132 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



present time certain of the results obtained by Mclndoo cannot 

 be reconciled with those of other workers, and it is evident that the 

 subject requires further exploration under carefully standardised 

 conditions. 



The Gustatory Sense. The probability that, in many cases, 

 insects do not experience " taste " and " smell " as separate and 

 distinct sensations has been previously referred to. Nevertheless, 

 the positions of certain sensillae in relation with the oral cavity 

 strongly suggest that such organs are adapted to perceive chemical 

 stinmli by actual contact only. Proof of such a contention is 

 extremely difficult to attain, since the location and minute size 

 of the organs concerned do not render their functions amenable 

 to determination by exact experiment. Sensillae situated on the 

 lining membrane of the epipharynx of mandibulate insects, the 

 two groups of sensory hairs mentioned by Deegener as situated on 

 the floor of the pharynx in Lepidoptera, and the elaborate 

 pharyngeal organ of Hemiptera appear to be almost certainly 

 gustatory in function. The organ in Hemiptera (Fig. 60, A) lies in 

 the clypeal region of the head and is located on the roof of the 

 pharyngeal duct. As described by Bugnion and Popoff (1911), the 

 cuticular lining of the duct at this point is differentiated into a 

 cribriform plate whose pores appear to be occupied by processes of 

 sense-cells. The organ has a definite nerve-supply, but its whole 

 structure, and its range of development in different families of 

 Hemiptera, need further elucidation. In some species the pores arc 

 few in number, but they range up to about 600 in others. This 

 organ, from its structure and location, appears to be adapted for 

 testing the qualities of the plant-sap passing up the pharyngeal 

 duct and before entry into the cavity of the pharynx ; there 

 seems little doubt, therefore, that it can only function as a contact 

 (taste) organ. The blood-sucking bug, Rhodnius, will seldom 

 imbibe salt solution through a membrane unless a small amount 

 of haemoglobin be added (Wigglesworth). It is probable in 

 this instance that the gustatory organ, just referred to, is 

 involved in testing any fluid taken up. 



The various sensillae that occur distributed on the mouth-parts 

 of different insects have also been regarded as gustatory organs. 



