52 SMITHSONIAN MISCELLANEOUS C0L;,ECTI0NS VOL. 'J'] 



Plate-like sense organs occur on the antennae of aphids, Coleop- 

 tera, and Hymenoptera, and have been described on the mandibles 

 and the maxillary palpi of Dytiscus. They have not been studied with 

 equal care in each of these three groups of insects, and it is possible 

 they are not all homologous structures. 



The organs on the antenuce of aphids consist externally of ellip- 

 tical, oval, or sometimes elongate membranous discs (fig. 23 A). 

 Those present on the insect at hatching are said to persist through 

 all the molts to the mature insect, including both asexual and sexual 

 forms ; others appear at the last molt, especially on the winged 

 forms, and are organs of the mature insect only. The internal 

 structure of the organs in the aphids has been but little studied. Flogel 

 (1905) gives a crude figure of a section through one of the discs 

 of Aphis ribis, beneath which he shows a mass of sense cells with 

 their distal processes spread over the entire under surface of the 

 membrane. Externally, he says, the membrane is surrounded by 

 a groove, which in turn is encircled by a chitinous ridge. The mem- 

 brane itself is from i to 1-1/2 microns in thickness, the surrounding 

 chitin being 7 microns thick, from which Flogel argues that the 

 membrane is capable of being traversed by a liquid that could absorb 

 odor substances. He believes, therefore, that the antennai organs 

 in aphids have an olfactory function. Organs of a similar external 

 appearance occur also on the legs and wing bases of aphids (Baker, 

 1917), but it is not certain that the latter are not campaniform 

 organs. 



Plate organs have been noted and described in several species 

 of Coleoptera. Vom Rath (1888) says that the antennai lamella; 

 of Cetonia aurata are thickly beset with them (" membrane canals "), 

 and he gives the form of the cuticular parts of one in section as 

 shown at C of figure 23. Ruland (1888) records plate organs on the 

 antennae of Necrophorus vespillo, and figures the chitinous 'parts as 

 shown at D. Hochreuther (1912) describes plate organs on the an- 

 tennas, mandibles, and maxillary palpi of Dytiscus marginalis under 

 the name of " chalice-form organs," the term referring to the shape 

 of the cuticular canal beneath the plate in vertical sections (fig. 23 B, 

 C). He says that the plates in Dytiscus are extremely small, being 

 from 6 to 8 microns in diameter, and that those of the antennae num- 

 ber from 4,500 to 5,000. They have the same essential structure 

 (B) as those of the antennas of Hymenoptera (I, K) except that the 

 margin of each is deeply inflected. The histological elements of the 

 plate sensilla of the Coleoptera have not been well distinguished, 

 though each is innervated through a group of sense cells. 



