82 The Larva of Chironomiis 



Mycetobia, however, is not peripneustic, but ampM- 

 pneastic, having the middle spiracles closed, and only the 

 prothoracic and terminal spiracles open. The larvae of 

 Rhyphns, some Tipulidae, and some Psychodidae are also 

 amphipneustic. Most Culicidae and Tipulidae, besides 

 the aberrant genus Dixa, are metapneustic, with spiracles 

 at the hinder end only. This gradual reduction in the 

 number of open spiracles is no doubt due to increasing- 

 obstruction by water or earth. 



As in other insects, initial tubes are usual in Nemoceran 

 larvae ; they lead inwards from the spiracles, one branch 

 to each spiracle. The initial branches subdivide in- 

 ternally, forming local systems in each segment. The 

 Chironomus-larva does not advance beyond this stage 

 (we are speaking of the bottom-feeding species), and its 

 imperfect tracheal apparatus consists at most of three 

 thoracic segmental systems of very small extent. The 

 local systems may be connected in a rather later stage 

 by longitudinal trunks, from which branches to the vis- 

 cera, body -wall, and limbs are given off. In the larva of 

 Mochlonyx ^ the boundaries of the segmental systems 

 of the abdomen are still marked by thin septa, which 

 stretch across the longitudinal trunks. 



In many Chironomidae, as well as in Corethra, Simu- 

 lium, and Blepharocera, the tracheal system no longer 

 opens at the surface of the body. The initial tubes 

 become impervious, and may perhaps disappear alto- 

 gether in some forms. The longitudinal trunks are 

 usually retained in those larvae which have once acquired 

 them, but in Corethra they subsequently become obli- 

 terated, two pairs of dilatations only persisting as 

 hydrostatic vesicles. 



Nemoceran larvae commonly bear the posterior spi- 

 racles on the eleventh segment, whether this is the last, 

 as in Phalacrocera and Pericoma, or the last but one, as 

 in Culex and Mochlonyx. In Dicranota and Ptychoptera, 

 however, it is the twelfth segment which bears the 

 spiracles. 



The spiracles are usually flush with the general surface 

 of the body, but may be sunk a little, as in Dixa, where 

 a respiratory cup is formed, like that of some aquatic 

 Coleopterous larvae (H^^drobius). In the Culex- and 

 Mochlonyx-larvae, on the contrary, the spiracles are 



1 Meinert, 1886, p. 60 (428). 



