NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 9 



terior to the one they enter. In the thoracic ganglia, however, Za- 

 warzin says, while the sensory nerves from the body of the segment 

 end in the same manner, i. e., in an anterior ganglion, those from the 

 legs terminate mostly in the ganglia of their respective segments. 

 This evidently has the effect of making each thoracic ganglion a 

 more independent center than are the individual abdominal ganglia. 



The intraganglionic connectives in the ganglia of the dragonfly 

 larva, as described by Zawarzin, comprise two sets of fibers and their 

 respective cytons. One set consists of transverse fibers (fiig. 3, c) 

 which intermediate, by means of basal collaterals and terminal arbor- 

 izations, between the two sides of the ganglion ; the other set consists 

 of T-shaped neurons {d), the two branches of which intermediate 

 between the dorsal motor neuropile and the ventral sensory neuropile. 



The fibers of the longitudinal commissures have their cytons 

 {e, f, g, h, i) in the lateral parts of the ganglion; the axons, after 

 giving ofif collaterals in the ganglion, proceed through the commis- 

 sures to other ganglia of the chain. Some of these are motor con- 

 nectives (e, f), sending their collaterals into the motor neuropile 

 of the ganglion ; others {g, h, i) are sensory connectives, sending 

 their collaterals into the sensory neuropile. Zawarzin distinguishes 

 three types of commissural fibers in the abdominal ganglia of the 

 dragonfly larva: tautomeric fibers (e, i) that leave the ganglion 

 through the commissure of the same side, after giving ofif one col- 

 lateral into this side; Jieteromere fibers (/, h) that give ofif one col- 

 lateral and then cross to the opposite side of the ganglion to enter 

 the commissure of this side; and hekateromere fibers {g) that cross 

 the ganglion but give ofi a collateral in each side. 



The commissural tracts pass superficially through the dorsal and 

 ventral parts of each ganglion. Zawarzin distinguishes on each side 

 of the dorsal part of each ganglion in the dragonfly larva a median 

 tract (fig. 3, /) which contains fibers that go long distances through 

 the nerve chain, and a lateral tract {k) that contains the shorter 

 dorsal fibers ; and, on each side of the ventral part, an external 

 median tract (/) of long fibers, an internal median tract (w) of 

 short fibers, and a lateral tract (n) of short fibers. Besides these, 

 finally, he says there are two internal ventral tracts {p) on each 

 side that contain the sensory fibers which traverse the commissures. 



The cytons and roots of the median nerves of the ventral ganglia, 

 according to Zawarzin, lie in the posterior parts of the ganglia ; but 

 we are not here concerned with these nerves. 



The brain and its nerves. — The brain of an insect consists of 

 the ventral ganglia of the first three head segments united into one 



