II 



1 2th larval segment, the part behind the bunches of setae being con- 

 stricted off." Figure 9 shows a dorsal view of the last few segments 

 of larva No. 1 1 (Appendix) with the more important muscles ; one 

 can see clearly how the reduction has taken place in C. pnsio (fig. 8) 

 and the same would apply to C. dorsalis, and the great majority of 

 other known species. The letters in figures 8 and 9 refer to what are 

 supposed to be homologous muscles in the two species. The anal 

 bristles are clearly seen, but are not very large ; in No. 13 (Appendix) 

 they are quite four times this length and very conspicuous, and enable 

 us to identify the position of the rudimentary segment with facility. 

 It might be said we have here to deal with a false segment, or a 

 modern variation from the type ; but when we remember that other 

 Diptera may have thirteen segments, as for instance Dicranota* 

 and that all Lepidopterous caterpillars when carefully examined are 

 found to possess this number, and that the typical number of abdominal 

 and thoracic segments in imaginal insects is thirteen, we must I think 

 conclude that thirteen is the true number, and regard the apparent 

 twelve segments of most Chironomus larvae as due to fusion of the last 

 two segments. 



Surrounding the anus are two pairs of blood-gills, in which ramify Anal blood-gills, 

 a dense network of fine tracheal tubes ; hence we may call them 

 tracheal gills. They differ from those of C. dorsalis in this and in 

 the absence of red blood. These organs vary a good deal in the 

 different species I have examined. For instance, in one very small 

 species (No. 16 of Appendix) they were nearly three times the usual 

 length and deeply constricted in the middle. In another species 

 (No. n) the gills were capable of almost complete retraction into the 

 rectum. 



There are no ventral blood-gills on the eleventh segment, and from Ventral 

 my own observation I should consider this form of gill of rare occurrence. 

 I have only come across one species (No. 9 of Appendix) bearing ventral 

 gills ; this larva resembles C. dorsalis very closely in most respects, 

 but as may be seen from fig. 19 the ventral gills are much longer than 

 usual, and much coiled. But more remarkable still, on segment ten 

 are found a pair of small gill-like organs laterally placed and projecting 



* MIALL. Aquatic Insects, p. 165, fig. 50. 



