NO. 12 FLYING APPARATUS OF BLOW-FLY — RITTER 7 



The scutum (pi. i, fig. i; pi. 2, figs. 3, 4; pi. 8, fig. 23; s) is 

 the largest plate of the thorax. It is about twice as broad as long, 

 slightly convex dorsally, and extends downward laterally in an 

 obtuse angle to the root of the wing. Like the praescutum, it 

 is provided with bristles which form six longitudinal and three 

 transverse rows. Besides these rows there are usually present 

 on its lateral parts, where the scutum bends downward, four bristles, 

 one of which is situated on the tip of the spina scutalis (pi. i, 

 fig. I ; pi. 2, figs. 3, 4; pi. 8, fig. 23, sp). It is to be noted that 

 the lateral parts of the scutum that slope downward toward the 

 roots of the wings are not so strongly chitinized as the central 

 part. Distally, these parts of the scutum are split into several parts, 

 directly connected with the roots of the wings (see below). The 

 scutum possesses two lateral, sharp, marginal projections, the spinse 

 scutales (pi. i, fig. i ; pi. 2, figs. 3, 4; pi. 8, fig. 22>, sp), which ex- 

 tend obliquely forward and downward into the cristse spinales, which 

 separate the two (right and left) fossae prsealares (pi. 8, fig. 23, f^) 

 from the fossae postalares (pi. 8, fig. 23, /). These two pairs of de- 

 pressions have a special relation to flight. When the wing beats 

 upward and backward, the rigid portions, the marginal veins and 

 the remigium, come to lie in the fossae prsealares, the flexible, poste- 

 rior portions in the fossae postalares. When the wing makes its great- 

 est excursion backward and upward the crista spinalis adapts itself 

 exactly to the notch between the rigid and flexible portions of the 

 wing. This view is opposed to that held by Lowne. That observer 

 correctly describes (1890-1892) the spina scutalis as separating the 

 fossae prsealaris from the fossa postalaris, but in an illustration {loc. 

 cit., pi. 7, fig. i) he shows the fossa postalaris in front of the spine, 

 nearly in the position of the fossa prsealaris. He correctly depicts 

 an anterior fossa (pi. 2, fig. 3 ; pi. 8, fig. 23, Z^) at the boundary 

 between the prsescutum and the scutum, but he omits the special 

 fossa prsealaris. If the unnamed projection which appears in his 

 drawing farther in front, near the scutum, is intended to represent 

 the spina scutalis, it is incorrect, as this projection is in reahty much 

 nearer the scutellum. 



The two processes which extend from the scutum to the root of 

 the wing, and which articulate with the latter, are the processus 

 prsealaris (processus pterahs thoracis I; pi. 2, figs. 3, 4, pti), cor- 

 responding to the fossa prsealaris, and the processus postalaris (pro- 

 cessus pteralis thoracis II ; the great alar apophysis of Lowne ; pi. 

 2, figs. 3, 4, ptil), corresponding to the fossa postalaris. The first 



