210 Life History of Wireivorms 



narrowed to the bluntly pointed apex. A pore, probably sensory, is 

 situated near the base of the segment, while a second, bearing a short 

 spine is present near the middle. The apex. Like that of the maxillary 

 palp, has an exceedingly minute tuft of short spines, which just break 

 the surface of the cuticle and are visible only under the microscope. 



In the basal segment there is a whorl of four moderately long setae 

 near the apex, two of these being ventral, the others dorsal. Two pores 

 are situtated near the inner margin of the segment, one behind the other, 

 a third in the median line near the base, and a group of two or three pores 

 near the base of the outer margin. 



The pores found on the palps appear to be more open than those on 

 the body of the labium. 



Spiracles of Larva, 



The two thoracic spiracles are situated in the pleurae of the meso- 

 thorax. They are larger than those of the abdominal segments but are 

 otherwise similar in structure. They lie almost, if not quite, parallel to 

 the longitudinal axis of the larva. 



The abdominal spiracles, which are present in each of the abdominal 

 segments 1 to 8, are placed in the anterior third of the dorsal scutum on 

 either side, just above the longitudinal groove separating the tergite 

 from the epipleura. In their case the long axis of the spiracle does not 

 He parallel to that of the long axis of the larva but is raised anteriorly 

 so as to form an angle of some 45 degrees. 



The shape of the complete spiracle is elongate oval (Plate IV, fig. d) 

 and it is broader in proportion to its length in A. obscurus than in 

 A. sputator. Each spiracle is provided with two sUt-Hke orifices, between 

 which there is a wide septum, widest at the surface (Figs. 3 a, 3 6). The 

 septum divides the spiracle completely into two chambers and is con- 

 tinued anteriorly beneath the cuticle. In this portion of the spiracle, 

 which I have named the antechamber, there are a number of pale 

 coloured processes or trabeculae, sometimes branched, which extend far 

 across the lumen of each antechamber. Internally another chamber 

 called the atrium (see Boving(3)) comes into apposition with the ante- 

 chambers and receives the air from them (Fig. 36). It is not, however, 

 so wide as the width of the two antechambers and the latter are continued 

 a httle beyond the opening of the atrium, leaving a sHght cul-de-sac in 

 each case. From the antechambers the atrium extends inwards {i.e. 

 away from the cuticle) for a distance of about double its wudth to meet 

 the trachea which carries the air thence to the main longitudinal trachea 



