128 PREDACIOUS AND PARASITIC 



the same purpose as the gastric juice in man, and I think we may 

 fairly describe these glands as "gastric glands." 



The investigation of the means by which the oil-globules of the 

 fovilla are rendered available for the nutrition of the insect presents 

 great difficulty, inasmuch as no organ presents itself as likely to 

 exercise pancreatic functions, and as yet I have failed to observe 

 the point at which the oily globules of the pollen disappear. 



Another interesting set of organs, which are remarkably well 

 seen in many of the Syrphidae, are those which I shall call the 

 "renal bodies." They are shown on PI. IX. both in situ, in 

 Fig. I, and under a higher amplification the individual organs are 

 depicted at Figs. 2 and 3, as seen on the surface and in section. 

 These are from Syrphiis luniger, but Fig. 4 (shown in situ in the 

 wall of the rectum) is from Eristalis tenax^ and with it are shown 

 some of the smaller pollen-grains (Lathyrus) floating loose in the 

 rectal cavity. These organs are also indicated on PI. IV., Vol. 5, 

 at /, t. These organs, which perform somewhat similar excretory 

 functions to the kidneys of animals, are four in number, and are 

 inserted through the walls of the rectum at a point below the valve, 

 at the termination of the lower intestine, where the former 

 enlarges to form a lemon-shaped pouch. 



The arrangement and construction of these organs are obvious 

 from the drawings. Each one of the conical bodies consists of a 

 hollow cone, composed outwardly of a delicate membranous 

 sheath, beneath which a thick layer of gland-cells are disposed. 

 These are somewhat angular from mutual pressure, and each 

 contains a nucleus, which, however, is generally very difficult to 

 demonstrate. 



The gland-cells are supported by a transparent and apparently 

 structureless membrane, which forms the inner surface of the cone. 

 A plentiful supply of oxygen is ensured by the large and direct 

 tracheal trunks, which connect these organs with the spiracles of 

 the abdomen. These trunks do not appear, as in the Musca vomi- 

 toria^ to divide into distinct sets of vessels supplying respectively 

 the inner and outer layers of the organ, but branch off in a tree-like 

 fashion, as shown in Fig. 3, ultimately ramifying into fine tubes, 

 which are disposed parallel to the exterior surfaces, beneath the 

 outer membrane, through which they may be seen by careful 

 focussing, as in Fig. 2. 



