THE LARVA OF TIPULA MAXIMA. 127 



Respiration. 



The respiratory system consists essentially of a pah* of large longitudinal tracheal 

 trunks lying dorsal to the pericardial diaphragm (PI. 27. fig. 19, tr.l.), and traversing 

 the body from the posterior segment to about the middle of the first body-segment, 

 where they end blindly in connection with the body-wall, at positions where the 

 respiratory trumpets of the pupa will open. From these longitudinal trunks, branches 

 arise segmentally to supply the body-wall and viscera (see PI. 27. fig. 19). 



The lateral trunks communicate with the exterior through the pair of spiracles on 

 the hind face of the posterior segment. 



The spiracles themselves are of complicated structure (see Pis. 24 & 25. figs. 4, 6, 7, 

 8, & 9). Externally they appear as broadly oval dark spots showing in surface view 

 two distinct regions : — 



1. A central area consisting of an imperforate disc of chitin occupying about one half 

 of the total diameter (sp.d.). 



2. A surrounding margin (sp.m.) formed of numerous rods of chitin radiating from 

 the central disc to the circumference of the spiracle, and lying side by side so closely as 

 to leave but very narrow slits between them. These slits appear further to be crossed 

 by numerous transverse connections, giving the whole a lattice-like appearance. Air 

 enters between the radial bars. Seen in sections this marginal lattice-work is formed 

 of three sets of parts : — 



1. Passing in a radial direction from the margin of the spiracle to the central disc, 

 but at a lower level than the outer surface (and hence not seen at all in surface view), 

 is a series of hollow chitinous radial bars (r.b.l.), irregularly oval in section, some bifur- 

 cating towards the centre, while others are joined with their neighbours by connecting 

 branches. 



2. Arising from these are the series of Y-shaped upstanding chitinous pillars (T.), 

 each of the radial bars bearing a complete series. 



3. Supported by the upper ends of contiguous Y-pillavs is a second series of radial 

 bars (r.b.n.), slightly flat-topped but wedge-shaped below. These being supported by 

 branches of neighbouring Y-pillars will necessarily alternate with the lower radial bars. 

 Further, these are the bars seen in surface view, the transverse connections being the 

 Y-pillars seen from above. 



To complete the structure the Y-pillars are connected together by very numerous and 

 excessively fine chitinous threads (f.h.), which branch and intercommunicate, the whole 

 forming a close network. 



This spiracle cover appears quite incapable of closing, and the arrangement seems to 

 be a complicated form of filtering apparatus, probably also preventing the entrance of 

 water to the spiracles when submerged. 



The spiracles open, not directly into the longitudinal tracheal tubes, but into two 

 " stigmatic chambers," which, in turn, lead into the tracheal tubes (PI. 24. fig. 6, st.ch.). 



The " stigmatic chambers " are tubular or cylindrical pits, almost circular in trans- 

 verse section, but expanded ventrally immediately within the spiracle. 



19* 



