298 Mr. J. A. Dell on the Structure 



eleven ganglia, not segmentally arranged. The hindmost 

 of the chain is small, and close to the one immediately in 

 front of it. The first body-segment contains, in addition 

 to the first ganglion of the ventral chain, the cerebral and 

 sub-oesophageal ganglia, which in most insects lie in the 

 head. The shifting of these ganglia into the thorax is no 

 doubt explained, as in the case of Chironomus, by the 

 reduction of the larval head. The large and complex 

 head of the fly cannot be developed within the 

 n a small larval head, and its rudiments, besides the 

 brain, which it will ultimately enclose, are trans- 

 ferred to the much more spacious prothorax.* 



Alimentary Canal of the Larva. 



The alimentary canal resembles that of 

 Chironomus f so closely that I shall merely 

 describe the points of difference between the 

 two. The fore-part of the stomach is separated 

 as a cardia, but has no caeca. Between its wall 

 and the reflected wall of the oesophagus arises 

 a thin chitinous membrane, the " peritrophic 

 membrane " of Balbiani.j 



The function of the peritrophic membrane is 

 to protect the mesenteric epithelium from 

 abrasion by inorganic particles, which the larva 

 swallows along with its food. It is apparently 

 cast periodically. In a living Psychoda larva 

 reversed peristaltic contractions have been 

 observed. The digested food has been seen to 

 be carried up into the space between the 

 peritrophic membrane and the mesenteric 

 epithelium by these peristaltic contractions. At 

 the hinder end of the mesenteron is a slight 



l 1 IG 3 . 



dilatation, into which open the five Malpighian 

 Nerve-cord tubules. Five is an unusual number of tubules 

 " among insects, but it is found in Blepharoceridss, 

 Culex, and Ptychoptera.§ 

 There is a pair of salivary glands situated in the fore- 

 part of the thorax. They are roughly bean-shaped, and 

 the duct arises from the part corresponding to the hilum 



* Miall and Hammond (1892, 1900). 



+ Miall and Hammond (1900). X Balbiani (1890). 



§ Midler (1895). 



