NO. I THE INSECT HEAD — SNODGRASS 43 



nerves originate in sense cells of the epidermis and enter the trito- 

 cerebral ganglia usually by way of the frontal-ganglion connectives. 

 In the Mallophaga, however, Buckup (1959) shows that they enter 

 the ganglia independently. 



Dorsal tegumentary nerves from the tritocerebrum of insects un- 

 doubtedly go to segmental regions of the head that are not tritocere- 

 bral, since this segment is practically eliminated in the adult head. In 

 the malacostracan Crustacea, according to Hanstrom (1928), a dor- 

 sal tegumentary nerve from the brain branches anteriorly to the eye 

 stalks and posteriorly to the whole cephalothorax. In Limulus Patten 

 and Redenbaugh (1899) describe and illustrate a pair of lateral 

 nerves from the tritocerebral ganglia that turn backward on the epi- 

 dermis of the leg segments and finally branch toward the first five 

 appendages of the abdomen. 



Tegumentary sensory nerves, therefore, do not necessarily iden- 

 tify segments or segmental appendages by the ganglion they enter. 

 The neurocytes of motor nerves, on the other hand, lie in the ganglia 

 and in general their function is to innervate the muscles of the cor- 

 responding body segments. Motor nerves are thereby more reliable 

 indices of segmental limits than are sensory nerves, but even here 

 there may be exceptions. Niiesch (1954) reports that in the thorax 

 of the moth Telea polyphenms the second ganglion gives off nerves 

 to the three thoracic segments. The deductions of both Henry and 

 Butt are thus not justified, since both are based on the sensory nerves 

 of the labrum, which are merely a group of sensory fibers from the 

 general preoral region of the head. Only in the tritocerebral ganglia 

 can these fibers make connections with motor neurons of the ventral 

 nerve cord. The function of sensory stimuli is to produce movement. 



Of greater significance than the sensory innervation of the fore- 

 parts of the head is the fact that the motor innervation of the labral 

 and clypeal muscles comes from the frontal ganglion, or its equiva- 

 lent in some arthropods known as the stomodaeal bridge. Chaudon- 

 neret (1950), for example, describes in Thermohia domestica an 

 elaborate innervation of the anterior head region from the frontal 

 ganglion. Dorsal nerves of the ganglion go to the muscles of the 

 mouth angles (hypopharyngeal muscles) and to the anterior dilators 

 of the pharynx. A median nerve goes to the labral muscles, and 

 lateral nerves go to the cibarial dilators (clypeal muscles) and the 

 transverse epipharyngeal muscles. 



The frontal ganglion is developed from the anterior wall of the 

 stomodaeum just before the mouth. The stomodaeum, however, is 

 an ectodermal ingrowth at the site of the mouth. The frontal gan- 



