Sensing taste and odours

The olfactory sensilla can be recognised from the multiple pores in their walls. It is hypothesised that these pores let the odour through to the inside of the sensillum.
The pore tubules (5 – 50 waxy tubules present in some pores with a pore kettle) can often be seen spreading towards the central lumen, when viewed using transmission electron microscopy. These tubules dangle freely in the lymph within the sensillum. As remnants of wax canals formed during cuticle generation they may be involved in carrying odour molecules into the sensillum.
Walls of the sensilla can be variable in thickness. Long trichoid types may have walls 0.5 µm thick or thicker. Pegs without grooves may be less than 0.3 µm in wall thickness.
Pores in the wall can be arranged in different ways. In grooved pegs (grooved sensilla basiconica) pores are arranged at the base of the grooves and usually not obvious from the surface view. In sensilla with a smooth surface, for example trichoid, basiconic and placoid sensilla, pores are spread across the surface evenly or in patterns. Pore densities vary. For the biting midge, Forcipomya Lasiohelea townsvillensis they range from 7 to 75 per µm2 across 6 types of trichoid sensilla on the antennae (Cribb, 1997) The functional significance of the variety in pore density has not yet been determined.
Not all grooved pegs have simple pores at the base of the grooves. Some grooved pegs have a more complex double-walled structure with spoke channels running the full width of the walls. So the pores of double-walled sensilla do not have pore tubules associated with them. The channels are filled with an electron-dense material for the entire length. Good images of grooved pegs and pores can be seen in Shanbhag et al. (1999).

Diagram through an olfactory sensillum (smooth-walled peg type). The internal lymph space is fluid-filled. Here the pore opens into a spherical bowl-shaped structure within the wall, referred to as a pore kettle. Epicuticle covers the sensillum and pore opening.
[Diagram: B.W. Cribb]

Diagram of a transverse section through a grooved peg showing the channels running from the base of the grooves. [Diagram: B.W. Cribb]

Taste sensilla (gustatory sensilla or contact chemoreceptors) have a single pore at the tip. This pore is bigger than pores in odour-sensitive sensilla and measures about 0.2µm in longest diameter. It can be circular, oval or slit-shaped. A porous plug may fill this hole.
Some gustatory sensilla can be hair-shaped but others are more complex like the styloconic sensillum (peg set on the end of a style) seen at right. Other gustatory sensilla are also present.
The shaft of the sensillum may have a socket but this has nothing to do with the chemosensory function. It just indicates that there is also a mechanoreceptive nerve cell at the base that responds to movement of the shaft.

Within the shaft of the gustatory sensillum is a tube called the dendrite sheath. This houses the dendrite projections from the sensory neurons below. There are different arrangements of the dendrite sheath. It can be a free-standing tube, as seen in grasshoppers (A), or a tube fused to the inside of the sensillum wall over to one side, as in flies (B).
[Diagram: B.W. Cribb]

Feeding Video

Insects use some of the body components such as legs and labial/maxillary palps to touch and taste substrates such as food. Where direct contact occurs we are sure to find touch and taste sensilla. The insects shown in this clip are the grasshopper, Valanga irregularis and the Monarch butterfly caterpillar, Danaus plexippus. See if you can see the hairs and spines. Pegs on the very ends of the grasshopper palps are too small to be seen but those further up the segments are visible. Note the long hairs on the antennae of the caterpillar.