The Sensory Physiology of the Harvest MiteTrombicula AutumnalisShaw
Authors: Jones B. M. 
Published: April 2021

Responses to stimuli

Light

In a strong beam of light the harvest mite will move directly towards the source, whereas in a weak light the tracks are at first inclined to be wavy, but as they approach the source the tracks straighten. The mite moves along the bisector of two intersecting lights of equal intensity, and when blinded on one side makes circus movements.

When offered a linear gradient of light intensity the mite avoids the darkened portion and moves towards the lightest part of the field. Its movement towards sunlight is a true response to light and not to heat. A sudden decrease of light intensity produces a questing response.

Temperature

The sensory perception of heat is poorly developed. The mite is incapable of locating a warm tube or the body of a young live mouse. On touching a heated object it displays a well-defined response to a temperature difference of about 15° C. In a linear or concentric temperature gradient it displays avoiding reactions to low and high temperatures and appears to prefer a range extending from 15 to 26° C.

Chemical stimulation

The mite is repelled at a distance of 0.5 cm. from phenol, methyl phthalate, dilute ammonia, xylene and a 3 % solution of glacial acetic acid.Toluene was repugnant at 1.5 cm., whilst a mixture of amyl acetate and water repelled the mite at 5 cm. Complete indifference was shown to the odour of skin, liver, sebum and cerumen, but perspiration induced an avoiding reaction.

Humidity

Depletion of the water content influences the response of the mite to humidity. A desiccated mite is active in dry air and inactive in moist air, but a normal individual will settle in either moist or dry air, while avoiding saturated air. The mite requires high humidities for prolonged survival, but avoids free water.

Touch

Unfed mites are very sensitive to touch. The extent of stimulation by contact with each other’s bodies, which is regarded as high, immobilizes them, and it is primarily responsible for the quiescent state of a cluster of mites. When the stimulation is low, for example, when only the tarsi are in contact with a surface, the mite responds by displaying a high state of activity. A mite lightly touched will immediately quest, a response induced equally by vibrations of the substratum.

Clustering

The gregarious habit of the mites is primarily a response to the touch of each other’s bodies. When the humidity is within the range 95-100% R.H. light will induce the mites to climb up a rod and form a cluster at the tip. Whether or not negative geotaxis also plays a part, it is difficult to say, because the evidence suggested that the mite is independent of gravity.

Sense organs

There are three types of sensilla: (1) tactile sensilla, both plumose and plain; (2) peg organs; (3) minute sensory rods, principally confined to the first leg. An elliptical lens, a discoid mass of red-pigmented oily substance, and a pronounced dark pigmented cup are conspicuous features of the better developed anterior eye of each ocular area.

Identification of the responses

Where possible the responses of the mite to various kinds of stimuli have been identified according to a recognized scheme of classification.

Behaviour in the natural environment

The responses to stimuli which the mite will encounter in the natural environment, and their value with respect to acquiring a host, are discussed.