]06 V. G. DETIIIFR, J. R. LARSEN AND J. R. ADAMS 



under ultraviolet light for 48 hr. Sections were cut with glass knives in 

 a Porter-Blum and LKB ultramicrotome. Micrographs were taken with 

 RCA-EMU-Z and Akashi TRS-50E electronmicroscopes at magnifications 

 of 1000 to 16,000 times. 



Receptors were also studied with a phase contrast microscope and after 

 staining with methylene blue according to the method of Grabowski and 

 Dethier (1954) and with Hohnes' silver technique (Larsen, 1960). 



ANTENNAL SENSILLA 



The majority of sensory structures in insects (internal proprioceptors 

 excepted) consist of a cuticular covering, which is a modified portion of 

 the general body cuticula, one to approximately fifty bipolar neurons 

 whose dendrites are associated with the cuticular covering and whose 

 axons pass into the central nervous system, and three or more accessory 

 epidermal cells which have no direct part in reception. A unit consisting 

 of these components is termed a sensillum. The individual neuron is the 

 receptor. A single sensillum may contain a number of receptors, each of 

 which mediates a different modality (e.g. a labellar hair, which houses a 

 water receptor, a sugar receptor, a salt receptor, and a mechanoreceptor). 



The sensilla of the antenna, among which are numbered the olfactory 

 receptors, are situated on the fleshy apical segment. Neither the basal 

 segment nor the conspicuous plumose arista bear olfactory receptors. The 

 sensilla are present on the entire surface of the segment and within large 

 pits which are a conspicuous feature of the antenna. 



Antennal pits are confined to the dorsal, ventral, and inner lateral distal 

 half of the segment, this area being unpigmented. The remainder of the 

 segment is fuscous. There is some variation in the number, size, and exact 

 location of pits from one individual to the next, but in general the males 

 possess from 9 to 11 and the females from 1 1 to 16. Some pits are simple, 

 pocket-like cavities (Plate I, Fig. 5) ; others are mukichambered (Plate I, 

 Fig. 3). Their orifices are ringed with short, stout, spinous projections of 

 the cuticle. Within the pits are found three types of sensilla : thin-walled 

 pegs, thick-walled pegs, and coronal pegs, so called because of their crown- 

 like appearance in transverse section (Plate T, Fig. 3). Only one type of 

 sensillum occurs in a pit. 



The surface of the antenna is thickly clothed with non-innervated spines 

 with which the numerous sensilla are interspersed (Plate III, Fig. 3). The 

 sensilla are of three general types : pointed, tapering, thick-walled pegs 

 (Plate III, Fig. 1) ; short, rounded, thin-walled pegs (Plate III, Fig. 1) ; 

 and minute stellate pegs (Plate III, Fig. 4). The thick-walled pegs, the 

 most numerous type, are more or less evenly distributed over the entire 

 surface of the segment, while the thin-walled pegs are absent or sparse on 



