172 THE RESPIRATORY SYSTEM [CH. 



a strong transverse branch lying in the eighth segment, by means 

 of which it becomes connected with the ventral trunk of the same 

 side. This cross-trunk is probably the highly developed homologue 

 of series (v) ; its greater size being correlated with the great enlarge- 

 ment of the corresponding spiracle. The genital duct crosses it on 

 the inner side. From it, two strong tracheae are given off to 

 supply the last two segments and the anal muscles. 



The dorsal trunks are easily recognized by their dark reddish- 

 purple or brownish colour, as well as by their large size. 



The Visceral Trunks (FT). These are smaller than the dorsal 

 trunks, and of a much paler colour, either whitish or lightly pig- 

 mented in brown or pink. They run closely alongside the whole 

 extent of the mid-gut, laterally. Anteriorly, each visceral trunk 

 bends inwards in a kind of " kink " in the region of the gizzard, then 

 bends outwards along the surface of the crop, which it crosses 

 dorsally, finally entering the dorsal trunk of the opposite side close 

 up to the thorax. The right visceral trunk usually passes over the 

 left, towards the anterior end of the crop. This point is called the 

 visceral crossing. Posteriorly, each visceral trunk turns downwards 

 in the eighth segment to join the ventral trunk, at its point of 

 union with the large cross-trunk mentioned above. Each visceral 

 trunk gives off the following branches : 



(i) A branch to the crop and gizzard. 



(ii) Six short branches to the mid-gut. Each of these, on 

 reaching the gut, bifurcates into two diverging portions, which 

 spread out anteriorly and posteriorly, ramifying over the surface. 



(iii) A strong branch to the zone of entry of the Malpighian 

 tubules. This divides into a number of short branches corre- 

 sponding with the separate Malpighian conduits. Each short 

 branch quickly divides into five or six fine tracheae, one of which 

 accompanies each Malpighian tubule along its entire length. 



(iv) A strong branch to the hind-gut. 



The Ventral Trunks (VNT). These are well developed and 

 darkly pigmented, but not so large as the dorsal trunks. They 

 lie ventrally in the abdomen, above the pleural membrane, one 

 on either side of the ventral nerve-cord. Anteriorly they become 

 very attenuated, and divide up into two or three small branches, 

 one of which appears to reach the metastigma, or a branch arising 



