LARVA OF TANYPUS MACULATUS. 85 



— one beneath the prothoracic ring, and the other at the termina- 

 tion of the body ; the former corresponding to the front pair of 

 thoracic Hmbs, and the latter to the anal pro-legs of caterpillars. 

 Both pairs are built on the same plan, and consist of coronets of 

 recurved booklets surmounting retractile fleshy footstalks. The 

 anterior pair can, perhaps, be scarcely described as a pair, since 

 the two branches coalesce into one common stem arising from the 

 centre of the under-surface of the segment. In other species, 

 however, the insertions are distinct. We may see occasionally the 

 two coronets withdrawn into their respective branches ; then these 

 are withdrawn into the common stem, and finally the whole 

 disappears into the interior of the body. These organs are used by 

 the larvae to grope their way among the flocculent sediment in 

 which they are found, and confer, as might be expected, but 

 little locomotive power upon their possessors. 



The whole course of the alimentary canal can be easily traced. 

 The anterior portion constituting the pharynx receives in its 

 passage through the head the insertion of a mass of powerful 

 muscles, which have their origin in the integument, and which 

 serve to dilate its cavity when occasion requires for the passage of 

 food. These muscles stand out in brilliant colours under the 

 action of polarized light. 



Immediately behind the head we find two large and delicate 

 sacs, lined with epithelial cells, and each terminating anteriorly in 

 a small duct which joins with its fellow of the opposite side. The 

 common duct thus formed, I have not been able to trace, but 

 analogy leaves little doubt that it enters the pharynx immediately 

 behind the mouth. The sacs are the salivary glands, and the duct 

 is the salivary duct. 



After passing the salivary glands, the oesophagus suddenly 

 widens into a large crop, the counterpart of what is known as the 

 sucking-stomach of the fly. In this may frequently be discerned 

 two or three small Crustacea, such as Chydorus sphcei-icus^ just 

 swallowed, lively enough as yet, and making vigorous but futile 

 efforts against the walls of their living prison. 'Jlie crop is suc- 

 ceeded by the proventriculus, an organ that corresponds to the 

 gizzard of the cricket, so much admired as a microscopic object 

 for its rows of horny teeth.* No such teeth, however, exist in 

 the proventriculus of the Diptera, but the organ is surrounded by 

 a number of casca or blind-appendages, as shown in the drawing. 



The proventriculus is followed by the ventriculus or true 

 digestive cavity. Here again we find Chydorus^ but its struggles 

 are now at an end, and under the action of the gastric juice it is 



* I observe that Newport says there is no gizzard in the Diptera. The organ 

 I have above described evidently occupies its place, howeyer different in appearance 

 and function. 



