Tracheal System 8i 



nomus dorsalis there are two pairs of segmental tracheal 

 systems in the thorax, of which the fore pair are much 

 the larger. The segmental systems appear first in or 

 close to the intersegmental boundaries, between the pro- 

 and mesothorax, and again between the meso- and meta- 

 thorax. At first they are independent of one another, 

 but in the end they become slightly connected by longi- 

 tudinal vessels. A very slender lateral or initial tube 

 passes from each segmental system to the integument, 

 but the spiracles are closed. The tracheae, with the 

 exception of the initial tubes, are filled with air ^. Each 

 of the four initial tubes is plugged with dark chitinous 

 deposit at the point where it reaches the skin ; at these 

 points the old tracheae are withdrawn during a moult as 

 separate bunches, the slender longitudinal vessels being 

 broken across. 



No insect is known to us which has more completely 

 departed from the habits and structure of an air-breath- 

 ing animal. Yet even here we find visible proof of 

 descent from a terrestrial insect with branching air-tubes. 



It is noteworthy that these larvae can live at very great 

 depths, where it is impossible for them to rise to the 

 surface (see p. 3). 



The account just given holds good of the blood-worms, 

 which we have more particularly studied, but not of all 

 larvae of the genus Chironomus, some of which have 

 well-developed longitudinal tracheae. 



Terrestrial Nemoceran larvae may have numerous Tracheal 

 spiracles disposed along the sides of the body ; such ^J^^l^^ ^^ 

 larvae are peripnetistic -. Bibionidae, Cecidomyidae, and Nemoceran 

 Myceto^^hilidae furnish many examples. The larva of ■^^'^^'*'^* 



* Forel (' Materiaux jiour servir a I'etude de la faune profonde du Leman, 

 Bull. Soc. Vm(d. de Sci. Nat, 1874, p. 57) says that Cliironomus-larvae brought 

 up from great depths in the Lake of Geneva always had the tx'acheae 

 devoid of air. 



= Haliday, 1857, p. 179. 



MIALL. Gr 



