150 The Pupa of Chtronomus 



narrow and pointed behind; it extends to the end of 

 tlie metatliorax, the oesophagus having now contracted 

 so much that the cardiac end of the stomach is close to 

 the head, from which it is separated only by the length 

 of the greatly reduced prothorax. The diverticulum is 

 an outgrowth of the oesophagus, and probably repre- 

 sents the sucking stomach or honey-bag of some winged 

 Diptera, Neuroptera, Lepidoptera, and Hymenoptera. 

 It has a distinct epithelium with small nuclei, a rather 

 thick muscular wall, and, like the oesophagus itself in 

 this stage, secretes no chitinous intima. The cavity, 

 which is at first conspicuous but not large, never contains 

 food ; it shrinks rather rapidly, and in the late pupa is 

 nearly obliterated, the epithelium being then irregularly 

 folded, and in course of disintegration. 

 The two The tube-dwelling Chironomus-larvae are distinguished 



Chirono- from the surface larvae by a number of adaptive charac- 

 ^Tpupir ters, so marked that it is a matter for surprise to find that 

 contrasted. |^^^-j^ groups cau be compriscd within one genus. The 

 tube-dwelling larvae usually have red blood, four ventral 

 and four anal blood-gills, and vestigial tracheae, reduced 

 to two almost independent intersegmental systems. The 

 pupa is furnished with tracheal gills on the prothorax. 

 The larval head is usually small, and the invaginations 

 for the compound eyes and antennae often extend far 

 into the prothorax, where the larval brain is situated. 

 In the surface larvae, on the other hand, the blood has 

 rarely any red colour ; the ventral blood-gills at least are 

 wanting ; the tracheal system may extend throughout 

 the whole length of the body, its various parts being- 

 connected by longitudinal trunks of fair capacity. The 

 pupa has prothoracic trumpets in place of tracheal gills. 

 The larval head is sometimes decidedly larger in propor- 

 tion to the body than in the other group ; the invaginations 

 for the compound eyes and antennae are therefore shorter, 



