38 PREDACIOUS AND PARASITIC 



brane. The effect is that of minute tubercles set irregularly on 

 the papilla. 



The inner papilla on each side terminates in a single tubercle, 

 of less diameter than the papilla on which it is set, and in shape 

 like the extremity of a sausage. In some of my specimens these 

 tubercles appear to be set with a few short spines ; but in others 

 three or four very minute bladder-like structures are visible, pro- 

 jecting from the tubercle, so close together as to impinge on one 

 another. 



It is impossible to predicate, in the slightest degree, as to the 

 precise function of these organs. I have some reason to think 

 that one or other of the pairs is of an olfactory nature, and ana- 

 logy would lead us to expect that one or both pairs should be 

 endowed with a sense of touch. That they are the seat of some 

 active perception of importance is shown both by their position 

 and by the nerve-fibres, which under favourable conditions may 

 be observed to pass into the base of the papillae. 



Alimentary System of the Imago. 



The entire alimentary system, from the proboscis to the rectum, 

 with the various glands forming its immediate appendages, is 

 shown on PL IV. at Fig. i. The drawing from which this plate 

 is taken was made from dissections of Eristalis tenax, this genus 

 presenting many advantages for the purpose in comparison with 

 the more delicate Syrphus luniger or others of the same genus. 



The internal structure of the former insect differs so little from 

 that of the aphidivorous genera, that it may be fairly taken as 

 conveying to the reader all the important characters of the latter, 

 while the larger size and tougher nature of the various organs 

 enables them to be dissected and represented with a completeness 

 almost impossible of attainment if the excessively minute and 

 delicate viscera of S. luniger or similar species had been employed. 



The principal difference in structure exists in the proboscis, 

 and in order to enable the reader to form a correct idea of that 

 organ as it is found in the genus Syrphus^ an outline drawing on a 

 small scale is given on the same plate at Fig. 2. 



The proboscis of Eristalis h longer and of more substantial and 

 horny structure than that of Syrphus. As in the Muscid(B—\.\iQ 



