THE INTERNAL ORGANS OF RESPIRATION. 177 



colon, from which they originate in a tuft, take their course to the 

 head, where they again furcate. On each side of the ventral portion 

 two smaller vessels lie, which are united to the dorsal vessels by means 

 of transverse branches. The upper one of these runs also to the head, 

 the lower one, taking its course nearly in the centre of the body, termi- 

 nates on the contrary in delicate ramifications * at the stomach. We 

 find also in the perfect insect both the ventral and dorsal stems, the 

 latter communicating by means of delicate canals with the seven spi- 

 racles of the abdomen. 



The tubular vessels, lastly, are found very generally in the larvae of 

 the Diptera. The larva of the common gnat (Cw/e.r) has two large 

 dorsal stems, which originate, already divided, from the above described 

 posterior air tube, and give off their fine branches to the internal 

 organs t. In the larva of Eristalis tenax, Meig., which has been called 

 the rat-tailed maggot, from its long air tube (PL II. f. 8.), both the 

 two great tracheal stems unite, previously to their passing into the inner 

 tube of the air tube, by means of a transverse branch, and remain for 

 a small space separated, lying convoluted in front of the internal aper- 

 ture of the tube, but it is only where they pass into the inner tube that 

 they are truly united together. In the body itself they are never again 

 united, but in the first segment in the membranous head there is ano- 

 ther connecting tube which proceeds directly behind the cerebrum. In 

 front of this connection they become considerably narrower, but behind 

 it each stem proceeds out of the head as a fine tube passing into a small 

 air tube placed at each side of the head, which were necessary for the 

 expiration of the previously inspired air. It is probable that such 

 anterior air tubes are found also in the larvae of other Diptera. A 

 similar structure is found in the larvae of all the flies ; but they want 

 the tail, and both the tracheal stems separately vent themselves at the 

 posterior obtuse surface of the body (PI. II. f. 1.). 



The larvae of the Hi/menoptera have also tubular main stems, but 

 which, as they are formed of small tubes that proceed from the spi- 

 racles, are never so large and developed. Two main stems consequently 

 proceed on each side of the body, united in each segment by means of a 

 transverse connecting vessel, but there orginate from them, at those 

 places where the tubes of the spiracles pass into them, innumerable 



* Suckow in Heusinger, f. 7. & 9. 



f Swammerdam Bib. Natuno, PI. XXXVII. f. 5. h. 



