42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I42 



Since the basic point in the arguments of both Henry and Butt 

 concerning the nature of the labrum is the "innervation" of the in- 

 sect labrum by nerves from the tritocerebral gangHa, the value of these 

 nerves as evidence must now be examined. In the insects a nerve 

 trunk goes forward from each tritocerebral lobe of the brain and 

 divides into a frontal-ganglion connective and a so-called "labral" 

 nerve. The latter nerve, however, does not restrict its branches to 

 the labrum ; it ramifies profusely to the epidermis of the frons, the 

 clypeus, the labrum, the mouth region and the epipharynx. If Henry 

 (1948), therefore, had taken into account the entire head area of the 

 insect supplied by the "labral" nerves, she should have included at 

 least the clypeus as well as the labrum in her "first segment," whereas 

 the clypeus is regarded as the "second segment." Chaudonneret 

 (1950) is more consistent in this respect, since he attributes the 

 median parts of both the labrum and the clypeus to the tritocerebral 

 segment, the lateral parts to a "superlingual" segment. However, he 

 regards the clypeolabral area as being only the sterna of these seg- 

 ments which have become preoral. Likewise invalidated by the wide 

 distribution of the "labral" nerves is the contention of Butt (1957) 

 that the tritocerebral innervation of the labrum identifies the labral 

 lobes with the tritocerebral appendages. Clearly the labral branches 

 of these nerves can have no specific value of any kind related to the 

 labrum alone. 



Furthermore, the tritocerebral nerves which are said to "innervate" 

 the fore part of the head have been shown to be integumentary 

 sensory nerves. Bretschneider (1914) says those of the cockroach 

 Periplaneta (Blatta) are entirely sensory. Josting (1942) illustrates 

 their elaborate sensory ramifications on the clypeus and labrum of 

 the larva of Tenebrio. According to Bierbrodt (1942) these nerves 

 in the larva of Panorpa come from the epidermal sense organs of the 

 frons, the clypeus, the labrum, and the mouth region. In the malloph- 

 agan Myrsidea the "labral" nerves are described by Buckup (1959) 

 as breaking up into sensory branches to the labrum, the clypeal 

 region, the cibarial sclerite, and the epipharynx. 



While it may be true that motor nerves commonly are restricted 

 to the segments of their respective ganglia, this is not necessarily true 

 of integumental sensory nerves. The neurocytes of sensory nerves 

 are peripheral, their axons grow inward to the ganglia. It has been 

 shown by Wigglesworth (1953, 1959) that as new sense cells are 

 developed in the epidermis of postembryonic instars of Rhodnius, 

 their axons grow inward, join with the first nerves they meet, and 

 accompany them to the central nervous system. The clypeolabral 



