The probing response of Stomoxys calcitrans (L.) (the stable fly) to vapours

Abstract

The probing response of the Stable Fly, Stomoxys calcitrans (L.) to vapours was studied using single flies in small still air chambers. Probing was clearly distinguished from proboscis extension alone and from the biting response. The function of probing in feeding behaviour was thought to be to increase the chances of encountering further stimuli which would induce biting and ingestion. The vapour above a 0·16 per cent, ammonia solution (1·1 mg. ammonia per litre air) was found to stimulate probing in over 50 per cent, of flies 24 to 30 hours after feeding to satiation on blood. Saturated water vapour had a lesser but still significant effect in stimulating probing. There was considerable variation in probing threshold between samples of flies. An ammonia vapour concentration of 2·2 mg. per litre stimulated more flies to probe for longer periods than an ammonia concentration of 1·1 mg. per litre but the response to an ammonia vapour concentration of 0·56 mg. per litre was not significantly greater than that to saturated water vapour. The vapours of a series of compounds were tested as probing stimulants using an ammonia vapour concentration of 1·1 mg. per litre as a standard for comparison. The vapours above the following compounds at the concentrations tested were ineffective as probing stimulants; iso-amyl amine solution, methyl amine solution, higher fatty acid mixture, nonylic acid, ammonium valerate solution, commercial ammonium carbonate, indole, L-a-alanine solution, L-lysine solution, carbon dioxide, fresh rabbit blood, and clove oil. The separate vapours above a 0·1 per cent, solution of n-propyl amine and above concentrated n-caprylic, n-caproic and valeric acid were found to resemble ammonia in their effectiveness as probing stimulants. The stimulating effect of the mixed vapours above 0·16 per cent, ammonia solution and above a 0·2 per cent, valeric acid emulsion was less than that of either of the vapours singly. The threshold of the probing response rose immediately after feeding to satiation either on withdrawn blood or on 0·5 M sucrose solution. Lower, but still readily acceptable sucrose concentrations, (0·18 M and 0·25 M) although imbibed in as large or larger volumes than a 2·0 M sucrose solution did not appreciably effect the probing threshold. There was a gradual fall in the probing threshold in the 24- to 30-hour period following feeding to satiation on withdrawn blood. The probing threshold was at a similar level 24 hours after feeding to satiation on either withdrawn blood or 0·5 M sucrose solution. The probing response was not effected by 24-hour rhythms of behaviour. There were considerable day to day fluctuations in the threshold of response in a population of flies of similar age, but the fluctuations did not appear to be of a cyclic nature nor obviously associated with reproductive events. There was no obvious association between ( a) weight of blood imbibed and subsequent probing response and ( b) gut contents 24 hours after feeding and the probing response. The probing response to saturated water vapour was not associated with water need. The probing behaviour of males and females was very similar within the age group used in these experiments (2 to 6 days post-emergence age). Neither larval conditioning nor crowding of the adults had an appreciable effect on the probing response. Removal of the antennae and the maxillary palps did not significantly alter the probing response in an ammonia vapour concentration of 1·1 mg. litre per. The increased probing response of the antennae-less flies in the control chamber, however, obscured the effects of antennae removal on the response to saturated water vapour. Tarsal lacquering reduced the probing response to ammonia vapour in both antennaeless and normal flies.