NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS — SNODGRASS 



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true cytons of the sensory fibers, according to which, all the sen- 

 sory nerves of insects are analogous to the olfactory nerves of 

 vertebrates, being the centripetal axons of nerve cells developed 

 in the peripheral ectoderm. 



The developmental origin of the sensory cells of Type I that 

 lie within the hypodermis has been studied by many investigators, 

 and all agree that they are specialized hypodermal cells. No one 

 has demonstrated the growth of a nerve axon from these cells, 

 while, on the other hand, several investigators have claimed that 

 a nerve fiber grows outward and unites with the sense cell. Berlese 

 (1909), for example, after studying the postembryonic develop- 



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Fig. 7. — Comparative diagrams of central and peripheral nervous 

 systems of an insect (A) and a vertebrate (B). In the vertebrate most 

 of the sensory cytons (SCy) are in the spinal ganglia (SpGng) ; in the 

 insect the apparent sensory cytons (SCy?) are in the hypodermis. 



mental stages of several kinds of sense organs, says that, in all 

 cases, it is found that a sensory nerve proceeds outward from the 

 central nerve chain and elongates until it attains the basement mem- 

 brane of the body wall at the point where the sense organ is to be 

 formed. Here it remains until the sense organ is ready to receive 

 it, when it penetrates the basement membrane and unites with the 

 sensory complex that has formed in the hypodermis. Vogel (1923) 

 says that in a mature larva of a wasp the antennal nerve has already 

 reached the tip of the antenna without penetrating the basement mem- 

 brane of its walls. During an early stage of the pupa, however, 

 branches of the nerve enter the hypodermis, where a single fiber unites 

 with the base of each sense cell. The base of the sense cell, Vogel says, 

 may elongate slightly toward the nerve, but the connection with 

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