Sensillum, pheromone-detecting

Fig. 3 Diagrammatic representation of a pheromone-detecting sensillum trichodeum of a moth antenna. Note the compartmentalization of the lymph and particularly its isolation from the hemolymph...

Fig. 5 Single sensillum recordings from the pheromone-detecting sensilla placodea on P. di-versa male antennae. Note a dose-dependent increase in spike frequency after stimulus application for 300 ms (bar)...

Kim J.-Y. and Leal W. S. (2000) Ultrastructure of pheromone-detecting sensillum placodeum of the Japanse beetle, Popillia japonica Newmann (Coleoptera Scarabaeidae). Arthropod Struct. Dev. 29, 121-128. 

How are these extremely low amounts of pheromone detected and processed As mentioned above, the detection takes place on the antenna. Here ORNs are housed within hairlike cuticular structures, called sensilla, shown clearly in Figure 3. Each sensillum houses between one and three ORNs which send a dendrite up into the hair. On this dendrite receptor proteins act as a lock for a certain odour key. It is thus the receptor protein that is the identification site for the odour molecule. 

Figure 2. Schematic drawing of a pheromone-detecting sensillum placodeum. In the Japanese beetle, these pheromone detectors house two olfactory receptor neurons (ORNs), one specialized for the detection of pheromone (5,7,8) and the other tuned to the behavioral antagtmist (5,7,8).

A number of chemo- and mechanoreceptors participate in the male behaviors. The female contact pheromone is detected by chemosensilla on the antennae and labial and maxillary palps (Ramaswamy and Gupta, 1981). The number of these sensilla increases dramatically during the metamorphic molt, and much more so in males than in females. Unfortunately, no electrophysiological recordings have been conducted, and the specific sensillum type that responds to the contact pheromone... 

After the olfactory receptor is activated, the semiochemical signal must be destroyed to prevent continued stimulation. Esterase, dehydrogenase, aldehyde oxidase, epoxidase, and glutathione-S-transferase activities have all been detected in the sensillum and could degrade the signal. A further example is found in the pale-brown chafer. Phyllopertha diversa. which uses 1,3-dimethyl-2,4-( liT,3//)-quinazolinedione 10 as its sex pheromone. 

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