NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS I3 



parts of the body. The similarity of the glomeruH of the antenna! 

 lobes of insects to those of the olfactory lobes of a vertebrate brain 

 has strengthened the idea that the sense receptors of the antennse 

 are chiefly organs of smell. 



The third division of the brain consists of the tritocerebral gan- 

 glia (fig. 4, sBr), but since the connecting commissure (^Coni) lies 

 beneath the oesophagus, these ganglia do not form an intimate part 

 of the brain. The tritocerebral segment of the head has a pair of 

 vestigial appendages in the embryonic stage of some insects, but 

 the appendages disappear in all adult insects, except possibly in 

 Campodea, one of the Thysanura. Each tritocerebral lobe gives 

 off two nerves, a labral nerve (LmNv) and a frontal ganglion nerve, 

 or frontal commissure (FrCom), but the two are usually united 

 at their bases into one labro-frontal, or tritocerebral, nerve trunk. 

 The labral nerve innervates the labrum and some of the dorsal 

 muscles of the pharynx. It is said to be composed of sensory fibers, 

 the roots of which, according to Jonescu (1909), in the honeybee, 

 are distributed in the protocerebrum, within and beneath the deu- 

 tocerebral commissure, and in the antennal lobes. Kenyon (1896) 

 says that some of its roots go also into the suboesophageal ganglion. 

 The frontal commissure is said to consist of motor fibers, the 

 cytons of which lie in the tritocerebral lobes. 



The suhccso phage al ganglion. — The composit ganglionic mass 

 lying in the lower part of the head, knowai as the suboesophageal gan- 

 glion, (fig. 4, ScoGng) on account of its position below the oesopha- 

 gus, consists at least of the united ganglia of the fourth, fifth, and 

 sixth head segments {4Gng, ^Gng, 6Gng). Its principal nerves 

 innervate the mandibles, the maxillae and the labium, but besides 

 the three pairs of mouth part nerves there is commonly a fourth 

 pair, and sometimes several pairs, issuing from the posterior part 

 of the ganglion. One of these posterior nerve pairs has been traced 

 in some insects to the salivary glands ; the others when present 

 go to the neck region and into the prothorax. The origin of 

 nerves other than those of the mouth parts from the subcesophageal 

 ganglion might suggest that this ganglion is a composit of more 

 than three primitive ganglia, and Verhoeff (1903) has argued that 

 here is positive evidence of the presence formerly of a neck seg- 

 ment, or " microthorax," in insects, homologous with the segment 

 of the poison fangs in the Chilopoda, the ganglionic center of which 

 is united with the ganglia of the true mouth part segments. Ontog- 

 eny, however, has not recorded the presence of a neck segment 

 rudiment in any insect embryo, and entomologists mostly still re- 



