454 ^"- E. MCINDOO. 



For the purpose of judging whether the antennal organs are 

 better adapted anatomically than the olfactory pores for receiving 

 odor stimuli, the former organs (Text-fig. 3, p. 453), of Dytiscus 

 marginalis have been copied from Hochreuther ('12). This work 

 of Hochreuther is a comprehensive study of the morphology of 

 all the chitinous sense organs of Dytiscus. Since it is perhaps 

 the latest and certainly the best study on the antennal organs 

 of beetles, these organs shall be briefly described. 



Each of the 11 segments in the antenna of Dytiscus carries a 

 number of sense organs. The farther from the base of the 

 antenna the more numerous they are. The distal half of the 

 antenna is covered abundantly with sense organs, while the 

 proximal half is sparingly covered with them. The first and 

 second segments are well provided with slender tactile hairs 

 (Text-fig. 3^, THr) which have been called Sensilla trichodea 

 by Schenk. These hairs are also found on all the other appen- 

 dages and even on the head, thorax and abdomen. Two groups 

 of sense bristles (Text-fig. 3-B, SB), called Sensilla chatica by 

 Schenk, lie at the proximal end of the second segment. These 

 hairs are also common on most of the other appendages, on the 

 head, thorax and abdomen. AH segments, except the first one, 

 are well provided with small massive pit pegs of the thick-walled 

 type (Text-fig. 3D, MPPg). All segments, except the first one, 

 are only sparingly provided with a second type of pit pegs. This 

 one is the hollow or thin-walled type (Text-fig. 3C, HPPg). 

 Only about six of these were found on each segment. Besides 

 being found on the antennae, both types of pit pegs are common 

 on all the mouth parts, on the mesothorax, around the spiracles, 

 on all the legs, and on the sexual apparatus. Pit pegs have 

 been called Sensilla coelloconica by Schenk. All segments, 

 except the first two, are abundantly supphed with the cup- 

 shaped or pore-plate organs (Text-fig. 3D, PorPl). For both 

 antennae they are estimated between 4,500 and 5,000. These 

 organs are also common on the palpus of the first maxilla. They 

 were first studied by Nagel on the antennae and maxillary palpi 

 of Dytiscidae. In the honey bee Schenk has called them Sensilla 

 placodea. Of the five antennal organs of Dytiscus, only the 

 hollow pit pegs are regarded by Hochreuther as probably ol- 



