1910] Anatomy of Cecidomyia resinicoloides 49 



ment 4 are quite small and inconspicuous, whereas the sets point- 

 ing backward become thicker anteriorly. We see then that these 

 rows of bristles are heaviest at either end of the larva and absent 

 or imperfectly developed along the middle of the body, there 

 being a gradual transition from the posterior — to the forward — 

 pointing bristles. The bristles thin out laterally and are more 

 numerous on the venter than on the dorsum. 



Although the larva is not an extensive traveller, it is never- 

 theless quite active and moves about considerably within the 

 limited space of the resinous mass in which it lives. It cannot 

 survi\'e xei'Y long when completely submerged in the gummy 

 exudation, but lives with its anal spiracles at the surface of the 

 mass or in a hollow therein, while its mouthparts are in close 

 proximity to the abraded cambium. It must also turn itself 

 around in its cocoon before pupation. It was observed that the 

 posterior portion of the larva is usually more extended and the 

 cephalic end often more contracted. When the larva desires to 

 advance, its anterior segments are extended, thus exposing the 

 strong rows of backward-pointing bristles, while the hinder end 

 is somewhat drawn in, thus concealing its bristles, and, by con- 

 tracting and expanding the anterior segments the desired move- 

 ment can be obtained. It would not seem of so much importance 

 that the larva move its whole body backwards, but that it extend 

 its caudal spiracles to an opening for air is quite imperative. 

 Thus it could firmly anchor its cephalic end to a desired spot, and 

 by means of the forward-pointing bristle rows extend its spiracles 

 to the surface. By sufficient contraction the rows of bristles at 

 one end could be completely concealed and those of the other 

 end fully exposed and brought into play. The pseudopods 

 may also assist in the movements of the larva. 



INTERNAL ANATOMY. 



Tracheal System. PI. VI, Figs. 4 and 5. This was very care- 

 fully worked out, the tracheal arrangement being followed to 

 the finest ramifications. As in other insects it consists of longi- 

 tudinal trunks connected with the spiracles by lateral branches. 

 Figs. 4 and 5 of plate VI shows this system from a dorsal and 

 ventral aspect, of a larva submitted to some pressure under a 

 coverglass. It consists of a pair of longitudinal dorsal trunks 

 beginning at the spiracles of segment i and terminating in the 

 caudal spiracles of segment 11. At G, Fig. 4, a stout branch is 



