NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 3 



insight into how these properties enable insects to maintain so suc- 

 cessfully their place in nature in spite of our efforts to dislodge 

 them. 



The following resume, therefore, of the known facts concerning 

 the structure of insect sense organs and the sensory nervous system 

 is offered in the hope that it will be found useful to the experimental 

 worker who is attempting to obtain knowledge of the life processes 

 of insects, with the purpose of rendering practical benefits to man- 

 kind from his discoveries, while the generalizations that are sug- 

 gested from a study of the assembled facts are given as an attempt 

 to further the science of pure morphology. 



I. THE NERVOUS SYSTEM OF INSECTS 



In complex animals the ectoderm is the body layer that comes 

 into direct contact with the environment. It may be supposed, there- 

 fore, that its cells retained from the first a higher degree of the 

 properties of irritability and conductivity than did the cells of those 

 layers that are infolded to form the strictly internal organs. It is not 

 surprising, then, to find that, later, from the ectoderm are developed 

 the sensitive, conductive, and receptor elements of the fully evolved 

 animal. 



Insects possess two nervous systems, which, though united with 

 each other in the mature condition, appear to be independent in their 

 origin. One constitutes the ventral nerve cord of ganglia and con- 

 nectives, including the brain ; the other consists of the ganglia of 

 the stomodeum and their nerves, forming the stomatogastric sysfcin, 

 or the so-called " sympathetic " system. 



THE ORIGIN OF NERVOUS TISSUE 



All true nervous tissue, comprising the nerve cells and their 

 prolongations known as nerve fibers, are formed from the ectoderm. 

 The investing tissue, or neurilemma, may be of mesodermal origin, 

 but some investigators claim that even this in insects comes from the 

 ectoderm. 



The ganglia and commissures of the ventral nerve cord arise in 

 the embryo from the ventral part of the germ band. At an early 

 stage of development there appear two longitudinal neural ridges 

 (fig. I A, NIR) on the under side of the embryo, with a median 

 neural groove (NIG) between them. The ectoderm (Ect) both of 

 the ridges and the groove proliferates from its inner surface a layer 

 of large cells, the neuroblasts (Nbl), which are to generate the tis- 



