among the trunk muscles and viscera. 
The 8th abdominal or terminal ganglion gives off four pairs of nerves (fig. 1). 
The first pair arises from the lateral margins of the ganglion near its anterior 
end and passes laterad to the muscles and viscera of the 8th abdominal segment. 
The second pair of lateral nerves originates so close behind the first pair that 
these two pairs can be regarded as having a common root. These two pairs of 
nerves go to the first pair of genital rudiments ( 1G ). The third pair of lateral 
nerves runs caudad into the 9th abdominal segment, breaking up meanwhile into 
several branches, some of which supply the second pair of genital rudiments ( 2G ). 
The fourth and last pair arises from the posterior margin of the ganglion, and 
these nerves run almost directly caudad to the 10th segment, where they break 
up into branches supplying the viscera and muscles of this segment. Three seg¬ 
ments, the 8th, 9th, and 10th abdominal, are therefore represented in the ter¬ 
minal ganglion on the basis of the number and distribution of the lateral nerves. 
The fourth segment, the 11th abdominal, is represented by only an insignificant 
ganglionic rudiment, and its corresponding segment disappears about the time 
embryonic development is completed. 
