NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS — SNODGRASS 



51 



The plates of the sensilla placodea often resemble the domes of 

 the sensilla campaniforma (Cf. ngs. 23 B, I and 20 E, D), and the 

 plate organs might be supposed to represent an evolutionary stage 

 beyond the campaniform organs in which the dome has been finally 

 flattened to a chitinous plate. A comparison of the internal struc- 

 ture of the two sets of organs (figs. 21 and 24), however, shows that 

 they belong to different series. In each sensillum placodeum there 

 is a group of sense cells (fig. 24, SCls), and also a cap cell (CCl) and 



"HO 



C 



D 



a- 



Fig. 23. — Cuticular parts of sensilla placodea in surface view and in 

 section. 



A, discs near end of third segment of antenna of grain aphis. B, sec- 

 tion of plate on antenna of Dytiscus (Hochreiither, 1912). C, same 

 from Cctonia atirata (vom Rath, 1888). D, same from Necrophorus 

 vespillo ■{'R.\.\\^nd, 1888). E, surface view of antennal plate of Ophion 

 lutcus (Ruland). F, section of antennal plate of Cynips gallcE tinctorice 

 (Ruland). G, section of antennal plate of Ophion luteiis (Ruland). H, 

 surface view of antennal plates of Vespa crabro (Ruland). I, trans- 

 verse section of one of same. J, surface view of antennal plate of 

 Apis- mcllif era. K, longitudinal section of same: a, outer ring of light 

 chitin ; b, inner groove. 



enveloping cell (ECl) of typical form and relation to each other 

 and to the sense cells. The external plate (PI), moreover, lies over 

 the cap cell, or hair membrane cell, and is, therefore, evidently the 

 chitinized hair mernbrane rather than the base of the hair. The plate 

 organs, hence, belong structurally to the chemoreceptive series, while 

 the campaniform organs belong to the tactile series. The campani- 

 form dome is probably the reduced hair ; the plate appears to be the 

 chitinized hair membrane. These statements are, of course, tentative 

 and must be tested by a further study of the histology of the organs 

 in question. 



