Pomona College Journal of Entomology 691 



The lic.'ul, as in related forms, is retractile .and thus ca|)al)le of being pro- 

 tected. The mouth |)arts are normal with the usual hooks, and nothing that is 

 at all abnormal. 



'I'wo methods were followed in studying the internal structure of the larva. 

 In order to see the tracheal system clearly, live maggots were put into melted 

 glycerine jelly directly, thus preventing the air within the tracheae from being 

 driven out. The trans])arency of the body and the perfect retention of the air 

 within them made the main trachea? and even the finer net-work of the system 

 beautifully distinct (Figure ^22 A). One straight, continuous tube extends from 

 each j)osterior spiracle to the corresponding one near the anterior end, giving off 

 in its course numerous branches outwardly which branch and rebranch many 

 times ; between and connecting the two main tubes are seven arches or loojis 

 directed forward, most of which give off minute branches at the apex of the 

 arch forward. Xear the base of each main tracheal trunk there is an inferior 

 branch, quite large, which extends down and forward, one branch extending back 

 to the anal tip, the remainder going toward the head end, supplying oxygen to 

 the ventral tissues. So far the respiratory system is quite normal. 



If there is any adaptational variation from the normal we would naturally 

 look for it at the entrance to the tracheal trunks, and, in fact, it is just here that 

 we find it. At the dorsal invaginating fold at the base of the posterior spiracular 

 processes the tracheal trunks each turn abruptly upward into the processes. 

 About the middle of the process the trunk enlarges to form a sub-globose pocket; 

 from this enlargement it extends on to the tip, opening out at five spiracular 

 foramena, or stigmata. 



Transverse and longitudinal sections through the tracheal trunks at the 

 posterior end show that the intima from the spiracle to the anterior end of the 

 globose pocket is densely lined with long cilia, which apparently serve to strain 

 the air, as it were, by removing the particles of dust and oil which might find 

 entrance (P'igure 222 D). The spiracular openings themselves are protected by a 

 thin, sieve-like, ciliate cover, easily visible in cross section. Looking at them 

 from above, in situ, they appear as five light pinkish spots on the black back- 

 ground, arranged four around a central stigma. As nearly as can be determined 

 this cover consists of many cilia radiating from the circumference. No traces of 

 a valve or of oeclusor muscles are visible, and this is readily explained bv the 

 substitution for these of the telescoping of the entire stigma. 



The anterior spiracles are much smaller, less consjjicuously chitinized, and 

 situated well forward near the posterior end of the head. These also are retrac- 

 tile, but, as already described, differently from the posterior pair. As in the 

 others the tracheal trunk enlarges abruptly into a large sac near the surface and 

 thence continues to the surface in tlie form of a slender, thickly chitinized tube 

 which ex|iands at the tip into a large cai)ut with a number of facet-like stigmata. 

 Internally the sac and tube are lined with cilia, as in the case of the posterior 

 trachea'. Extending in as far as the base of the tracheal sac is an invagination 

 of the euticular wall. When the spiracles are extended for breathing, the invag- 

 ination is continued outward in an evagination to the base of the tracheal caput. 



