Morphology of a newly discovered sensory array on the mouthhooks of Gasterophilus larvae

The distal mouthhooks of Gasterophilus intestinalis (DeGeer) (Diptera: Gasterophilidae) and G. nasalis (Linnaeus) (Diptera: Gasterophilidae) larvae were studied with scanning electron microscopy to determine the morphology of a previously unknown sensory array. The design of the 3rd stage G. intestinalis sensory array was used as the model for comparison to the 2nd stage of this species and the 2nd and 3rd stadia of G. nasalis. Some components forming the sensory array of the 3rd stage G. intestinalis mouthhook were found in 3rd stage G. nasalis. A major difference between these species was the replacement of shallow pits with sensilla-laden troughs in G. nasalis. Second-stage G. intestinalis has a distal mouthhook that varies considerably from the model type, lacking shallow pits and associated peg-like sensilla. By contrast, the sensory array of the 2nd stage G. nasalis larva is the most elaborate yet encountered, with extensive sensilla and troughs. The existence of other types of sensilla on the mouthhook is proposed, as well as a sensory role for the unsculptured surface. Possible uses of the sensory array by the parasite are discussed.