STRUCTURE OF LARVA OF GLOSSINA PALPALIS 387 



pigmented, and is marked by a number of longitudinal striations 

 projecting backwards from this there is a pair of large processes, 

 gently rounded in shape, and with a roughened or shagreened surface. 

 These caudal protuberances oppose one another, and form the bound- 

 aries of a deep pit. Their internal surfaces are marked by two slight 

 linear depressions, which divide the inner surface into three lobes, the 

 whole structure corresponding, as Roubaud has pointed out, to sup- 

 pressed stigmal plates. The true openings of the lateral tracheae 

 are situated in the pit between the protuberances. 



The internal structure of the larva is specially modified for its 

 peculiar mode of life, especially as regards alimentation. The alimen- 

 tary canal shows the usual divisions, but each of 

 these is adapted for the exceptional conditions. The of the (arva 

 pharynx, which is situated at the extreme anterior 

 end of the body, immediately behind the buccal orifice, forms the 

 sucking organ. In its dorsal wall there is a stout median process, 

 which projects freely into the cavity anteriorly, and forms a sort 

 of tongue, composed of longitudinal muscles bilaterally arranged, 

 and some transverse fibres. The opening of the cavity is directed 

 towards, and is in close apposition with, the papilla on which the 

 duct of the accessory glands opens, and the larva obtains its nourishment 

 by actively sucking the secretion provided by the glands. The papilla, 

 in fact, serves as a teat. The oesophagus is an exceedingly delicate tube, 

 which passes through a ring of nervous tissue in the ordinary manner, 

 ending in a small proventriculus. The mid-gut is very highly modified, 

 both in its structure and in the part it plays in the metabolism of the 

 insect. It consists of three parts, of which the first and last are simple 

 and tubular, and are slightly convoluted. The middle portion, on the 

 other hand, is dilated into a voluminous sac, occupying almost the entire 

 space within the body cavity, and in life is always distended with the 

 granular milky fluid obtained from the accessory glands. The walls of 

 this sac are of great delicacy, and contain very little muscular tissue ; 

 they are lined by a single layer of cells which resemble, as Roubaud 

 points out, those of the fat body rather than ordinary digestive cells, and 

 are constantly loaded with globules which are probably of a fatty nature. 

 In the tubular portions of the mid-gut these cells are columnar and 

 project freely into the cavity of the organ, but in the middle distended 

 part they are flattened by the pressure of the contents until they form a 

 very thin lamina. The saccular part thus resembles an organ for the 

 storage of food material rather than a digestive chamber. 



