1910] Alimentary Canal of Psychoda Alternata 281 



it is developed into four distinct papillae which surround the 

 anus (Fig. 1, a. p.) 



The Malpighian vessels join the alimentary canal at the 

 union of the mid-intestine and ileum. They are five in number, 

 two on the latero-ventral side, two on the dorso-lateral side, 

 and the fifth on the dorsal surface of the canal (Fig. 1, m. t.) 

 The vessels all have an expanded reservoir-like basal portion, 

 w^hich in living larvae is snow-white, but in alcoholic specimens 

 fades out (Fig. 27, c.) The two latero-ventral vessels extend 

 far forward to a point opposite the middle of the mid-intestine 

 where they make an abrupt turn backward for a short distance. 

 The two dorso-lateral ones start forward but in the middle of 

 their expanded basal reservoirs they turn abruptly upon them- 

 selves and their distal ends become closely associated with the 

 reproductive organs opposite the middle of the colon. The 

 dorsal tubule follows the same course, its tip lying free above 

 the middle of the colon. Beyond the expanded basal reservoir, 

 the vessels are much constricted and heavily charged with a 

 reddish-brown pigment in life. The vessels are slightly motile. 



Wheeler ('93) claims that the primitive number of Mal- 

 pighian vessels in insects is six. Where there are less than six, 

 he claims that the reduction is due to a loss or fusion of certain 

 ones, while where there are more than six, he explains it as being 

 due to a division of some of them. However this may be, it is 

 well known that the vessels are usually paired, that is, there is 

 an even number of them. It is rare to find an odd number of 

 Malpighian vessels in insects. Wheeler reports seven for a 

 Neuropterous larva, probably Chauliodes, and the same number 

 is found in the larva of Corydalis cornuta. Five Malpighian 

 vessels have been found in Culex and Psychoda. I find this 

 same number in Pericoma. In Psychoda alternata there seeins 

 to be no difference in form or structure between the single dor- 

 sal vessel and the other four paired vessels. They all join the 

 alimentary canal singly and are about equi-distant. The same 

 number and arrangement are found in the pupal and adult 

 stages. 



Surrounding the anal opening are four prominent fleshy 

 lobes. These are direct prolongations from the rectum and have 

 a respiratory function. Numerous fine tracheae extend down- 

 ward and forward to these, from a constriction in the large 

 longitudinal tracheae just caudad of the vent. These lobes are 



