38 INTERNAL ANATOMY. 



surfaces to which they are attached give room for the 

 growth of muscles of any desirable number and of 

 greater comparative size, increased freedom of mo- 

 tion and so on. This point can be roughly illustrated 

 by putting the arm into a stiff paper cylinder, and 

 supposing the muscles to be attached to it, instead of 

 having their attachments crowded together centrally 

 upon a small axis of internal bones. 



An instructive feature in the anatomy of insects is the 

 system of tracheae and air-sacs. On either side of the 

 prothorax (PL II., Fig. 15, p. 38; PI. I., Fig. 6, j-i, p. 10), 

 mesothorax (PI. II., Fig. 15, PI. I., Fig. 7, s~), and first eight 

 abdominal rings (PI. II., Fig. 15, PI. I., Fig. 7, j3_j-io^ is a 

 pair of spiracles. These are openings of tracheae, as al- 

 ready stated. According to Packard ^ the main system 

 of tracheae in the abdomen consists of six tubes, two 

 dorsal (PI. II., Figs. 15, 16, r//), two ventral (Fig. 15, 

 vi), and one at either side (Figs. 15, 16, st^. From the 

 latter branch small tubes whose external openings are the 

 spiracles. Besides these main tubes there are three pairs 

 of dilated tracheae (Fig. 16, abt^., abfi, abt^) near the end 

 of the abdomen. The air-tubes are found in the thorax 

 and head (Figs. 15, 16, ct, cephalic tracheae, of, ocular 

 tracheae) ; they also extend into the wings and legs. The 

 colorless blood flows from the heart into great lacunae 

 or 'cavities without proper walls. From thence a por- 

 tion of it, at least, passes to the wings, where it has been 

 seen flowing in a network of definite channels.'^ While in 

 the wings, according to some authors, the blood absorbs 

 oxygen from the air in the tracheae of these organs and 

 becomes purified, so that the wings are not only locomo- 

 tive, but also in part respiratory organs. 



1 See First An. Rep. U. S. Ent. Com., 1877, Chap. IX. 



2 See The Cockroach, Miall and Denny, 1886, Chap. VIII. 



