48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. yj 



Pflugstaedt (191 2) describes and figures a large cell beneath the dome 

 of each organ on the halteres of Diptera ; and Sihler (1924) says 

 that the sense cell of the cereal campaniform organs of Orthoptera 

 is always accompanied by a dome-forming cell. The "dome cell" 

 (fig. 21, CI) in its relation to the dome {Do) of a campaniform or- 

 gan corresponds closely with the cap cell of a sensillum placodeum 

 (fig. 24 A, CCl) in its relation to the plate {PI) ; but the two sets 

 of organs are only superficially similar, and the campaniform dome 

 cell, as figured by Sihler in the cockroach (fig. 21, CI), suggests the 

 enveloping, or trichogenous, cell of a hair sensillum (fig. 13 B, ECl). 

 The single large cell in the sense organs of the rectum of Lamelli- 

 corn beetle larvae (fig. 22, CI), which appear to be organs of the 

 campaniform type, is described by Orlov (1924) as a gland cell. 

 The question, therefore, as to whether the dome of a campaniform 

 organ represents the reduced hair or the hair membrane of a sensil- 

 lum trichodeum must be regarded, for the present, as an unsettled 

 one. 



It is to be noted that the campaniform organs, having each a 

 single sense cell, fall structurally in the class of the tactile oigans of 

 the hair and peg varieties. 



The campaniform organs are known to occur on the head, thorax, 

 abdomen, the antennae, mouth parts, legs, wing bases, cerci and 

 sting of various adult insects in all the principal orders, and they 

 have been found on the larvae of some species. Their external struc- 

 ture was first described by Hicks (1857, 1859) who call d them 

 simply " vesicles." Since then, both the external and the internal 

 structure of the organs, described under various names, have been 

 more closely studied by many other investigators. The distribution 

 of the organs over the various parts of the body and appendages of 

 insects in all the principal orders has been described extensively by 

 Mclndoo (1914-1920). The structure of the organs of the wing 

 bases has been particularly studied by Guenther (1901), Freiling 

 (1909), and Erhardt (1916) ; those of the halteres of Diptera, by 

 Hicks (1857, 1859), Weinland (1891), Pflugstaedt (1912), and Mc- 

 lndoo (1918). Hochreuther (1912) describes the organs on the 

 head, antennae, mouth parts, and legs of Dytiscus marginalis, and 

 Lehr (1914) those on the wing bases of the same species. Sihler 

 (1924) gives an account of the campaniform organs on the cerci 

 of Orthoptera. Janet (1904) describes the organs of ants, and Mc- 

 lndoo (T914) and Trojan (1922) record their presence on the sting 

 of the honeybee. 



The shape of the campaniform dome varies in different organs. 

 It may be strongly convex and evenly rounded (fig. 20 A), or its 



