NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 53 



It is in the Hymenoptera that the sensilla placodea are best known 

 and, apparently, best developed. They have been found only on the 

 antennae in this order. In Cynips and Ophion, according to Ruland 

 (1888), the plates are greatly elongate in form (fig. 23 E). The 

 outer surface in Cynips is slightly elevated above the general surface 

 of the antenna and is surrounded by a deep groove ; the under surface 

 presents two longitudinal ridges (F). In Ophion (G) the plate is 

 a thin, arched, chitinous membrane, beneath which project two ridges 

 from the walls of the cuticular canal, leaving only an axial slit open be- 

 tween them. In Vespa crabro the plates are also elongate and nar- 

 row (H), and each is surrounded by a deep furrow, as shown well 

 in a cross-section (I). 



In the honeybee the plate organs are closely distributed over the 

 entire inner and ventral surfaces and on the dorsal surface of the 

 distal half of the last eight segments of the flagellum of each antenna. 

 There are about 30,000 on both antennae of the drone, 5,000 to 

 6,000 in the worker, and 2,000 to 3,000 in the queen. In the Vespi- 

 dae, solitary bees, and bumblebees, according to Schenk (1903), the 

 plate organs are but little more numerous in the males than in the 

 females. Each plate, in the honeybee, is elliptical in shape (fig. 23 J) 

 and from 12 to 14 microns in longest diameter, which is lengthwise 

 on the antenna. The surfaces of the plates are flush with the an- 

 tennal wall, but each plate is surrounded by a line of clear chitin 

 (a) which may be marked by a slight groove in some cases, but 

 is certainly not excavated to form a deep furrow around the plate, 

 as indicated in figures by Ruland (1888), Schenk (1903), and Mc- 

 Indoo (1922). Within the margin of the plate is a second concen- 

 tric light line {b), due to a submarginal groove on the inner surface 

 (K, b). The cuticular canal beneath the plate is large but contracted 

 proximally (K), its wall nearest the base of the antenna being ap- 

 proximately vertical while the other slopes inward toward the an- 

 tennal base. 



The inner structure of a plate sensillum in the Hymenoptera 

 is similar to that of the pegs and the thin-walled hairs of the chemo- 

 receptive series (fig. 18). In a plate organ of the honeybee (fig. 24 A) 

 most of the space in the cuticular cavity beneath the plate is occu- 

 pied by a large cap cell {CCl) which projects below the inner sur- 

 face of the cuticula. On the side toward the base of the antenna 

 the cap cell is perforated by a tubular canal which contains the distal 

 parts of the enveloping cell (A, B, ECl) and the terminal strand 

 {TS) of the sense cells. The strand expands proximally into the 

 compact, cylindrical bundle or fasciculus (Fas) of sense cell proc- 



